How to design a heating system CIBSE Knowledge Series: KS8
Principal author Gay Lawrence Race
Editors Helen Carwardine Ken Butcher
CIBSE Knowledge Series — How to design a heating system Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Contents
1 Introduction ...... 1 1.1 Use of this guidance ...... 2
2 The heating design process ...... 4 2.1 The design process ...... 4 2.2 Heating system design process ...... 5 2.3 Key heating design calculation sequence ...... 8 2.4 Thermal comfort ...... 9
3 Key design steps ...... 10 3.1 Step 1: pre-design and design brief ...... 10 3.2 Step 2: gather design information ...... 11 3.3 Step 3: design data ...... 12 3.4 Step 4: building thermal performance analysis ...... 13 3.5 Step 5: heating system option analysis and selection ...... 15 3.6 Step 6: space heat losses and heat load ...... 20 3.7 Step 7: equipment sizing and selection ...... 23 3.8 Step 8: heating load analysis ...... 25 3.9 Step 9: plant sizing and selection ...... 27 3.10 Step 10: system analysis and control performance ...... 27 3.11 Step 11: Final value engineering and energy targets assessment 29 3.12 Step 12: design review ...... 29
4 Developing the design — key issues ...... 31 4.1 Design data ...... 31 4.2 Design margins ...... 31 4.3 Energy efficiency ...... 32 4.4 Quality control ...... 33
References ...... 34
Bibliography ...... 34
CIBSE Knowledge Series — How to design a heating system 1 Introduction
In cooler climates the provision of heating is an essential part of creating Heating comfortable internal environments, and therefore heating system design is a fundamental part of building services design. In 2005:
Heating is a major sector within mechanical building services. There are some ● 1.65 million new domestic boilers ● 21 million domestic properties in the UK with gas-fired central heating, and a 23,500 commercial boilers ● further 200,000 commercial properties with heating. The UK market for 9 million radiators ● 22 million metres of underfloor heating systems is substantial, with around 1.65 million new domestic boilers heating pipe
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 installed per year and around 23,500 commercial boilers. There are around 9 million radiators installed per year with a further 22 million metres of were installed in the UK alone. (1) underfloor heating pipe (2005 figures) . Sources: BSRIA domestic boiler marketing report March 2006, BSRIA commercial boiler marketing report Heating is also a major consumer of energy within the UK, with space heating March 2006. accounting for over 40% of all non-transport energy use and over 60% of domestic energy use(2), rising to over 80% if hot water is included (see Figure 1). As major energy users, heating and hot water also generate a substantial
proportion of CO2 emissions, delivering almost half the CO2 emissions from non-domestic buildings.
Given the current requirements to limit energy consumption and CO2 production, good design of heating systems is essential to ensure that systems Figure 1: operate efficiently and safely and make effective use of energy. Historically UK non-transport energy there have been problems with oversizing of heating systems which can lead use (2002 figures) million to inefficient operation, particularly at part load operation, to control tonnes of oil equivalent problems and to a reduction in plant operating life(3). The energy consumption for oversized plant can be 50% more than necessary. 11·8 Although heating is often considered to be a simple, basic system, there are 2·4 4·4
many options and permutations to be considered. The majority of UK 41·4 buildings will require heating but different building types and locations will 15·0 have very different requirements and constraints — consider for example the choices possible for a small ground floor flat in a city centre development 9·7 against those for a holiday cottage in one of the National Parks, or the 12·9 3·3 choices for an urban industrial unit against those for a rural agricultural unit
and farm shop. Space heating Water Cooking/catering The fundamental components of any heating system are: Lighting appliances Process use Motors/drivers — a means of generating heat, i.e. the heat source Drying/separation — a means of distributing the heat around the building or buildings, i.e. Other non-transport the distribution medium and network Source: DTI Energy consumption tables: overall energy consumption. URN No: — a means of delivering the heat into the space to be heated, i.e. the 05/2008 Table 1.2 Non-transport energy heat emitter. consumption by end use, 1990, 2000, 2001 and 2002
CIBSE Knowledge Series — How to design a heating system 1 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Good design There are many possible options to be considered, some of which are listed in Table 1 below. These can give many permutations, from the simple use of
Good design of heating systems is electric panel heating, using electricity both as the heat source and essential to ensure that systems operate distribution medium, to a conventional gas boiler system distributing low efficiently and safely and make effective use of energy. temperature water to a convector system. A more complex system would be one serving various buildings by using oil as the heat source to generate high temperature water for the main distribution, which is then reduced in temperature and pressure to low temperature water, via heat exchangers, to serve a radiator system.
Table 1: Heat source gas CHP Heating systems LPG solar oil biomass coal off-peak electricity electricity wind air or water via heat pump ground via ground source heat pump Distribution medium water: low, medium or high temperature air steam electricity Factors to consider Emitter radiators ceiling panels forced convectors natural convectors Building type: panel heaters underfloor heating coils ● domestic ● school ● apartment building Whilst heating systems may seem relatively simple, in practice there are many ● retail factors to be considered during the design process, in order to achieve a ● hospital well-designed system that delivers both the required comfort conditions and ● factory level of control whilst still minimising energy consumption. This publication, ● office together with other CIBSE guidance, aims to assist the designer in achieving that aim. Location:
● city centre 1.1 Use of this guidance ● urban ● suburban This publication provides a clear, step-by-step overview of the whole heating ● rural design sequence:
— section 2 maps the heating design process, with flowcharts illustrating the design steps sequence, and sets this in the context of the overall building process — section 3 outlines the key design procedures for each design step, and provides guidance on data requirements and sources, design outputs, key design issues and potential problem points
2 CIBSE Knowledge Series — How to design a heating system — section 4 addresses additional design issues that affect the design process.
The publication links to the CIBSE Design Guides and also cross-references other key industry sources of design procedure guidance. Other relevant titles in the Knowledge Series are:
— KS04 Understanding controls — KS06 Comfort — KS09 Energy efficient heating. Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 This guidance is intended to enable and assist building services engineers involved in design, installation and commissioning to appreciate the key decisions and design steps involved in heating system design. It is likely to be of particular benefit to junior engineers and those whose main experience lies within other sectors of building services design. It can also be used by building services engineers to facilitate discussion on design requirements and design decisions with their clients.
The publication answers the following questions, which can be used to help you find the most relevant sections to you:
— What are the key stages in the heating design process? (Section 2.2) — What are the design criteria for thermal comfort? (Sections 2.4 and 3.3) — What should I consider when selecting a heating system? (Section 3.5) — How do I determine preheat requirements? (Section 3.6) — What should I consider to determine the required heating load? (Section 3.8) — When should I consider load diversity? (Section 3.8) — What else should I consider during design? (Section 4).
Finally, a selected bibliography is provided for those who want further reading on the subject, subdivided to cover the main design steps and key topics such as design data, design calculations, design checks, heating plant and controls. Detailed technical information on heating system design and design data can be found in CIBSE Guide A (2006) and CIBSE Guide B (2001-2), chapter 1.
CIBSE Knowledge Series — How to design a heating system 3 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
2 The heating design process
2.1 The design process
Design involves translating ideas, proposals and statements of needs and requirements into precise descriptions of a specific product(4), which can then be delivered. (See Figure 2.) Two major features characterise the design process in general. Firstly, design tends to evolve through a series of stages during which the solution is increasingly designed at greater levels of detail, moving from broad outline through to fine detail. Secondly, design tends to contain iterative cycles of activities during which designs, or design components, are continually trialled, tested, evaluated and refined. Feedback is therefore an essential component of the design process, as shown in Figure 2.
Figure 2: 1. Client Feedback/ The design process need Inform review
Implement Design performance
4. Design 2. Design The design process delivery requirements
Feedback/ review
Select Develop 3. Design
Within construction, design is a part of the larger construction process, as shown in Figure 3. Both the RIBA Plan of Work Stages(5) and the ACE Conditions of Engagement Agreements A(2) and B(2)(6), which are commonly used for mechanical and electrical building services design, divide design into the separate stages of outline design, scheme design and further/detail design. In practice, therefore, the construction design process is invariably iterative, with many design steps being revisited and revised as the design evolves and develops, and this necessitates constant communication and clarification between team members.
4 CIBSE Knowledge Series — How to design a heating system ACE Agreements A(2) & Figure 3: RIBA plan of work (1999) B(2) (2002) Construction process A Inception/Identification of C1 Appraisal stage stages client requirements C2 Strategic briefing B Strategic brief Pre-design C Outline proposals C3 Outline proposals stage D Detailed proposals C4 Detailed proposals stage E Final proposals C5 Final proposals stage
F Production information C6 Production information Design stage
G Tender documentation C7 Tender documentation and Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 H Tender action tender action stage J Mobilisation/Project C8 Mobilisation, construction planning and completion stage K Construction to practical
completion Construction L After practical completion
2.2 Heating design process
The problem with the standard design process is that it is both complex and lacking in design task details. Although design is a clear part of the process, detail of the design tasks involved is not given beyond global statements such as ‘develop the design and prepare sufficient drawings…’.
Therefore, a simple straightforward design sequence for heating design has been developed (see Figure 4 over the page) to both clarify the process and allow detail of specific design tasks to be added. This gives a simplified linear design sequence, from the pre-design stage through the various analysis, decision and calculation steps through to the final solution, enabling design tasks to be clearly linked to both preceding and succeeding actions. Although some feedback loops are shown, in practice there are often feedback loops between all tasks and even within specific tasks, reflecting the more iterative nature of real-life design. Further detail on all of these steps is available in section 3.
It is important to still set this in the context of the full design process. In practice there are several design repetitions within the various stages, and overlaps from one stage to another. For example, information on overall space requirements and plant structural loadings is often required by other team members at the outline design stage. This degree of detail is unknown at this early stage therefore often assumptions and approximations have to be made in order to provide information. It is vital that these are checked as the design progresses.
CIBSE Knowledge Series — How to design a heating system 5 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Figure 4: Heating system design process Step no. Key design steps Design tasks
1 Pre-design Obtain design brief. Identify client and building user needs and requirements. Refer to feedback and lessons learned from previous projects
2 Gather design information Gather information about site, including utilities provision and fuel options. Obtain information on use of building, occupancy hours and on possible building form, fabric, etc
Establish and confirm key design requirements including Regulations and Codes of Practice. Establish planning conditions for use of on-site renewables
3 Design data Establish the key design data and parameters that relate to the design of the heating system, including building air tightness data, and potential use of renewables. Develop room design data sheets
Check that design parameters comply with legislation, energy targets, etc
4 Building thermal performance Analyse building – establish fabric thermal performance and infiltration analysis Determine whether intermittent operation is likely and consider potential pre-heat requirements
Estimate approximate building total heat loss to inform system selection process
5 Heating system option Consider zoning requirements. Consider alternative heat source (fuel) and heating analysis and selection system options. Establish contribution from renewable sources
Consider operating and control strategies, and building usage and layout data. Assess options against client requirements, performance, risk, energy use, etc
Select proposed system
6 Design calculations Calculate space heat losses. Assess ventilation requirements and provision. Assess Space heat losses and heat load HWS provision
Check system selection choice still appropriate. Determine pre-heat requirements
7 Equipment selection and sizing Consider suitable emitter positions and connections. Check distribution layout considering balancing and regulating requirements. Consider circuit layouts and connections and pumping choices – variable or constant volume. Develop control requirements
Size and select emitters and distribution network and determine any distribution losses
8 Design calculations Determine other loads such as HWS and process. Heating load analysis Calculate main heating loads. Analyse load diversity and pre-heat requirement and determine the total heating load
9 Plant sizing and selection Consider any standby requirement. Determine number of boilers /modules required and size and select main plant. Finalise control requirements
Check layouts and services co-ordination for clashes and ease of commissioning and maintenance
10 Design calculations Review system design and check predicted system performance. System analysis Check part load performance Control performance Check that the selected controls are capable of achieving the required level of control, response and energy efficiency, particularly at part load
11 Final value engineering and Check that final system and components meet client requirements for energy targets assessment performance, quality, reliability, etc at acceptable cost; and also meet required energy targets and comply with Regulations, such as meeting the seasonal efficiency requirements
12 Review Design review
6 CIBSE Knowledge Series — How to design a heating system As the design develops, these design steps are revisited and further detail added with more accurate analysis as additional information becomes available. The steps and amount of repetition involved will differ from design to design but an example is illustrated in Figure 5. This uses the same design steps numbers as Figure 4 to show how the different steps are repeated and Figure 5: revisited as the design develops. The detailed design tasks at each step have Heating design process been omitted to keep the diagram to a manageable size. mapped against the main design work stages Design stage Step no. Key design steps Key outputs
Pre-design 1 Pre design: obtain client brief. Refer to feedback and lessons learned from previous projects Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 Outline design 2 Gather design information and establish key design requirements. Design brief Establish planning requirements Outline drawings and schematics. 3 Establish key design data Provisional cost plan
4 Initial building thermal performance analysis. Approximate heat loss
5 Heating system – consider options and fuel choices
7 Consider system requirements, potential layout, etc
9 Approximate total loads and plant size to arrive at cost plans, provide space requirements and structural load information, etc.
Scheme / Detail design 2 Gather further necessary design information and establish key design Design drawings and requirements schematics. Cost plan 3 Establish key design data
4 Detailed building thermal performance analysis
5 Heating system choice and selection
6 Design calculations: space heat losses
7 Equipment selection and sizing – emitters and distribution network. Control requirements
8 Design calculations: heating load analysis, possibly including thermal modelling
9 Initial plant and control selection
11 Value engineering workshops
12 Interim design review
Design development/Final 4 Further building thermal performance analysis, to assist in modelling Design drawings and proposals/Production dynamic building and system performance (if required) specification for tender information purposes. 7 Final equipment selection and sizing Possibly co-ordination drawings. 8 Final heating load calculation and analysis Final cost appraisal
9 Plant selection. Control requirements. Preparation of detailed design drawings and specifications for plant and equipment
10 Design calculations. System performance analysis, including part load performance and predicted energy use. Possible final dynamic modelling of building and system performance. Control performance
11 Final value engineering exercise
12 Final design review
Post-occupancy review
CIBSE Knowledge Series — How to design a heating system 7 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
2.3 Key heating design calculation sequence
Within the overall heating design sequence there are some specific calculations that will need to be carried out, and the sequence of these can also be illustrated as shown in Figure 6. These mainly take place during steps Figure 6: 4, 6 and 8 — building performance analysis, heat losses and load analysis; Key steps for heating continuing into system and equipment sizing in steps 7 and 9, and system design calculation analysis in step 10. sequence
Building air- Internal and external Fabric tightness details design conditions details
Condensation 'U' values Site weather risk analysis data Infiltration Fabric heat heat loss loss
Natural ventilation Space heat Internal gains (only Building thermal load (if any) loss if both heating and response analysis gains are continuous)
Emitter Space heating sizing load Pre-heat margin
Distribution system Infiltration load sizing diversity Intermittent operation assessment Distribution system Maximum simultaneous HWS losses space heating load load
Load diversity Process analysis load Part load performance Total heating Central fresh air load ventilation heating load
Boiler/heating Standby capacity plant sizing (if required) Final system and control performance analysis Flue Fuel supply sizing system sizing
8 CIBSE Knowledge Series — How to design a heating system 2.4 Thermal comfort Figure 7: Design output For heating design, thermal comfort could be regarded as the main output of Input Output the design process, as shown in Figure 7. Certainly most clients do not ask Client Design Thermal for a heating system as part of their design brief — their focus is on what need process comfort systems deliver and not how they do it. What clients really require is the building services design to deliver comfortable working or living conditions to enable their business to function efficiently. An understanding of thermal comfort is therefore central to good heating system design.
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 Although there are many factors to take into account, thermal comfort is fundamentally about how people interact with their thermal environment. Generally, a reasonable level of comfort is achieved where there is broad satisfaction with the thermal environment, i.e. most people are neither too Thermal comfort hot nor too cold.
The four main environmental factors that affect thermal comfort are: ‘That condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation.’
— air temperature (ta) ASHRAE Standard 55-2004 — relative humidity
— mean radiant temperature (tr) — air velocity (v).
All of these are affected by the choice of heating system and the way it delivers heat to the space.
Building designers should aim to provide comfortable conditions for the greatest possible number of occupants and to minimise discomfort. This is achieved by considering comfort requirements and setting appropriate design criteria.
For the thermal environment, these would usually be the operative Key factors in thermal comfort temperature and humidity, together with a fresh air supply rate. A typical initial winter design condition might therefore be written as 21 °C and 50% ● temperature RH for operative temperature and relative humidity respectively, with 10 l/s ● humidity per person of fresh air required. More often some variation is allowed, i.e. ● air movement 21 °C ±1 °C and 50% RH ±10%. Example design criteria for a range of ● air quality. building types are given in section 3.3.
For a further discussion of comfort, see CIBSE Knowledge Series KS06 Comfort, and CIBSE Guide A, chapter 1.
CIBSE Knowledge Series — How to design a heating system 9 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
3 Key design steps
This section covers the key steps in the heating design process given in sections 2.2 and 2.3 in more detail to give some further guidance. Key design outputs from each stage are summarised and additional reference sources provided.
3.1 Step 1: pre-design/design brief
Depending on the type of project, the design brief may evolve during the course of the initial project stages. However, design briefs do not usually ask for specific heating systems, they tend to concentrate on the outcomes that must be achieved, i.e. the internal conditions that must be delivered. The brief may simply ask for a heated building, with specific comfortable working conditions. Design of any system must therefore relate to the functional brief, and be seen in the context of the full design requirements.
During the initial design process the building services engineer can potentially provide input on ways to optimise building performance and reduce energy loads, including advice on:
— building form and orientation to optimise the impact of solar gain — building air tightness, to reduce infiltration — fabric insulation — optimisation of glazing, balancing daylighting needs against thermal performance — building thermal mass.
Much design data and information can be gained from the client brief and occasionally additional input will be needed from the client to clarify points or to provide missing data in order to develop the design brief. Some client briefs will include the necessary initial design data such as internal design conditions, in some cases this will need to be advised. In both cases it is sensible to check any data provided against good design practice.
Input to the design brief can include advice on:
— future need design requirements — comfort requirements — ventilation strategy — spatial requirements — standards and regulations — energy strategy — operating strategy including facilities maintenance requirements — plant life expectancy and replacement strategies
10 CIBSE Knowledge Series — How to design a heating system — control strategy. Key design outputs for step 1: pre-design
Information required from the design brief can include: ● functional design brief.
— required functional performance — occupancy — usage details and potential internal loads — internal design conditions — cost plan.
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 (Further detail of this is given in step 2.)
3.2 Step 2: gather design information
A large amount of information is necessary to inform the various design stages, and as such this task is ongoing throughout the design stages. Much of the information is available from the original client brief or statement of requirements, and additional information can be sought by additional questions. Other data must be gathered from other sources such as site visits, etc. Some key initial information is given in Figure 8.
Specific information required Outputs Figure 8: Location: Geographical location and height above sea level Information gathering Local microclimate, wind Information on local conditions – pollution, External design noise conditions Site Orientation: Details of surrounding information buildings, shading, etc
Services: Utilities provision and positions Available services
Access: Access to site
Possible comfort Functional performance: Specific or energy deliverables requirements
Operating strategy: Client approach to Possible system building design and operation including constraints or sustainability, energy strategy, control, requirements maintenance, etc Cost budgets Costs: Cost plans and budgets and constraints Client brief Internal design Key design outputs for step 2: Occupancy: Information on occupancy conditions information gathering activity and density Assessment of Hours of occupation, etc intermittent system operation ● Building use: Tasks, office equipment, etc Internal loads – key design requirements small power, lighting, etc ● necessary information to establish Future needs: Future proofing and internal and external design data flexibility requirements Additional system ● requirements site assessment and utility provision ● statutory and regulatory design Standards and Statutory and regulatory Design requirements requirements and targets. regulations requirements Energy targets, including % energy to be provided from renewable sources
CIBSE Knowledge Series — How to design a heating system 11 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Building Regulations Part L 2006 The building services engineer will also need to provide information to other design team members throughout the project. As outlined in section 3.1, at Heating systems should be designed to the initial design stages this can include advice on optimising building minimise carbon emissions and make it performance, and can also include information on potential spatial easier for the whole building to achieve a requirements, which can be refined as the design develops. building CO2 emission rate (BER) lower than the set target (TER) and thus comply with Part L requirements, which implement the EPBD directive. The new Building Regulations Part L (2006) requires that both fabric and services heat losses are limited and that energy efficient services with effective controls are provided. Details are provided in the second tier documents such as the Non-domestic heating, cooling and ventilation compliance guideand the Domestic heating compliance guide.
3.3 Step 3: design data
The fundamental initial design data needed for design of a heating system to deliver comfortable conditions are the:
— internal design conditions — external design conditions.
The design conditions selected can have a substantial impact on both system loads and subsequent system performance and therefore care must be taken to select appropriate values. See section 4.1 for further discussion.
Internal design criteria may be specified in the brief, or a required functional performance may be asked for and the designer will have to specify the Key design outputs for step 3: required conditions. In either case these will need to be checked against good design data practice design standards.
● internal thermal comfort design conditions Table 2 gives example winter internal design conditions for thermal comfort ● schedule of internal design for a range of common building types. More detailed guidance for a wider criteria for each space (e.g. on range of building and room types is given in CIBSE Guide A, Table 1.5, which room data sheets) ● external design conditions. also relates the design guidance to the expected clothing and metabolic rates of occupants to achieve a predicted percentage persons dissatisfied (PPD) of around 5%. For design purposes reference should be made to the full table together with the associated footnotes.
12 CIBSE Knowledge Series — How to design a heating system Suggested air supply rate Table 2: Winter operative temp Building/room type l/s per person range °C Recommended winter thermal (unless stated otherwise) comfort criteria for some Dwellings selected building types bathrooms 20-22 15 l/s (Source: CIBSE Guide A, Table 1.5) bedrooms 17-19 0.4-1ACH halls, stairs 19-24 - kitchen 17-19 60 l/s living rooms 22-23 0.4-1ACH Offices conference/board rooms 22-23 10 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 computer rooms 19-21 10 corridors 19-21 10 drawing office 19-21 10 entrance halls/lobbies 19-21 10 general office space 21-23 10 open plan 21-23 10 toilets 19-21 >5ACH Retail department stores 19-21 10 small shops 19-21 10 supermarkets 19-21 10 shopping malls 12-19 10 Schools teaching spaces 19-21 10 Notes: 1. ACH stands for air changes per hour. 2. For design purposes, please refer to the full Table 1.5 in CIBSE Guide A. External design conditions
Selection of appropriate external design criteria requires information on the Appropriate design criteria should be site location, development details and local microclimate, as outlined in agreed with the client, taking into consideration the acceptable risk of section 3.2, as well as meteorological data. The type of building and the exceedence of design conditions. thermal inertia will also help to determine what may be an acceptable risk of exceedence of conditions, and this will need to be discussed and agreed with the client. Further guidance is provided in CIBSE Guide A, chapter 2 and in CIBSE Guide J (2002).
3.4 Step 4: building thermal performance analysis
The thermal performance of the building will need to be established to enable the calculation of heat losses, assess preheat requirements and calculate the heating loads.
CIBSE Knowledge Series — How to design a heating system 13 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Figure 9: Specific information required Outputs
Building form and fabric Layout information to Building plan and form: Details of inform services building plan and form location, zoning Building orientation and shading strategy, etc Building layout Glazing locations, etc Room dimensions Constraints on Building Internal layout: Layout drawings emitter information Potential space use and fit out positioning Partitioning Constraints on distribution space Plant and distribution space: Potential location and space required/ Possible system available (should be discussed and agreed constraints or with rest of design team as early as requirements possible in the design)
Fabric: Detail of building materials Thermal mass and construction assessment Glazing height Fabric thermal performance (heavy or light weight) Glazing: Glazing information – type, Fabric and glazing Fabric U-values information dimensions, including glazing height, Glazing height influences comfort within and thermal performance Fabric admittance the occupied space both due to Y-values downdraughts and to cold radiation Window leakage Air tightness: Construction quality rates which affects the mean radiant Building air tightness prediction temperature. Infiltration data
Calculation procedures and data required to establish the fabric thermal properties, including the transmittance details, i.e. the fabric and glazing ‘U’ values, are given in CIBSE Guide A, chapter 3, together with ‘U’ values for standard constructions. This information, together with the design conditions from step 3 (section 3.3), and site data from step 2 (section 3.2), will also enable the analysis of condensation risk, if this is part of the agreed design duties. Key steps in the calculation sequence related to this and the building thermal response are shown in Figure 10 in dark blue. Figure 10: Building air- Internal and external Fabric Key steps to analyse building tightness details design conditions details thermal performance
Site weather Condensation 'U' values data risk analysis
Infiltration Fabric heat heat loss loss
Space heat Building thermal loss response analysis
14 CIBSE Knowledge Series — How to design a heating system Key design outputs for step 4: building thermal performance As the thermal insulation performance of the building fabric has improved, the infiltration component of heat loss can now comprise a substantial ● fabric thermal transmittance percentage and therefore needs to be estimated as accurately as possible. details, i.e. the fabric and glazing Although building air leakage testing will be required for most buildings, and ‘U’ values will form part of the design requirements, this sets an expected standard, ● building thermal response (and dynamic thermal performance generally specified for a specific applied pressure difference such as 50 Pa, characteristics including and therefore does not provide data for infiltration calculations. Methods for admittance values, if required) estimating infiltration rates are given in CIBSE Guide A, chapter 4, with ● infiltration assessment for individual spaces and for the additional guidance in CIBSE AM10. whole building
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 ● assessment of intermittent An initial assessment of building use and hours of occupancy will determine if operation to inform preheat requirements intermittent, rather than continuous, operation is likely. Details of the overall ● estimation of approximate total building thermal response will be needed to determine the likely preheat building heat loss. requirements and the impact on heating system performance (see also section 3.6). More detailed modelling of the building and system dynamic
performance can then be carried out at a later design stage if required. Infiltration estimation
An initial estimate of total building heat loss can be useful at this stage to help A useful cross check for infiltration inform system choices, just to give an approximate global figure. The system estimation is to convert the estimated choices that are reasonable for a 50 kW loss can be very different from those infiltration total to a room or whole building air change rate, as appropriate. for a heat loss of 1,500 kW, for example.
3.5 Step 5: heating system option analysis and selection
Heating system choice depends on many factors. These can be loosely grouped into two areas relating to practical system installation and to performance and use factors. Zoning Installation factors include:
Zoning strategy needs to be agreed with — space required/available — both for plant and for distribution the client. Some variation in internal conditions may be acceptable, which can — potential plant room locations related to the spaces to be served help to minimise the number of zones — cost plan — capital cost of installation and improve operating efficiency. — zoning requirements — flexibility — any requirements for future change of use or changes in fitout — ease of installation — access, materials, etc — ease of commissioning.
Performance and use factors include:
— cost — comfort — control
CIBSE Knowledge Series — How to design a heating system 15 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
— convenience. To determine the most appropriate system to meet the client’s requirements, an assessment of options against some of these factors can be helpful. System choices can be compared using, for example, a ranking and weighting matrix to assess suitability using some of the key usage factors related to system choice. Information on the client’s operating and control strategy will also inform the decision process. Table 3: Cost operating and maintenance costs System performance and use energy efficiency factors carbon emissions and energy usage. Comfort balance of radiant and convective heat output to provide comfort conditions time taken to achieve comfort conditions from start up evenness of heat distribution throughout space noise level. Key design outputs for step 5: Control ability to provide accurate control of space temperature heating system selection ability to provide localised control speed of response to changing conditions. ● zoning strategy for building — to give details of building zones and Convenience ease of use required operating conditions — location hours of use and internal design conditions potential lettable/usable space taken up by emitters/outlets and distribution ● selection of heating system(s) in principle — fuel/heat source, ease of maintenance. system, distribution medium and emitter types.
Key design decisions will include the choice of:
— heat media and distribution system — system — centralised or de-centralised — heat emitter
Table 4: Heat media the balance between radiant and convective output Heating system design choices required from the system space required for distribution speed of response to changing conditions, and on start up. System centralised or de-centralised – potential plant locations. Heat emitter characteristics including the balance between radiant and Low and zero carbon technologies convective output location to provide uniform temperatures noise level Part L (2006) of the Building Regulations encourages the use of low and zero space required. carbon (LZC) technologies, such as Heat source conventional boilers or other heat sources such as heat renewables, CHP and heat pumps, as a pumps, CHP, etc way of meeting the required carbon emission reductions, and implementing boiler and fuel type, any storage requirements the requirements of the EPBD directive. central plant location. Many local planning authorities also encourage the use of these technologies, — heat source. in some cases making it a specific planning requirement. The following tables provide further information on some system options,
16 CIBSE Knowledge Series — How to design a heating system giving some characteristics and relative advantages and disadvantages together with some selection flow charts for heating systems and fuels. Although not included on the diagrams, note that, in addition to CHP, other low and zero carbon technologies such as renewables should also be considered as heat source options. Further information on heat emitters and Figure 11: heating systems is given in CIBSE Guide B, chapter 1. Selection chart: heating systems Note : This selection chart is intended to give initial guidance only; Start here it is not intended to replace more rigorous option appraisal N Y Constraints on combustion appliances in workplace? Considering CHP, waste fuel or local community N Y heating system available as source of heat? Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 Most areas have similar heating requirements N Y in terms of times and temperatures?
Decentralised system Centralised system N Y N Y Significant spot heating (>50% of heated space)? N Y N Y Above average ventilation rates? N Y N Y Non-sedentary workforce? Radiant heat acceptable N Y to process? N Y N Y
Convective Medium or high temperature Convective Low temperature system radiant system system radiant system Source: CIBSE Guide B, chapter 1, Figure 1.2, itself based on the Carbon Trust Good Practice Guide 303(7)
Figure 12: Selection chart: fuel Decentralised system Centralised system
Waste fuel or local community heating N Y available as source of heat? N Y N Y Strategic need for back-up fuel supply? N Y N Y N Y Natural gas required?
Radiant heat required? N Y
Natural Natural gas + Electricity for gas oil back-up Community Community high temperature or waste with or waste Oil or systems, LPG Oil + LPG Community oil or LPG with gas LPG for medium electricity or waste heat back-up back-up temperature systems back-up Source: CIBSE Guide B, chapter 1, Figure 1.3, itself based on the Carbon Trust Good Practice Guide 303(7)
CIBSE Knowledge Series — How to design a heating system 17 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Table 5: Heat distribution media
Medium Principal characteristics Advantages Disadvantages Air Low specific heat capacity, low No heat emitters needed Large volume of air required — density and small temperature No intermediate medium or large ducts require more difference permissible between heat exchanger is needed distribution space supply and return, compared to Fans can require high energy water, therefore larger volume consumption needed to transfer given heat quantity Water High specific heat capacity, high Small volume of water required Requires heat emitters to density and large temperature — pipes require little transfer heat to occupied space difference permissible between distribution space supply and return, compared to air, therefore smaller volume needed to transfer given heat quantity. Usually classified according to water temperature/ pressure: — LTHW (LPHW) Low temperature/pressure hot Generally recognised as simple Output is limited by system water systems operate at to install and safe in operation. temperatures temperatures of less than 90 °C Use with condensing boilers to (approx.), and at low pressures maximise energy efficiency that can be generated by an open or sealed expansion vessel — MTHW (MPHW) Medium temperature/pressure Higher temperatures and Pressurisation necessitates hot water systems operate at temperature drops give smaller additional plant and controls, and between 90–120 °C (approx.), pipework, which may be an additional safety requirements with a greater drop in water advantage on larger systems temperature around the system. This category includes pressurisation up to 5 bar absolute — HTHW (HPHW) High temperature/pressure hot Higher temperatures and Safety requires that all pipework water systems operate at over temperature drops give even must be welded, and to the 120 °C, often with higher smaller pipework standards applicable to steam temperatures — perhaps up to pipework. This is unlikely to be a 200 °C, with even greater cost-effective choice except for temperature drops in the the transportation of heat over system. These temperatures will long distances require pressurisation up to around 10 bar absolute Steam Exploits the latent heat of High maintenance and water condensation to provide very treatment requirements high transfer capacity. Operates at high pressures. Principally used in hospitals and buildings with large kitchens or processes requiring steam
18 CIBSE Knowledge Series — How to design a heating system Table 6: Centralised versus non- centralised systems
Centralised Non-centralised Capital cost Capital cost per unit output falls with increased Low overall capital cost, savings made on minimising capacity of central plant. the use of air and water distribution systems Capital cost of distribution systems is high Space requirements Space requirements of central plant and distribution Smaller or balanced flues can often be used systems are significant, particularly ductwork Flueing arrangements can be more difficult in some Large, high flues needed locations Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 System efficiency Central plant tends to be better engineered, operating Energy performance in buildings with diverse patterns at higher system efficiencies (where load factors are of use is usually better high) and more durable As the load factor falls, the total efficiency falls as distribution losses become more significant System operation Convenient for some institutions to have centralised May require more control systems plant Zoning of the systems can be matched more easily to Distribution losses can be significant occupancy patterns System maintenance Central plant tends to be better engineered, more Can be readily altered and extended and operational life durable Equipment tends to be less robust with shorter Less resilience if no standby plant provided operational life Plant failure only affects the area served Maintenance less specialised Fuel choice Flexibility in the choice of fuel, boilers can be dual fuel Fuel needs to be supplied throughout the site Better utilisation of CHP, etc Boilers are single fuel Some systems will naturally require central plant, e.g. heavy oil and coal burning plant Based on data from CIBSE Guide F (2004), chapter 10.
CIBSE Knowledge Series — How to design a heating system 19 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Table 7: Common emitter/system types
Design points Advantages Disadvantages Radiators Output up to 70% convective Good temperature control Fairly slow response to control Check for limit on surface Balance of radiant and convective Slow thermal response temperature in some applications, output gives good thermal comfort e.g. hospitals Low maintenance Cheap to install Natural convectors Quicker response to control Can occupy more floor wall space Skirting or floor trench convectors Can get higher temperature can be unobtrusive stratification in space Underfloor heating Check required output can be Unobtrusive Heat output limited achieved with acceptable floor Good space temperature Slow response to control surface temperatures distribution with little stratification Fan convectors Can also be used to deliver Quick thermal response Can be noisy ventilation air Higher maintenance Occupies more floor space
Warm air heaters Can be direct fired units Quick thermal response Can be noisy Can get considerable temperature stratification in space Low temperature radiant Ceiling panels need relatively low Unobtrusive Slow response to control panels temperatures to avoid discomfort Low maintenance High temperature radiant Can be direct gas or oil fired units Quicker thermal response Need to be mounted at high level heaters Check that irradiance levels are Can be used in spaces with high to avoid local high intensity acceptable for comfort air change rates and high ceilings radiation and discomfort
3.6 Step 6: design calculations: space heat losses and heat load
The next step in the design sequence is to take the information on the building fabric and infiltration performance from step 4 and use this to establish both infiltration and fabric heat losses for each space to give an individual heat loss for each building space that will require heating. Information on the type of heating system and emitter selected is also required, as both manual calculations and the majority of software packages will require information on the relative radiant and convective outputs as part of the input data.
Heat losses CIBSE Guide A, chapter 5 provides details of the required calculation procedures for heat losses, covering both a steady state heat loss approach A useful cross check for heat losses is to and a dynamic approach which can provide more detailed analysis if required, 2 convert the calculated values to W/m or including modelling of building and system thermal response. Section 5.6.2 of W/m3 figures to check against reasonable benchmarks. CIBSE Guide A provides a worked example for the steady state heat loss calculation.
Key steps in the calculation sequence for space heat loss are shown in Figure
20 CIBSE Knowledge Series — How to design a heating system Figure 13: Building air- Internal and external Fabric tightness details design conditions details Key steps to establish individual space heat losses
Site weather Condensation 'U' values data risk analysis
Infiltration Fabric heat heat loss loss Heat losses — temperatures
Space heat Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 loss Care needs to be taken when considering the temperatures to use for heat loss calculations. Design criteria are usually 13 in dark blue. given as operative temperatures (to). Fabric heat losses should use the internal environmental temperature (tei) and With better fabric insulation the infiltration heat loss can now account for up infiltration loss the internal air temperature (tai). These can differ to 50% of the total heat loss in some smaller buildings and therefore substantially for some buildings and some infiltration rates need to be estimated as accurately as possible — see section heating types. CIBSE provides a method for steady state heat losses that applies 3.4. correction factors F1 and F2 to enable the design internal operative temperature to be used — see CIBSE Guide A, section To move from the heat loss to the heat load for a space, additional factors 5.6.2. (Note: for very well insulated need to be considered, including any additional loads within the space and buildings, without large areas of glass, and with low air change rates, there is often little difference between operative, Infiltration Fabric environmental and air temperatures.) heat loss heat loss
Building thermal Figure 14: Space response heat loss analysis Key steps to establish space heating loads
Natural ventilation Internal gains (only Intermittent load (if any) if both heating and operation gains are continuous) assessment
Space Pre-heat heating load margin
any preheat requirements, as shown in Figure 14. Radiant systems
An assessment of ventilation provision is required at this stage, as although For high temperature radiant systems the this is likely to be met by a separate system in most buildings, it will in some standard heat loss calculation methods cases be met by natural ventilation, in which case it will add an additional heat are not appropriate for equipment selection. Instead the distribution of load directly to the space. Further information on naturally ventilated radiant energy in the space should be buildings is given in CIBSE AM10 and on mixed mode buildings in CIBSE determined, utilising a radiant polar diagram for the emitter. Further guidance AM13. is given in CIBSE Guide A, section 5.10.3.7 and CIBSE Guide B, section 1.4.6. Medium and low temperature A preliminary assessment of other loads that may also need to be met by the radiant systems can be sized using the main heating source, such as any HWS load, can also be made at this stage to usual heat loss calculation methods.
CIBSE Knowledge Series — How to design a heating system 21 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
HWS provide information for the next calculation step (see also section 3.8). Internal gains HWS requirements and options should be assessed, e.g. consider whether Normally no allowance would be made for internal gains in establishing space storage or instantaneous water heating is more appropriate. For hot water storage heating loads as a worst-case scenario is always considered, i.e. to bring the consider the options of a dedicated boiler unoccupied building up to temperature. However, exceptionally, if the heating or a standalone hot water generator (direct-fired storage system). For will be operating continuously and there are constant heat sources such as instantaneous hot water consider the electric lights and occupants in a continuously occupied building, then the choice and availability of fuel and whether point-of-use provision or multi-outlet is steady state heat requirement can be reduced by the amount of the constant more suitable. gains. However the risks of this should always be made overt to the client as if any gains are removed or reduced or the building is operated intermittently then the system may not be able to achieve the design temperatures.
Preheat requirements
The building thermal capacity will affect the way the building responds to heat input, meaning the rate at which it warms up and cools down. For any building that is heated intermittently this will need to be considered as the building will cool down during the unoccupied periods and then need to be brought back to temperature. For heavyweight buildings with a high thermal capacity, and/or those intermittently occupied, some additional heating capacity will be required to ensure that the building can warm up and achieve the design temperature before the start of the occupied period; the preheat time (see Figure 15). This additional capacity is required by the space heating
Figure 15: Preheat Design inside temperature erature Intermittent operation p
Preheat
Intermittent heating occurs when the Inside tem time heating plant is switched off at or near the end of a period of occupancy and then turned back on at full capacity prior to the next period of occupancy in order to bring the building back to the design Plant off Optimised Start of temperature. There are two main types start time occupancy of intermittent operation: Time
● normal intermittent operation is where the output is reduced system, i.e. the emitters, as well as by the main heating plant. when the building is unoccupied In order to assess the preheat requirements, information on both — for example to a level of 10 °C to protect the building intermittent operation and on the building thermal response is needed. For fabric and contents normal intermittent operation the plant and equipment will need to be larger ● highly intermittent is where the building is occupied for short than the steady state requirements, with the required capacity calculated by periods only and therefore needs applying an ‘intermittency factor’ F3, based on the thermal response factor to be brought back to temperature quickly prior to use. for the building and the total hours of plant operation:
22 CIBSE Knowledge Series — How to design a heating system Plant size ratio
Peak heating load = F3 x space heat load
The intermittency factor F3 can also be Details are given in CIBSE Guide A, section 5.10.3.3 and Appendix 5.A8, and expressed as a plant size ratio (PSR) defined as: in CIBSE Guide B, section 1.4.7.3. PSR = installed heat emission design peak steady state If the calculated value of F3 is less than 1.2, CIBSE suggests that the value be heat load taken as 1.2 to ensure that a reasonable margin of 20% for preheat is applied, although other values may be used, for example by using a dynamic simulation model to more accurately assess the required excess capacity. Full
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 analysis of building thermal response can require dynamic rather than steady state modelling and this is discussed further in CIBSE Guide A, chapter 5.
Key design outputs for step 6: space CIBSE suggests in Guide B, chapter 1 that acceptable values for F3 lie in the range heat losses and heat load 1.2–2.0, with research(8) indicating that values over 2.0 cannot be economically justified for most buildings and could result in considerably oversized plant. The ● schedule of individual space and same research found that a value of 1.5 was a more typical economical value for zone heat losses, subdivided into fabric and infiltration losses, the cases investigated. For small buildings and small plants the optimum values together with details of the will be even lower. The use of optimum start control, as illustrated in Figure 15, internal design conditions ● assessment of preheat can help to ensure adequate preheat time in cold weather. requirements for the building ● schedule of space heating loads. For highly intermittent systems, a steady state heat loss is inappropriate to size the system and a dynamic simulation model that considers the way heat is absorbed by the building fabric is required. Details are given in CIBSE Guide A, section 5.10.3.3.
3.7 Step 7: equipment sizing and selection
Once the individual room losses and space heating loads have been determined and decisions have been made on the system, emitters, etc, then the system can be sized and emitters selected. Key steps for this are shown in Figure 16 below. It is possible that alternative solutions are still being investigated at this stage, in which case further comparison in terms of cost,
Figure 16: Emitter Space heating Preheat sizing load margin Key steps for emitter and distribution system sizing
Distribution Infiltration Heat transfer correction factors system sizing load diversity Distribution system losses The type of heat emitter can have a significant effect on the calculated design Maximum steady state heating load, so it is essential simultaneous that appropriate values for the heat space heating load transfer correction factors F1 and F2 were used at step 6.
performance and energy efficiency may be required to reach a final decision.
CIBSE Knowledge Series — How to design a heating system 23 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Heat emitters The heat output from the emitter, and therefore the size required, will be Check that the manufacturer’s published affected by its position within the space and local effects such as furniture data is applicable to the conditions at which the emitter will be operating and positions, etc. For example if emitters are situated behind furnishings then apply any relevant corrections for space most of the immediate radiant heat output will be lost, and in some cases temperature, water temperatures, etc. Note that manufacturers’ outputs are even the convective heat output can be obstructed and reduced. Although based on particular space and water much of the heat will eventually enter the space it may not be available temperatures which may differ from the design operating conditions. during preheat and therefore an allowance may be need to be made and the required heat output increased to compensate. Details are given in CIBSE Guide A, section 5.10.3.2.
Some heating systems, such as warm air, can lead to considerable temperature stratification in the space — see Figure 17. This means that the inside temperature at high level is much higher than that used in heat loss calculations and therefore the heat loss through the ceiling/roof will be greater than anticipated. A correction to the heat loss, to allow for the height of space and system used, will need to be applied — for example a 5–15% increase in the fabric component of heat loss for a low level forced warm air system used in a space 5–10 m high. Further guidance is given in CIBSE
Figure 17: Radiator Underfloor heating Warm air heater at high level 3·0 Vertical air temperature gradients for different heating types 2·0 Source: CIBSE Guide A, Figure 5.6.
Room height / m 1·0
0 15 20 25 15 20 25 15 20 25 Air temperatures / °C
Key design outputs for step 7: Guide A, section 5.10.3.2 and in Table 5.15. emitter and distribution system sizing These corrections can now mean that, for certain heating systems, the required emitter load is larger than the original space heating load. Once the ● schedule of emitters with required output, and with surface emitters have been sized then the distribution layout can be determined and and water temperature for the system sized. Guidance on pipe and duct sizing is given in CIBSE Guide C hydronic systems ● initial control requirements (2001), chapter 4. When determining the most appropriate layout for the ● layout drawings with emitter distribution system, balancing and regulating requirements should be positions considered, e.g. the use of reverse return pipework layouts to aid system ● schematic of pipework layouts balancing during commissioning. with required flowrates for hydronic systems. The system distribution losses will need to be assessed. Those from within the space can contribute to the required space heating load. However any
24 CIBSE Knowledge Series — How to design a heating system non-useful distribution losses will need to be allowed for within the overall heating load for the building. Whilst for energy efficiency distribution losses should be minimised, for example by insulating pipes that run through non- occupied areas, an allowance will still need to be made. Guidance is given in CIBSE Guide C, chapter 3.
3.8 Step 8: design calculations — heating load analysis
Once individual space heating loads have been determined, and the emitters and distribution system sized, an overall heating load can be determined. This
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 will require establishing all the various heat loads that may need to be met, such as:
— space heating loads — any system distribution losses —HWS load — central fresh air ventilation heating load — if ventilation air is provided centrally by mechanical ventilation systems — any potential process load.
The first step is to establish the maximum simultaneous space heating load — see Figure 18. Having already considered the preheat requirements for the space(s), and sized the emitters, an allowance needs to be made for any non-
Figure 18: Emitter Space heating Preheat sizing load margin Key steps to establish the maximum simultaneous space heating load Distribution Infiltration system sizing load diversity Distribution system losses Maximum simultaneous space heating load useful distribution losses, as discussed in step 7.
Infiltration load diversity
For individual spaces the maximum heat loss is always required to size any emitters for that space. However when considering the total space heating load for sizing central plant, some diversity can be applied to infiltration, to allow for the fact that infiltration of outdoor air will only take place on the windward side of the building at any one time, with the flow on the leeward side being outwards. This suggests that the total net infiltration load is usually about half of the summation total for the individual spaces, although the infiltration patterns for individual building configurations should always be
CIBSE Knowledge Series — How to design a heating system 25 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
considered carefully. This exercise is important as, given current high levels of fabric insulation, the infiltration component of heat loss is now substantial, often accounting for up to 50% of the total in small buildings. CIBSE Guide A, chapter 4 provides further guidance on infiltration.
The next step is to consider the other loads that may need to be met by the heating plant and carry out an assessment and analysis of load diversity — see
Figure 19: Preheat Key steps to establish the total margin heating load Maximum simultaneous space heating load HWS load
Part load Load diversity Process performance analysis load
Central fresh air Total heating ventilation load heating load
Figure 19.
Load diversity analysis
Key design outputs for step 8: heating load analysis An analysis of load diversity is needed as the maximum demands for each separate part of the overall load are unlikely to coincide. In addition to the
● assessment and analysis of load infiltration diversity within the total space heating load, there can be zone diversity diversities, perhaps due to differing hours of occupancy. Process loads could ● total heating load to enable boiler be intermittent and the HWS load could perhaps peak at the middle or or other heating plant to be sized. towards the end of the occupied period, rather than the beginning.
The individual and zone space heating loads should be reviewed to check when the peak demand occurs. While it is most likely that the worst case scenario will be for all spaces to require heating at the same time it is possible in certain buildings that there could be spaces or zones which only have very occasional use and do not coincide with the main demand times from other areas.
For intermittent heating, the period of maximum demand for the heating systems will be during the preheat period. In practice the preheat periods for all spaces and zones will generally be co-incident and therefore the maximum space heating load will be the sum of these, after considering infiltration diversity as discussed above.
For continuous heating some diversity can be expected between the various zone heating loads. This is discussed in CIBSE Guide A, section 5.10.3.5, with
26 CIBSE Knowledge Series — How to design a heating system Table 5.18 suggesting that central plant diversity factors ranging from 0.7–1.0 may be appropriate depending on building type and system control.
3.9 Step 9: plant sizing and selection
Once the overall heating load has been determined, then the heating plant can be sized and selected, see Figure 20, together with other plant items
Figure 20: Part load Total heating performance load Key steps for boiler/heating plant Standby capacity sizing and selection Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 (if required) Final system and Boiler/heating control performance plant sizing analysis Fuel supply system sizing Flue sizing
such as the flue and fuel supply system if required. Key design outputs for step 9: plant sizing and selection Standby capacity ● schedule of plant, giving required output, flowrates, etc Occasionally standby capacity may be required so that, in the event of partial ● control requirements system failure or plant maintenance, the main loads can still be met and the ● schematic of plant layout, building continues to function. The decision on this can require risk connections, etc. assessment. However this can add still more additional capacity to the system increasing the overall risk of oversizing and poor performance, therefore this should be considered together with the load diversity analysis as there may already be sufficient capacity within the system. Where further capacity is Control system required careful consideration is needed of the load breakdown to ensure Both the heating system and its control that the various expected load combinations can be met efficiently, for system should be appropriate for the example considering the optimum module size for modular boiler requirements of the building and the operation it supports. Ideally the installations. If the heating plant consists of modular boilers then adding one approach should always be to use the extra module may be sufficient to both meet the requirement and still ensure simplest control system that meets building owner, operator and user needs system operating efficiency. and capabilities, and efficiently delivers the required quality of system operation. Control requirements should be finalised, considering the required system operation. With the main system design layouts completed, the final layouts and services co-ordination should be checked again for any clashes and for ease of commissioning and maintenance.
3.10 Step 10: system analysis and control performance
With the system selected and plant and equipment sized and plant selected, it is now possible to more accurately predict system performance and check energy performance targets are still met.
CIBSE Knowledge Series — How to design a heating system 27 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Normal system operation Predicted system performance, including part load performance, should be The initial system design is often based investigated to check that the selected systems can operate efficiently under on design conditions that occur for less than 1% of the occupied time. For the all predicted load conditions, see Figure 21. This is particularly important if majority of the heating season occupied additional capacity has been added, for example for preheat or standby, as period the system will be operating on a fraction of the installed load and this effectively adds a margin. It is important to check that this does not therefore it is essential to ensure that the unduly oversize the system, leading to poor performance at normal operating system can operate efficiently at these low load conditions. conditions. It is also essential to check whether other margins have been added at any stage in the design process, including those that will occur by
Maximum simultaneous Figure 21: space heating load System analysis Load diversity analysis
Part load Total heating performance load
Final system and Boiler/heating control performance plant sizing analysis
Flue sizing
selecting standard plant sizes.
Key design outputs for step 10: system analysis System control performance
● analysis of system part-load In order to achieve an energy efficient building that delivers the required level performance of functionality and occupant comfort it is essential to form a clear and ● system control strategy statement integrated control strategy at a very early design stage. In all cases the control and flowcharts strategy should be set out first so that the control options can be evaluated ● schematics of plant and systems against the required level of functionality. As such, the controls should be ● required control system functionality considered at an early stage as an integral part of the system design. ● control system specification. At this design stage the task is to carry out a final evaluation of the controls, now that the final system design is complete and part-load performance evaluated, to ensure that they can deliver the required level of control, response and energy efficiency.
Controls are discussed further in CIBSE KS04 Understanding controls, which also explains terms such as weather compensation, optimum start controls, etc; with further information on heating system controls given in CIBSE Guide B, chapter 1, CIBSE Guide F, CIBSE Guide H (2000), and in other texts such as Heating systems plant and control, A Day, M Ratcliffe, K Shepherd (2003).
28 CIBSE Knowledge Series — How to design a heating system 3.11 Step 11: final value engineering and energy targets assessment
Value engineering Final value engineering assessment
A systematic approach to achieving the Value engineering should be carried out at several stages within the project to required project functions at least cost without detriment to quality, ensure that the design is on track to meet the client requirements for performance and reliability. performance, quality, reliability, etc at least cost. For example, value engineering workshops can be held during both the scheme and detail design stages to ensure that the design decisions made are the ones that achieve
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 best value. Key design outputs for step 11: value engineering and energy targets Energy targets
● The final system performance will need to be checked again to ensure it value engineering review ● energy target and emission value complies with regulations and meets required energy targets, for example calculations. meeting the seasonal efficiency requirements and achieving a building emission rate (BER) less than the target emission rate (TER).
3.12 Step 12: design review Safety in design There are a number of different interim reviews that can be done throughout the design stages of a project, from the feasibility and innovation review to Reviews should include consideration of safety in design to ensure that the straightforward progress reviews, culminating in a post-project review after provision of the design can be project completion which can provide valuable feedback lessons to inform constructed, operated, maintained and de-commissioned safely, to comply with future work. the Construction (Design and Mangement) Regulations (CDM) requirements. Helpful guidance on During the design stages there should be review meetings of the design team designers’ responsibilities under CDM is at regular intervals to review design progress, agree changes, check given on the HSE website: www.hse.gov.uk/construction/designers/ compliance with the brief, etc. The intent of these is to monitor the progress index.htm. of the design against the programme and cost targets, anticipate potential problems, and ensure that required information will be available when needed. Review meetings can involve one or several design disciplines.
Some design practices hold a formal peer group in-house design review near the end of the design stages, presenting to other design teams, perhaps from other regional offices. This can be a useful part of the project quality checks, and provide additional valuable cross-checks on the proposed design solutions, as well as sharing experience and expertise within the organisation.
Post-project review is usually held by the in-house design team at the end of the project, after completion and handover, to review the inputs and outcomes and provide the opportunity to summarise key points learnt. This can provide the opportunity to review both the technical content of the design and the management of the design process to provide feedback to
CIBSE Knowledge Series — How to design a heating system 29 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Key design outputs for step 12: inform future work, including the provision of design benchmark data for design review future projects. A post-project review meeting can also be held with the entire project team. ● quality checks on the design technical content Sometimes there is the opportunity to obtain further feedback after handover ● feedback lessons and design benchmark data to inform future and occupation, e.g. via post-occupancy surveys. The client may also require work. additional duties to include monitoring system operation. For example, the energy performance of the system can be monitored using the CIBSE logbook approach, and the actual operation of the system and comfort performance monitored for compliance with the intended design operation. This can provide valuable feedback to inform briefing and design guidance for future projects. Further guidance on feedback can be found in BSRIA AG 21/98 Feedback for better building services. design
30 CIBSE Knowledge Series — How to design a heating system 4 Developing the design — key issues
This section covers some key areas relevant to the overall design of heating systems.
4.1 Design conditions
The choice of both internal and external design conditions can have a substantial impact on initial system loads and subsequent system
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 performance. These are a fundamental part of heating load calculations and the choice should be very carefully considered. For example the difference between using a temperature difference of 21 K (-1 °C to 20 °C) and one of 25K (-4 °C to 21 °C) for a particular building is a 20% increase in the heat loss. By the time allowance has been made for reduction in emitter output and preheat requirements the difference could be as much as 40%. When considering energy efficiency the fundamentals need to be considered first.
It is also important to consider what system performance criteria are acceptable and agree this with the client. Establishing the required system performance criteria at the briefing stage is one of the most critical tasks in the design and it is vital that clients and their designers have a thorough understanding of what conditions are required and what can practically be achieved. For example the difference between specifying an internal condition of 21 °C±1 °C or a condition of 21 °C±2 °C can have a considerable impact on energy consumption, control choice and system performance. The closer the control the more expensive the system. If conditions can be relaxed a little and allowed to vary (within reasonable limits) the system can be simpler and cheaper to install and to operate.
Further guidance can be found in CIBSE KS06 Comforton practical issues on temperature and design criteria, etc, with guidance on design conditions in CIBSE Guide A, chapters 1 and 2, and on the margins that can occur at different design stages in CIBSE RR04 Engineering design calculations and the use of margins(1998).
4.2 Design margins
Margins should never be added during a calculation process without an adequate reason for doing so and only with the approval of a senior engineer. Excessive margins can result in system oversizing and poor operational performance and control. If any margins are used they should be clearly identified and a justification given for their use, which should be recorded in the design file. It is also important to check for any inbuilt assumptions and margins in software calculation packages. The use of margins should be
CIBSE Knowledge Series — How to design a heating system 31 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
reviewed at several stages during the design process to check their Figure 22: appropriateness and avoid any duplication or excess, e.g. at the end of a The impact of oversizing on calculation procedure, at design review stage, etc. Figure 22 illustrates the heating system performance Constant flow If terminal units are oversized, space constant temperature Return water temperatures are lower temperatures drift higher than than expected if oversized constant required and energy is wasted. If temperature (constant flow) coils are oversized, too much water radiators are installed. Boilers can is pumped through the system and corrode if they are not protected performance and control is Variable flow constant compromised if laminar flow results temperature when flow rate is reduced Variable temperature circuit and variable flow circuit return water temperatures are higher than expected Variable temperature Oversized pumps consume excess energy as too much cold water is Boilers that are oversized will cycle pumped and/or they are inefficient at maximum demand. Under because they are not operating at medium and low loads burner their most efficient operating point. fraction on-time is small (especially They can often cause balancing if cycling rates are high) and problems during morning start-up reduction in plant dynamic and constant temperature pumps efficiency occurs. Operating costs may turn off and on at maximum increase because of the reduced demand plant load operating efficiency. Oversized plant permanently operating at low loads can reduce plant life. Accelerated wear can also arise from unstable control caused Oversized valves reduce effective by plant oversizing. For example: control and fail prematurely. They many oversized steam traps fail can often cause balancing problems prematurely because they operate during morning start up Boiler plant too close to their closed position
(Source: BSRIA AG 1/2000 Enhancing the performance of oversized plant by Barry Crozier, consequences of oversizing for heating system performance. BSRIA 2000)
(For more information on the use of margins in engineering design refer to Design Checks for HVAC a quality control framework for building services engineers, topic sheet number 1 Design margins and CIBSE Research Report RR04 Engineering design calculations and the use of margins.)
4.3 Energy efficiency
Energy efficiency should be considered throughout the design process. In general, energy efficient heating should:
— incorporate the most efficient primary plant to generate heat/hot water — optimise the use of renewable energy sources — ensure that heat/hot water is distributed effectively and efficiently — include effective controls on primary plant and distribution systems to ensure that heat is only provided when and where it is needed and at the correct temperature — be responsive to changes in climate, solar gains, occupancy, activity and
32 CIBSE Knowledge Series — How to design a heating system internal gains.
Designers should:
— select fuels and tariffs that promote efficiency and minimise running costs — segregate hot water services generation wherever possible — consider de-centralised heating and hot water services generation plant on large sites to reduce standing losses and improve load matching — locate plant to minimise distribution system and losses
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 — ensure distribution systems are sized correctly to minimise pump and fan energy consumption — insulate pipework, valves, etc effectively — ensure the base load is provided by the most efficient plant — utilise condensing boilers where feasible and appropriate — consider energy recovery where feasible, e.g. from air exhaust streams.
Further guidance is given in CIBSE Knowledge Series KS09 Energy efficient heatingand CIBSE Guide F, chapter 10.
4.4 Quality control
The design information, including the design calculations, is part of the design process and therefore will form part of the project design file and records and be subject to standard in-company quality assurance (QA) and quality control (QC) procedures. As such all information and data should be properly recorded and checked. Good practice includes:
— clearly identify and record all data sources to enable input information to be adequately verified — clearly state all assumptions, and identify, and flag, where more accurate data will be required (e.g. from client, manufacturer, etc) as the design progresses — review any assumptions as the design progresses to check they are still valid, and replace with more accurate information as received — clearly identify, record and review the required design inputs and design outputs — record calculations clearly, with sufficient detail to ensure the work can be followed by others (be aware that if a problem arises on a project this could mean revisiting calculations several years after they were originally done) — identify and record calculation checks and cross-checks clearly — verify the design to ensure it can meet the design requirements — review the overall design.
CIBSE Knowledge Series — How to design a heating system 33 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
Further guidance on design quality control is given in BSRIA AG 1/2002 Design checks for HVAC. References
1 BSRIA domestic boiler marketing report (Bracknell: BSRIA Ltd) (March 2006) and BSRIA commercial boiler marketing report (Bracknell: BSRIA Ltd) (March 2006)
2 DTI UK Energy Consumption in the United Kingdom: www.dti.gov.uk/energy/statistics/ publications/energy-consumption/page17658.html
3 Crozier B Enhancing the performance of oversizedBSRIA plant AG 1/2000 (Bracknell: BSRIA Ltd) (2000) and Brittain J Oversized heating plant BSRIA GN 12/97 (Bracknell: BSRIA Ltd) (1997)
4 Cross N Design: principles and practice product planning and the des brief(Open University) (1995)
5 RIBA Plan of Work (London: Royal Institute of British Architects) (1999)
6 ACE Agreement A(2) 2002 and B(2) 2002 (revised 2004) Mechanical and Electrical Engineering Services (ACE 2004)
7 The designer’s guide to energy-efficient buildings for industry GPG 303 (Carbon Trust) (2000)
8 Day A, Ratcliffe M and Shephed K Sizing central boiler plant using an economic optimisation model(Proc CIBSE National Conference) (2001)
Selected bibliography
Overall heating system design process Heating, ventilation, air conditioning andCIBSE refrigeration Guide B (London: Chartered Institution of Building Services Engineers) (2001-2), chapter 1 Design checks for HVACG Lawrence Race, BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002) A practical guide to HVAC building servicesG Lawrence calculations Race, S Mitchell, BSRIA/CIBSE BG 30/03 (2003)
Comfort ComfortCIBSE KS06 (London: Chartered Institution of Building Services Engineers) (2006) Environmental DesignCIBSE Guide A (London: Chartered Institution of Building Services Engineers) (2006), chapter 1
CDM guidance for designers www.hse.gov.uk/construction/designers/index.htm
Design data Environmental DesignCIBSE Guide A (London: Chartered Institution of Building Services Engineers) (2006), chapters 1 and 2 Reference dataCIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2001) Weather, solar and illuminanceCIBSE Guidedata J (London: Chartered Institution of Building Services Engineers) (2002)
34 CIBSE Knowledge Series — How to design a heating system Design management Project Management HandbookC Parsloe, L Wild, BSRIA AG 11/98 (Bracknell: BSRIA Ltd) (1998) Allocation of design responsibilities for building Cengineering Parsloe, BSRIA services TN21/97 (Bracknell: BSRIA Ltd) (1997) (New edition due in 2006)
Design margins Engineering design calculations and the useCIBSE of Research margins Report RR04 (London: Chartered Institution of Building Services Engineers) (1998)
Colin Campbell, [email protected], 3:16pm 03/09/2013, 1 Design quality control Design checks for HVACG Lawrence Race, BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002)
Design review and feedback Feedback for better building servicesG Lawrence design Race, C Pearson, T de Saulles, BSRIA AG 21/98 (Bracknell: BSRIA Ltd) (1998)
Domestic heating Domestic heating design CIBSEguide Domestic building services panel (London: Chartered Institution of Building Services Engineers) (2003)
Energy efficiency Energy efficiency in buildingsCIBSE Guide F (London: Chartered Institution of Building Services Engineers) (2004) Energy efficient heatingCIBSE KS09 (London: Chartered Institution of Building Services Engineers) (not yet published)
Fabric thermal performance Environmental DesignCIBSE Guide A (London: Chartered Institution of Building Services Engineers) (2006), chapters 3 and 5
Heating design calculations with worked examples A practical guide to HVAC building servicesG Lawrence calculations Race, S Mitchell, BSRIA/CIBSE, BG 30/03 (2003) Environmental DesignCIBSE Guide A (London: Chartered Institution of Building Services Engineers) (2006), chapters 3 and 5 Heating, ventilation, air conditioning andCIBSE refrigeration Guide B (London: Chartered Institution of Building Services Engineers) (2001-2), chapter 1 Model demonstration projectJ Sands, C Parsloe, D Churcher, BSRIA BG 1/2006 (Bracknell: BSRIA Ltd) (2006)
Heating plant and controls Heating, ventilation, air conditioning andCIBSE refrigeration Guide B (London: Chartered Institution of Building Services Engineers) (2001-2), chapter 1 Building Control SystemsCIBSE Guide H (London: Chartered Institution of Building Services
CIBSE Knowledge Series — How to design a heating system 35 Colin Campbell, [email protected], 3:16pm 03/09/2013, 1
General textbooks covering heating Engineers) (2000) systems and design aspects Understanding controls CIBSE KS04 (London: Chartered Institution of Building Services Engineers) (2005) Faber and Kell s Heating and air- conditioning of buildings, 9th ed, D Heating systems plant and Acontrol Day, M Ratcliffe, K Shepherd (Oxford: Butterworth- Oughton and S Hodkinson (Oxford: Heinemann) (2003) Elsevier) (2002) Heating and water services design in buildingsK Moss (London: Taylor & Heating systems Francis) (2003) Heating, ventilation, air conditioning andCIBSE refrigeration Guide B (London: Chartered Heating systems plant and Institution of Building Services Engineers) (2001-2), chapter 1 controlA Day, M Ratcliffe, K Shepherd (Oxford: Butterworth-Heinemann) Underfloor heating designCIBSE guide Domestic building services panel (London: Chartered (2003) Institution of Building Services Engineers) (2004) HVAC simplified, S Kavanaugh, ASHRAE (Atlanta: American Society of Underfloor heating the designersJ Sands guide BSRIA AG 12/01 (Bracknell: BSRIA Ltd) Heating, Refrigerating and Air- (2001) Radiant HeatingR Brown, BSRIA AG3/96 (Bracknell: BSRIA Ltd) (1996)
Infiltration estimation Environmental Design CIBSE Guide A (London: Chartered Institution of Building Services Engineers) (2006), chapter 4 Natural ventilation in non-domestic buildings CIBSE AM10 (London: Chartered Institution of Building Services Engineers) (2005)
Natural ventilation Natural ventilation in non-domesticCIBSE buildings AM10 (London: Chartered Institution of Building Services Engineers) (2005) Mixed mode ventilationCIBSE AM13 (London: Chartered Institution of Building Services Engineers) (2000)
Value engineering Value Engineering of BuildingG Hayden,Services C Parsloe, BSRIA Application Guide 15/96 (Bracknell: BSRIA Ltd) (1996)
36 CIBSE Knowledge Series — Variable flow pipework systems