Library of System Control Strategies

A BSRIA Guide www.bsria.co.uk

By A J Martin and A P Banyard

AG 7/98 Application Guide AG 7/98

LIBRARY OF SYSTEM CONTROL STRATEGIES

A J Martin C P Banyard

The Building Services Research and Information Association Old Bracknell Lane West, Bracknell, Berkshire RG12 7AH Tel: + 44 (0)1344 426511 Fax: + 44 (0)1344 487575 e-mail: [email protected] www.bsria.co.uk All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without prior written permission of the publishers.

ISBN 0 86022 497 X ©BSRIA 78040 April 1998 EXECUTIVE SUMMARY

EXECUTIVE SUMMARY

The Library of System Control Strategies provides a reference document for use when specifying, developing and configuring control strategies. The Library is published in five chapters, each containing control strategies relating to the following main types of plant:

· Chapter 1: BMS plant start /stop signals · Chapter 2: Heating systems · Chapter 3: Air handling plant · Chapter 4: Pumping · Chapter 5: Cooling systems

In each chapter the control of the principal plant items in current use are described. Where there are many plant items which carry out the same function but using different techniques, for example, water heating, the modular format of the Library allows a ‘pick and mix’ selection of plant modules to be made to provide the required configuration of a complete system.

Each plant module is shown in the form of a schematic and is described in terms of its general function and how it is controlled. Specification clauses are included which the system designer can copy into a particular specification document. Italics are used in the specification clauses to indicate where options are available from which the user can select the one required. Underlined text is used to indicate default values of setpoints and other parameters which the user should change for the particular specification.

Total digital and analogue input and output points are given in a BMS points list for each plant item and a further table summarises the modes of plant operation for each of the controlled devices.

Finally, each strategy has a flow chart which presents the control strategy information in an easy to follow format as an aid to understanding the sequence of control events.

The ‘standardisation’ initiated by the Library of System Control Strategies will lead to easier system specification, configuration, commissioning and witnessing due to the better understanding of and familiarity with the control software provided. Subsequent operational problems will also be easier to identify. This will provide more effective system design, installation, operation and maintenance and consequent productivity gains that such performance improvements bring about.

Note: This document is also contained on the accompanying diskette.

©BSRIA AG 7/98 Library of System Control Strategies i ACKNOWLEDGEMENTS

ACKNOWLEDGEMENTS

BSRIA would like to thank the following for their financial contribution to the development of the Library of System Control Strategies:

Department of the Environment, Transport and the Regions Andover Controls Europe Ltd Caradon Trend Ltd Sauter Automation Ltd TAC [UK] Ltd

Acknowledgement is also given to Mrs Sheila Mills who drew all the schematics and flow charts, and to the following individuals who provided technical comment:

Dr D Arnold (Troup Bywaters+Anders) Mr C Banyard (Hoare Lea and Partners) Mr R Brailsford (Oscar Faber Plc) Mr C Flitcroft (Bovis Lelliott Ltd) Mr L Ford (Sauter Automation Ltd) Mr T Godding (Symonds Group Ltd) Ms D Hobbs (MEPC UK Ltd) Mr A Langford (Andover Controls Ltd) Mr B Martin (TAC [UK] Ltd) Mr C Monson (Caradon Trend Ltd) Mr A Norén (TAC [UK] Ltd) Mr N Pennell (Land Securities Properties Ltd) Mr N Skemp (Building Design Partnership) Mr K Shenstone (Meica Services) Mr B Stevens (Max Fordham and Partners) Mr A Wallenborg (TAC AB) Mr C Wood (Glaxo Welcome).

Every opportunity has been taken to incorporate the views of the sponsors and technical panel but final editorial control of this document rests with BSRIA.

ii Library of System Control Strategies ©BSRIA AG 7/98 CONTENTS

CONTENTS

1 GENERAL CONTROL STRATEGIES ...... 13 1.1 BMS start/stop control...... 13 1.1.1 Time control...... 13 1.1.2 Low temperature interlock signals (plant protection)...... 3 1.1.3 Low inside air temperature interlock (building fabric and contents protection) ...... 5 1.1.4 Optimum start heating...... 6 1.1.5 Optimum stop heating ...... 8 1.1.6 Boost - fixed time ...... 9 1.1.7 Domestic hot water (DHW) demand...... 10 1.1.8 Optimum start cooling...... 11 1.1.9 Optimum stop cooling...... 13 1.1.10 Night cooling...... 14 1.1.11 Fan overrun ...... 19 1.1.12 Pump overrun ...... 19 1.2 Other control signals...... 20 1.2.1 Hand/off/auto selection switch...... 20 2 HEATING SYSTEMS ...... 21 2.1 Water heating plant...... 22 2.1.1 General boiler control strategy based on a single conventional boiler ...... 22 2.1.2 Single boiler installation - condensing boiler with split heat exchanger ...... 33 2.1.3 Sequencing boiler installation with 2 or more parallel low water content boilers...... 39 2.1.4 Sequencing boiler installation with 2 or more boilers with isolating valves...... 48 2.1.5 Sequencing boiler installation with 2 or more boilers, each with individual pump and bypass circuit ...... 58 2.1.6 Sequencing of non-identical groups of boilers ...... 67 2.1.7 Modular boiler installation ...... 77 2.1.8 Combined heat & power (CHP)...... 84 2.1.9 District heating...... 93 2.2 Pressurisation ...... 97 2.2.1 Static head pressurisation ...... 97 2.2.2 Sealed expansion vessel pressurisation ...... 99 2.2.3 Gas pressurisation ...... 101 2.2.4 Pump pressurisation...... 103 2.3 Hot water distribution (Secondary circuits) ...... 105 2.3.1 Constant volume / variable temperature...... 105 2.3.2 Constant temperature / variable volume ...... 112 2.3.3 Non-storage calorifier ...... 117 2.3.4 Storage calorifier ...... 122 2.3.5 Water heated underfloor system ...... 126 2.4 Self contained heating systems...... 130 2.4.1 Gas fired heater ...... 130 2.4.2 Radiant heating systems ...... 134 2.4.3 Electrical underfloor heating...... 137 3 AIR HANDLING SYSTEMS ...... 141 3.1 Damper control...... 141 3.1.1 Full outside air damper system ...... 141 3.1.2 Mixing damper system ...... 144 3.1.3 CO2 demand damper override ...... 149 3.2 Pre-heat coil / frost protection ...... 152 3.2.1 Low temperature hot water (LTHW) pre-heat / frost protection coil...... 152 3.2.2 Electric heating battery - pre-heat / frost protection ...... 156

©BSRIA AG 7/98 Library of System Control Strategies iii CONTENTS

3.3 Heat recovery devices ...... 160 3.3.1 Thermal wheel - variable speed ...... 160 3.3.2 Thermal wheel - constant speed with bypass damper control...... 164 3.3.3 Run around coil...... 168 3.3.4 Fixed plate heat exchanger ...... 172 3.4 Cooling coil ...... 176 3.4.1 Chilled water (CHW) cooling coil ...... 176 3.4.2 Direct expansion (DX) cooling coil...... 180 3.5 Dehumidification ...... 183 3.5.1 Chilled water dehumidification coil - recirculated air available...... 183 3.5.2 Chilled water dehumidification coil - no air recirculation available ...... 190 3.5.3 Direct expansion (DX) dehumidification coil - no air recirculation available ...... 194 3.5.4 Chilled water dehumidification coil - face and bypass dampers control...... 198 3.6 Heating...... 202 3.6.1 Low temperature hot water (LTHW) heating coil ...... 202 3.6.2 Electric heating battery...... 205 3.7 Supply fan...... 209 3.7.1 Constant volume supply fan...... 209 3.7.2 Two-speed supply fan...... 213 3.7.3 VAV main duct static pressure system - supply fan speed control...... 217 3.7.4 VAV branch duct static pressure system - supply fan speed control...... 222 3.7.5 VAV system pressure totalisation - supply fan speed control...... 227 3.8 Humidification...... 231 3.8.1 Steam humidifier ...... 231 3.8.2 Atomised spray humidifier ...... 235 3.9 Extract fan ...... 239 3.9.1 Constant volume extract fan ...... 239 3.9.2 Two-speed extract fan ...... 243 3.9.3 Variable volume extract fan...... 246 3.10 Terminal units ...... 250 3.10.1 Constant volume air terminal unit with reheat ...... 250 3.10.2 Fan coil unit - airside control...... 254 3.10.3 Fan coil unit - 'waterside' control...... 259 3.10.4 Pressure dependent VAV terminal unit ...... 263 3.10.5 Pressure independent VAV terminal unit ...... 268 3.10.6 Pressure independent intelligent VAV terminal unit ...... 273 3.10.7 Pressure dependent, fan assisted VAV terminal unit - series fan...... 279 3.10.8 Pressure dependent fan assisted VAV terminal unit - parallel fan...... 285 3.10.9 Induction VAV air terminal unit ...... 291 3.11 Natural ventilation...... 297 3.11.1 Mixed mode system ...... 297 3.12 Filtration...... 302 3.12.1 Air filter with pressure sensor ...... 302 3.13 Zone air temperature control...... 304 3.13.1 Zone temperature setpoint adjustment during unoccupied periods (occupancy and night setback control) ...... 304 3.14 Supply air temperature control ...... 306 3.14.1 Constant air volume (CAV) system - zone temperature control of primary supply air temperature...... 306 3.14.2 VAV primary supply air temperature setpoint reset - intelligent terminal units ...... 310 3.14.3 VAV primary supply air temperature setpoint scheduled to fan speed ...... 315 3.14.4 Supply air temperature setpoint scheduled to outside air temperature...... 319 3.15 Fire / smoke control...... 322 3.15.1 Fire/smoke control, plant shutdown...... 322 3.15.2 Fire/smoke control, extract position ...... 322

iv Library of System Control Strategies ©BSRIA AG 7/98 CONTENTS

3.16 Control strategies for complete air handling systems...... 324 3.16.1 Constant air volume system ...... 326 3.16.2 Variable air volume system...... 337 3.16.3 Fan coil unit system ...... 349 3.16.4 Displacement ventilation system ...... 359 4 PUMPING ...... 368 4.1 Pump control...... 368 4.1.1 Pump control...... 368 5 COOLING SYSTEMS ...... 373 5.1 Water chillers...... 373 5.1.1 Packaged air cooled water chiller (multi-stage single compressor)...... 373 5.1.2 Packaged air cooled water chiller with heat reclaim ...... 379 5.2 Heat pumps ...... 387 5.2.1 Reversible air to air heat pump ...... 387 5.2.2 Unitary water to air reverse cycle heat pump system ...... 395 5.3 Cooling towers...... 404 5.3.1 Open circuit (direct) cooling tower ...... 404 5.3.2 Closed circuit (indirect) cooling tower ...... 413 5.4 Chilled ceilings ...... 421 5.4.1 Chilled ceiling panels and beams ...... 421

1 LIST OF APPENDICES

APPENDIX - A Glossary of terms...... 427 APPENDIX - B List of signal name abbreviations ...... 433 APPENDIX - C Control system sensors...... 439 APPENDIX - D Near optimal control strategy for cooling tower fan speed control ...... 447 APPENDIX - E Bibliography...... 449 APPENDIX - F Comment form...... 451

LIST OF TABLES

Table 1.1 - Dates, days and times of plant operation ...... 2

LIST OF FIGURES

Figure 3.1 Energy recovered from variable speed wheel...... 161 Figure 3.2 Psychrometric chart showing the dehumidification process utilising a mid damper position...... 184 Figure 3.3 Psychrometric chart showing the dehumidification process utilising an extreme damper position (i.e. full fresh air) ...... 185 Figure 3.4 Pressure dependent VAV terminal unit control ...... 264 Figure 3.5 Pressure independent VAV terminal unit control ...... 267 Figure 3.6 Pressure independent intelligent VAV terminal unit control ...... 275 Figure 3.7 Pressure dependent, series fan-assisted VAV terminal unit ...... 281 Figure 3.8 Pressure dependent, parallel fan assisted terminal unit control ...... 287 Figure 3.9 Induction VAV air terminal unit control ...... 293 Figure 3.10 Supply air temperature setpoint scheduled to outside air ...... 319 Figure 4.1a Constant speed pump control with bypass valve...... 368 Figure 4.1b Variable speed pump under differential pressure control ...... 368

©BSRIA AG 7/98 Library of System Control Strategies v INTRODUCTION

INTRODUCTION

The Library of System Control Strategies provides a reference document for use when specifying, developing and configuring control strategies. The consulting engineer and other parties specifying control systems can use the library as an independent source of control solutions with a complete description of plant operation and control strategy. A standard specification clause for each individual control strategy can be quoted or referenced to form a detailed specification.

It is intended that the Library of System Control Strategies will provide control manufacturers and system specialists with a detailed specification that they will become familiar with, allowing the control application software for the HVAC system to be designed, configured and tested in less time. With the development of an in-house database of Building Management System (BMS) control solutions, implementing a control system for a specific application will take the form of interconnecting the required modules and assigning setpoint and other parameter values to suit the system.

In addition for use for a BMS, the library is also appropriate for use, with suitable engineering, for stand-alone controls.

The ‘standardisation’ initiated by the Library of System Control Strategies will lead to easier system commissioning and witnessing due to the better understanding of, and familiarity with, the control software provided. Operational problems will be easier to identify by referring to the descriptions and control strategies within the Library. This will also make for more effective system operation and maintenance by engineers who may not have been involved in the plant design or installation.

The provision of the Library does not, however, remove the onus from designers using the documents to ensure that an appropriate working solution is produced.

Using the Library

The Library has been developed using a modular system allowing control strategies to be selected on a ‘pick and mix’ basis according to the particular plant used. The major plant systems are sub-divided into a number of ‘plant functions’. For example, the plant function headings for ‘heating systems’ are listed in Table I.

The plant functions are further sub-divided into ‘plant modules’, see example in Table I for water heating plant. The term ‘plant module’ is also used to refer to strategies that are purely control ‘routines’ rather than plant items, for example, timeclock on/off, optimum start, weather compensation, primary supply air temperature control, plant changeover etc.

©BSRIA AG 7/98 Library of System Control Strategies vii INTRODUCTION

Table I Modular format of the Library Library of system control strategies

Plant systems

1. General control strategies 2. Heating systems 3. Air handling systems 4. Pumping 5. Cooling systems

Plant functions

2.1 Water heating 2.2 Pressurisation 2.3 Hot water distribution 2.4 Self contained heating (see contents list) (see contents list) systems (see contents list) Plant modules

2.1.1 Single boiler installation - conventional boiler 2.1.2 Single boiler installation - condensing boiler with split heat exchanger 2.1.3 Sequencing boiler installation with 2 or more parallel low water content boilers 2.1.4 Sequencing boiler installation with 2 or more boilers with isolating valves 2.1.5 Sequencing boiler installation with 2 or more boilers, each with individual pump and bypass circuit 2.1.6 Sequencing of non-identical groups of boilers 2.1.7 Modular boiler installation 2.1.8 Combined heat & power (CHP) 2.1.9 District heating

This modular approach allows various plant options to be described in terms of the selected individual plant modules. The intention of the Library is that for each whole plant system such as an air handling unit or complete heating system, the user has a number of alternative plant functions from which appropriate plant modules are selected to meet the requirements for a particular specification. The individual plant modules and their associated control strategies are therefore available as building blocks to develop the control strategy for a complete system.

The Library describes a number of ‘operating modes’ that a heating, cooling or ventilating plant could operate in at different times, as follows:

· low outside air temperature interlock (for plant protection) · low return water temperature interlock (for plant protection) · low zone air temperature in the conditioned zone (for building fabric and contents protection) · plant shutdown (‘off’) · fan overrun (air systems) or pump overrun (water systems) · boost heating · optimum start heating · optimum start cooling · night cooling · normal (often corresponding to the occupied period) · fire.

viii Library of System Control Strategies ©BSRIA AG 7/98 INTRODUCTION

These operating modes are described in section 1. Not all of the operating modes are applicable to all plant items. Further, the user of the Library may not wish to use all of the above operating modes and so references to them may be deleted in the specification document produced for a particular contract.

The general format and content of each control strategy is as follows:

· Schematic of plant module · Plant description · Control strategy · Specification clause · BMS points list · Summary of plant operation · Control flow chart.

The Plant schematic is a simple drawing of the plant module that is being controlled. The drawings were created using the relevant symbols contained in British Standard BS 1192 Construction drawing practice; Part 3 Recommendations for symbols and other graphical conventions. 1987.

The Plant description provides an outline of the plant itself, its function and how it is operated.

The Control strategy is a description of the recommended method of control for the plant, including essential interlocks. Some strategies include figures and tables.

The Specification clause contains the same information as that contained in the Control strategy except that it is written in a style that is commonly used in specification documents and it includes default values of setpoints and other parameters which the user can change as required to provide initial values for the particular specification.

The BMS points list is a table of the physical inputs from sensors, thermostats and other input devices and of the physical outputs to controlled devices that are associated with the control strategy or plant module. The physical inputs and outputs are connected to one of the BMS outstations or controllers. The BMS points list gives the total number of analogue and digital inputs and outputs, both for the ‘recommended’ inputs and outputs and the ‘optional’ inputs and outputs that are in italics and totalised separately (in brackets). The BMS points list also gives an abbreviated signal name for each point. These signal names are used in the Flow charts.

The Summary of plant operation gives the status of each physical output for each of the plant operating modes that are relevant to the plant module, assuming that no plant failure interlocks are active.

The Control flow chart is a graphical, step by step guide to the control and interlock structure of the control strategy.

Many of the strategies contain all seven of the above elements, and all strategies contain at least a Specification clause which the user can copy into a particular specification document. However, for some strategies not all of the above elements are necessary or relevant. For example, the strategies which relate to the control of the parameter ‘primary supply air temperature’ of an air handling unit do not contain a Plant schematic. The strategies concerned with scheduling of the primary supply air temperature setpoint do not relate directly to a BMS physical output to a particular controlled device and so it is not appropriate to include a Summary of plant operation for them.

Within many of the individual control strategies some options or alternatives are included that concern details of the strategy and the controlled plant. For example, for air handling plant the main control parameter may be the primary supply air temperature delivered by the plant or it may be the temperature of the conditioned zone. Where there are such alternatives, they are indicated in the text of the strategy by italic text with the symbol ‘/’ used to separate the alternatives. In cases where alternatives affect the BMS points list or Summary of plant operation, they are also shown by italic text. It is intended that the user of the Library select which options and alternatives are required for a particular specification document, and delete text and points from the BMS points list that relate to unused alternatives.

Default values for setpoints and other parameters have been incorporated in the text. These are indicated by underlining the value eg 18°C. The user of the Library should select appropriate values for their particular specification. General specification clauses such as 'all setpoints in this specification shall be adjustable' are assumed to be provided as part of the general BMS specification and are not included here.

A list of abbreviations for signal names used in the Library is provided in the Appendices.

Creating a specification document

To create a specification document for a particular plant from the individual control strategies available within the Library, the user should take the following steps:

· decide which ‘plant functions’ are needed to make up the whole system, (see table 1)

· decide for each plant function which of the available plant module options is to be used

· for strategies which offer alternatives or options, decide which of these are to be used.

The Library documentation of the selected control strategies can then be copied by the user into the particular specification document as required.

A section of the air handling systems’ section of the Library (Section 3, section 3.16) gives four examples of how control strategies for complete air handling systems can be produced by combining the strategies of individual plant modules. This section shows in detail how elements of individual control strategies may be combined to produce a specification for a whole plant.

The Library details only control strategies. The onus is on the designer to ensure that the selected strategies are integrated to form a working solution and that all supporting specification documentation such as equipment and other schedules, standard specifications and tender documentation is provided.

Other sources of complementary guidance by BSRIA include the following:

1 Standard Specification for BMS Application Handbook AH1/96 Volume 1.

This is a functional specification and details a standard system specification along with an installation specification.

2 Guide to BMS Standard Specification Application Handbook AH1/96 Volume 2

Provides guidance on how to structure a BMS functional specification, covering issues such as collection and presentation of information, assembling the full specification and the tendering process.

3 BMS Specification Guidance (1998) Application Guide AG 6/98

Provides background information and guidance relating to the issues to be considered when specifying a BMS.

4 Code of practice for the Commissioning of BEMS Application Handbook AH 2/92

Provides guidance on recommended procedures and considerations to ensure effective commissioning.

5 BEMS Performance Testing Application Guide AG 2/94

Highlights indications of poor BMS performance and provides guidance on appropriate performance testing procedures. xii Library of System Control Strategies ©BSRIA AG 7/98