Design Guide to Underfloor Air Distribution

Home , Cooling load, Displacement ventilation, Plenum space, Raised floor, Room air distribution

1 Design Guide to Underfloor Air Distribution

INTRODUCTION types of office moves is often referred to as churn. The concept of underfloor air is not new, however changes in office space usage, The costs associated with these churn moves sustainable design and indoor air quality are one of the largest costs an owner / tenant issues have sparked considerable recent may face. The costs associated with cabling interest in the concept. Underfloor air moves and to a lesser extent HVAC system conditioning had its start in computer rooms, are the major portion of these move costs. at a time when mainframe computers So a system that could greatly simplify the generated considerable heat and had a costs associated with churn moves could considerable amount of complex cabling help reduce total system life cycle cost. The required by the computers. Access floor advantage of access flooring is that cable systems allowed plenty of open space to run and HVAC moves are greatly simplified. cabling and a generous pathway to supply large quantities of cooling air under the intense heat of the electronics. The natural Improved comfort convection currents of warm air rising allowed cool air to enter low, cool the The second feature of underfloor air equipment and remove the warm air near the distribution (UFAD) was that the same ceiling. Of course today’s computer rooms principle of warm air rising from hot have changed and the need for this type of electrical gear could also be applied to warm cooling has decreased. However, access air around people. BOMA (Building Owners floor systems have found new markets. and Managers Association) reported in their “What Tenants Want Survey” that thermal and indoor air quality concerns were two of Reduced life cycle cost the top concerns and least met expectations of tenants. Traditional overhead air The recent interest in access floor systems distribution systems are designed to do a has been spurred by other applications that very good job of mixing the air in the space can make use of the two principles that these and prevent stratification in the room. earlier computer room systems addressed. Without increased complexity and cost First, the large open floor plenum was a delivering thermal comfort to every convenient space to run large amounts of occupant while also providing personal cable for power and communications. The ventilation may be cost prohibitive. greatest change in offices in the last 15 years Overhead systems may require more fan has been an ever-growing need for voice, energy to overcome static losses of data and power connections to every generating the mixing, airflow patterns worker’s workstation. This coupled with one required within the space. other trend in office space design that is the use of more open plan space and a trend to Delivery of ventilation air requires sufficient move workers into cross-functional mixing to assure that the ventilation workgroups with great regularity. IFMA component is delivered to the occupied (International Facility Management breathing zone. By applying the warm air Association) reported in one of their recent rising principle, air can be provided below surveys that on average an office worker the occupants and discharged directly into experiences a move every 5-½ months; this the breathing zone at relatively low velocity. is 44 percent move rate. The term for these The people warm the air and by natural

Design Guide to Underfloor Air Distribution convection the air rises toward the ceiling. comfortable, and provides better ventilation Since people only breath air in a zone from and improved indoor air quality. approximately the floor to 6 feet, the space above this zone can be treated as a stratified The application of an underfloor air air layer and the load components in this distribution system requires the system zone treated differently. The result is that air designer to view the design very differently. provided underfloor can be supplied at low It requires special consideration throughout pressure and the energy for space the design process from design schematic conditioning can be reduced. The ventilation stage, to load estimate, to commissioning. air can be provided in the zone where it is This document is intended to provide the needed most with pollutants moved gently designer with some of the guidelines to toward the return. Additionally, adjustable follow in systems design of underfloor air diffusers, which discharge air to small areas distribution, UFAD. Further in depth in the space, may be adjustable by the analysis of all aspects of UFAD design can occupant and can result in an improvement be found in the ASHRAE UFAD Design in personal thermal comfort. This results in Guide. a system that can be more energy efficient,

Figure 1 – Typical Underfloor Air Distribution System

3 Design Guide to Underfloor Air Distribution

UFAD System Benefits without primary usage of the flooring systems is to special air-handling provide a service and utility space to run cabling for voice, data and power. As a Several types of underfloor system designs result, the floors can be as little as 4 inches exist. This manual covers a system offered over the structural floor or they may be by Carrier Corporation (see Fig. 3c). This much higher. When the floor space is to be approach employs standard air handling used as a supply plenum for air distribution, equipment and uses duct supply floor heights are normally at 12 to 18 inches temperatures in the normal air conditioning high. range, which can best handle spot cooling requirements that often arise on every project. The air is distributed in the space through a pressurized plenum with swirl style floor grilles. The plenum is supplied through a zone parallel fan power box with a special DDC control to modulate temperature and controls plenum pressure. The primary damper provides the minimum ventilation air to the zone and maintains plenum pressure. The ECM style fan motor in the box modulates to mix return and Figure 2 – Access Flooring System primary air to maintain required temperature. Heating and cooling Access floor systems are successfully requirements for specialized high load applied to many types of buildings both new spaces and perimeter zones are handled with construction and renovation. However, when conventional overhead systems, underfloor applied to retrofit situations a few special series fan boxes or fan coil units. considerations are required. Elevator lobbies and bathrooms which are built on the APPLICATION original structural floor need to be raised to the new floor elevation or ramps provided to CONSIDERATIONS get from the structural floor heights to the raised floor height. Access flooring systems An access floor is a flooring system that is In new construction one architectural installed above the standard structural floor consideration of the floor systems is that a of the building. We will reference Haworth ceiling space could be reduced allowing a Access Flooring System in our discussions floor-to-floor height one foot less than (see Fig 2). The systems consist of a matrix standard. This can be accomplished by of adjustable pedestals mounted normally on either the use of open ceiling space with two-foot centers, which support the weight only lighting and sprinklers in the space, or of the access floor panels and the loads on by reducing the ceiling plenum space since the floor above. The two-by-two, large supply ducts are not required. Then for approximately 1-1/8 inches thick concrete every 12 stories of building an additional floor panels are laid into this grid system. story could be added with minimal The structural floor below is normally sealed additional cost in the building shell cost. In to provide a moisture and dust barrier. The buildings less than 12 stories the reduction

4 Design Guide to Underfloor Air Distribution in height would result in a reduction of 3.) The third concept is to provide overall building shell cost. airflow from underfloor that is controlled at the workstation. This method is called Task Ambient Basic Concepts Conditioning. The control is A traditional underfloor air distribution normally accomplished with a small system uses the floor space between the fan that directs an air jet at the structural floor and the access floor as a occupant within the workstation. supply air plenum. We will use the UFAD approach in this There are three concepts normally manual. The plenum may be configured associated with providing floor level using one of two approaches. Either the cooling, each with distinctive entire underfloor plenum is pressurized to a characteristics: relative low pressure of approximately 0.10 1.) Displacement ventilation discharges inch WG or air is discharged into the air horizontally near the floor at very plenum at zero or neutral pressure and fans low velocities and near laminar flow near the outlets draw the supply air from the conditions. The goal is to use only plenum and discharge it to the space. the buoyancy effects to create air Neutral pressure plenums have a reduction motion within the space and in leakage around floor tiles and plenum maintain the stratification layer penetrations, but fan wiring and energy costs above the controlled zone that is not negate some of the system’s inherent mixed. While this provides excellent benefits. The use of the underfloor plenum ventilation effectiveness in the has two major impacts on system design. occupied zone it is not the optimum solution for thermal comfort. This is First, since the air is brought in at floor because of wide temperature level, a chance exits for a cold floor or for variance, greater than the, ASHRAE cold discharge temperatures near the (Std 55) accepted, 3 to 5 degree occupant’s feet. Both conditions are spread from head to toe. objectionable so the air must be provided at a much warmer temperature than normal air 2.) Under Floor Air Distribution conditioning levels. The accepted level is (UFAD) uses the same buoyancy between 61 F to 65F. This impacts principles as displacement however equipment selection and operation, and the air is discharged into the space impacts air distribution devices. The second with adequate velocity and air issue is that the large surface area and mass pattern to provide mixing only of the structural slab provide both a thermal within the occupied zone. The mass that shifts load patterns and creates system still tries to maintain the significant amounts of thermal decay to stagnant zone above the breathing diffusers located far from the point at which zone. The system provides a more supply air enters the plenum. uniform temperature variation in the occupied zone to stay within the Conditioned air is handled in one of two comfort envelope. methods. The first method is when the space return air is removed near the ceiling and the entire volume returned to the air-handler.

5 Design Guide to Underfloor Air Distribution

This method requires special treatment by Figure 3 – Basic System Design the air-handler to bypass the required air and Options condition the required supply amount without losing control of temperature and 3a. Conventional Overhead Distribution particularly humidity in the space (see 78°F Return to AHU 55°F Supply from AHU fig.3b). The second approach is to use a mixing box located near the space that 78°F draws a portion of the return air near the ceiling and mixes it with the supply air to maintain the floor plenum pressure and temperature. The Carrier system discussed here uses the second method (fig.3c). 75°F 75°F

Perimeter zones and spaces such as conference rooms present some interesting challenges. These spaces have loads which vary greatly with time and which may have air

requirements that cannot be handled by 65- 3b. UFAD with Full Return degree air. To provide for these spot cooling 84°F Return to AHU situations either a fan powered box is provided 62°F Supply to AHU

under the floor to increase airflow or a

separate unit that can use standard 55-degree 84°F air can be used. A fan coil unit located under RETURN RETURN

the floor can also be used to provide for the

cooling and heating needs. When 55-degree

air is supplied to zone mixing units, the 75°F 75°F system may also employ traditional methods

used with overhead air distribution, to meet 62°F high load requirements. 62°F

Heating along the perimeter also presents challenges for UFAD systems. To provide the necessary heat, terminal units are frequently provided to addresses this situation. One very 3c. UFAD with Zone Mixing Unit 84°F Return to AHU common method is to use a fan coil unit that 55°F Supply from AHU may be placed under the floor and near the perimeter. The fan coil contains a heating coil, electric or hot water and it is controlled to 84°F overcome transmission losses.

84°F

75°F 75°F

62°F

6 Design Guide to Underfloor Air Distribution

Product Overview A second type of diffuser is used with underfloor mixing boxes and perimeter fan Floor plenum - The space between the coil units. This rectangular grille is used to access floor and the structural floor is the provide an air curtain washing exterior floor plenum. Using this space as an air surfaces with warm or cold air. These grilles distribution plenum requires some special depend on much higher velocities and can considerations. First the floor panel seams be used in various lengths to match the must provide a tight seal to prevent air airflow requirements of the mixing box or leakage from the plenum into the space. In fan coil unit. Either a 0° or 15° deflection some cases ductwork is run through the blade can be used to provide the required space to special variable load areas and the throw pattern. A version of the rectangular space may need partitioning for life safety or grilles can also be installed in a plenum that thermal zoning. A typical underfloor has a control damper to provide variable plenum is between 12 and 18 inches in volume control to selected spaces. The height. rectangular grilles can also be used to allow

return air from above the floor back down to Diffusers – Room air distribution is the underfloor mixing boxes if desired. If accomplished through one of two types of used directly in the pressurized plenum, a floor diffusers. Interior spaces are controlled standard balancing damper can be added to through a system of adjustable passive floor adjust airflow. mounted diffusers. Each diffuser consists of an adjustable swirl plate located in a mounting basket and mounts through a hole cut in the concrete floor panel. The basket catches any dirt or spilled liquids and prevents contamination of the space below the floor. The adjustable swirl plate allows the occupants to adjust their diffusers to a comfortable level in their space. The diffusers are design to achieve mixing of the approximately 65 F degree supply air within a very tight radius of the diffuser and at very low velocity. Typical diffuser airflow is about 60 to 100 CFM per diffuser.

Figure 5 Rectangular grille w/ plenum – Model 35BF-D

Figure 4 Swirl Plate Style – Model 35BF-R

7 Design Guide to Underfloor Air Distribution

Underfloor series fan powered Fan powered zone-mixing unit – mixing boxes – These units are One box is used per plenum temperature designed to fit underfloor in the space zone and is mounted in a closet or utility between pedestals and the grids of the space to control the airflow to the underfloor access flooring system. Size ranges from plenum. The product uses a standard Carrier 280 CFM to 1200 CFM. They are used for DDC controller configured for UFAD on a perimeter spaces, conference rooms and parallel fan powered box to control the other spaces with a highly variable load temperature and pressure of the zone. The pattern. They typically use supply air from box uses standard 55°F supply air from an the floor plenum and increase the volume air source and blends the air with return air delivered to the room to meet the variable from the zone to provide the 63°F discharge needs. The same style box can be used with air required in the plenum. The ECM motor ducted primary air when that option is in the box varies speed as required to control required to meet high thermal load plenum temperature by varying the amount requirements. Heating can be provided with of recirculated air introduced into the either hot water coils or with electric plenum, while the primary air damper heaters. simultaneously maintains the plenum pressure by controlling the amount of primary air introduced into the plenum. Minimum position stops assure that ventilation air is always provided. Plenum pressure is maintained at approximately 0.05-0.10 - inch WG.

Figure 6 – Underfloor Series Fan Power Mixing Box – Model 45UC

Underfloor perimeter fan coil boxes – These are another option to handle spaces with variable loads patterns and are well suited to addressing heating requirements of perimeter zones. Available in size from 325 to 2800 CFM these fan coil units also fit underfloor in the space between pedestals and the grids of the access flooring system. Heating is provided in these with either a hot water coil or an electric heater.

Figure 8 – Fan Powered Zone Mixing Unit – Model 45XC

Figure 7 –Underfloor Perimeter Fan Coils – Model 42KC

8 Design Guide to Underfloor Air Distribution

Central equipment – Since this system high perimeter to interior ratios or buildings uses a zone mixing concept and air is with many spaces, which have highly supplied at normal unit design conditions, variable loads. Normally, the best special air-handling equipment is not applications in new construction spaces are required. Mixing boxes may be used with offices with open plan construction. UFAD air-handlers in chilled water or DX type may also be used successfully in other types systems or with many types of packaged of space with mostly internal loads and open VAV equipment as well. Careful plan construction, as is done in some schools psychrometric analysis should be made of spaces. coil entering conditions to be sure the coil will meet the required leaving air conditions The second application is in retrofit particular to what may be higher than situations where the building characteristics normal entering sensible heat factors. Part make overhead air distribution systems load operating conditions in terms of coil difficult to apply. Recent trends to control or staging should be evaluated to be rehabilitate old warehouse space into offices sure room humidity levels can be maintained and downtown revitalization of existing below 60% RH. Economizers should be structures may fall into this category. The used with all system types where local issues with elevator and plumbing fixture climate permits and should use an integrated connections must be addressed. However, differential enthalpy type control. Because the savings and ease of providing the of low static pressure requirements, fan sizes cabling services and air distribution under may be lower than conventional overhead the floor may well pay for the cost of access systems. Due to varying air volumes to the flooring. The benefits of improved thermal zones, the use of VFD’s is recommended, comfort and indoor air quality may make the which will also improve system efficiency. space even more attractive to tenants. The use of high-quality filtration is recommend since a prime consideration of A third area where underfloor distribution the UFAD system is good indoor air quality. may make sense is in buildings with very high ceilings and no place to run overhead Types of Buildings air distribution. Some places of assembly Underfloor air systems are not the answer to (meeting rooms, churches, auditoriums) fit every building but may represent significant into this category. Again, open plan and life cycle cost savings when applied to the internal load concentration are the keys to appropriate buildings. successful applications.

The solution is most often related to three System benefits and cautions different types of applications. First, UFAD (+) Thermal comfort – Since UFAD can be applied in new construction office systems allow the occupant a degree of space where frequent office changes are control over ones own individual climate anticipated. Owners can benefit from the through adjustment of the diffuser; there reduced costs of moving offices particularly is a higher degree of perceived comfort. as related to the cost of communications Air distribution diffusers have also been cable and wiring. In general small spaces shown to provide good head to toe within a building are not well suited to temperature variations without underfloor air systems. Nor does UFAD disrupting the benefits of a stratification make good sense in a building with very zone.

9 Design Guide to Underfloor Air Distribution

associated with office churn, UFAD (+) Ventilation effectiveness – Fresh systems may have better total life cycle air supply is introduced to the space low costs. In addition, this energy efficient to the floor. As the air warms the air design will also increase the life cycle rises and pollutants are carried toward cost benefit and may make the system the ceiling by the warm air. Rather than look more appealing. mixing the air within the entire space, fresh air is provided to the breathing (+) Reduced Floor-to-Floor Height zone. The effect of this ventilation – The ability to reduce the floor-to-floor method may actually result in height by up to one foot per floor can efficiencies greater than 100%, but has result in additional floor space or not yet been documented. ASHRAE reduced shell construction cost. The Standard 62, (Table 6.2) lists the reduction comes from reducing or ventilation efficiency of UFAD systems eliminating ceiling plenums. However, at 100%. The perceived indoor air this reduction must be evaluated based quality is better and may potentially be on the need for ceilings to hide light used to reduce total ventilation airflow, fixtures, piping for sprinklers, and to although this is not generally help improve room acoustics. recommended. (+) Improved productivity and health – It is often a controversial thing to prove but recent studies have shown that productivity can be improved through good indoor air quality and comfort. Since the costs associated with employees far out weigh all other operating costs, savings can be significant.

(-) Cold floor – One of the most sensitive points of thermal comfort on the body is the ankle. Control of floor temperature can be a critical issue in order to avoid

Figure 9 – Thermal Plume Effect complaints due to the radiant effects from a cold floor. Also, the sensitivity to cold drafts as the result of floor diffuser (+) Energy usage – UFAD systems placement is an issue that should be may have significant energy savings adequately addressed. when compared to overhead air systems because of lower fan static requirements. (-) Code issues – UFAD technology as Economizer hours of operation may it is being applied is relatively new and increase energy savings as well, but is not all building code officials may know highly dependent on the local climate. how to interpret the codes as applied to the technology. The result is that some (+) Reduced Life Cycle Costs – life safety components such as fire Based on the reduction in costs resistant materials, sprinklers and smoke

10 Design Guide to Underfloor Air Distribution

detectors may be required on certain This standard sets green building objectives projects. in 5 categories that offer a total of 32 credits, from which a design can accumulate up to (-) Condensation – Even though the 69 points. The rating system is based on the thermal mass effects of the plenum floor number of points awarded and goes from a slabs can have a positive effect on certified level at 29 to platinum level at 52. sensible load swings, care needs to be Some prerequisite requirements are exercised with control schemes that pre- required, but other than that, the points may cool the slab and then are allowed to come from any of the 5 categories. HVAC bring in large quantities of potentially related systems account for 40% of the moist outdoor air. Air supplied to the possible points. The points are based on the underfloor plenum must be kept above entire building as a system. No particular the dewpoint temperature of the floor product or system is certified by the slab. program but rather the building in which they are used. Underfloor air systems have (-) Humidity control – The impact of potential in at least seven areas, primarily local climate and part load control within the environmental and indoor air methodologies must be carefully quality areas. evaluated. The impact of higher supply air temperatures has the potential to The largest points area in the program is negate some of the central system’s optimizing energy performance. This credit natural latent heat removal capability. varies from 1 point for exceeding ASHRAE Design analysis should evaluate the 90.1 energy cost budget (ECB) model by effects of part load latent control and 15%, up to 10 points for a 60% reduction. In control methodology to make sure room most UFAD projects the analysis would relative humidity is kept below 60%. involve using the ASHRAE energy cost budget to compare the underfloor system to Sustainability (LEED™ points) the energy costs of a VAV reheat system. There is a nationwide trend toward constructing buildings that are more UFAD systems may have a distinct environmentally friendly to meet green advantage in this area since they incorporate building or sustainable design construction many potential energy saving features. standards. The best known green building Lower fan static requirements as the result standard is the LEED-NC™ standard issued of low plenum pressure distribution will by the U.S. Green Building council, but reduce required fan energy. Mechanical additional standards like the LEED-CI for cooling requirements and hence equipment interior systems is also being prototyped. size and part load efficiency may be The LEED (Leadership in Energy and improved due to the use of higher discharge Environmental Design) standards are a temperatures. Economizer usage may be voluntary rating system that rates the extended to get more free cooling hours and building design on its environmental impact help reduce part load energy requirements. on the community, the site, the water In general, UFAD systems have the efficiency, the energy efficiency and the potential to show significant energy savings indoor environmental quality for the over the required base system used in the occupants. energy budget model.

11 Design Guide to Underfloor Air Distribution

The UFAD system may also qualify for comfort envelope, the system can qualify for points under the material and resources this credit as well. category. Within this category 3 points can be earned for building reuse. If the project UFAD has shown itself to be an excellent involves an existing building, the percentage system to consider on projects seeking of building shell reuse would help obtain LEED certification. Remember, however, these credits. UFAD systems lend that certification is based on the total themselves very well to certain retrofit building system and the points and overall situations where the building is being rating for a specific project are dependent on renovated to meet the requirements of new the design of that project. technology without destroying the current building structure. UFAD gives the architect a range of options to achieve these goals. Then, depending on the location, it is possible that the floor system used would qualify for the regional credit if the project were within 200 or 500 miles of the equipment manufacturing location.

Looking at the Indoor Environmental Quality category, UFAD systems have the potential to qualify for points in 3 areas. This is the area where many of the existing certified projects have relied on points from UFAD systems. The first is the area of ventilation effectiveness. UFAD systems are not rated by ASHRAE as having any advantage in ventilation effectiveness as we noted earlier. However, in the current LEED documentation, it is implied to have ventilation effectiveness greater than one. As a result, the use of UFAD systems may be a qualifying point. The second area addresses system controllability, where the LEED requirement is for interior zones to provide individual control to at least 50% of the occupants. Since UFAD diffuser provides individual control inherently, the system again is nearly an automatic qualification for this point. The third area relates to thermal comfort. Since UFAD systems have been shown to provide a high degree of personal comfort with space control parameters that fall within the guidelines of the ASHRAE standard 55

12 Design Guide to Underfloor Air Distribution

DESIGN PROCESS So for a typical side-fed core location the maximum zone size 2 Design Decisions would be 120 ft by 60 ft, or 7200 ft or less. When beginning the design of a UFAD • Open office interior zones, those that system certain key differences and concepts have fairly uniform loading, lend need to be remembered to avoid themselves to pressurized plenums incorporating elements of traditional using passive swirl diffusers. overhead mixing systems: • Open area zones with loading • Underfloor systems work best when differences are hard to “isolate” from a stratification layer is established at one another without plenum barriers 6 feet above the access floor. This beneath the access floor. Too many creates a partial displacement barriers destroy the future ease of ventilation effect within the room rezoning and utility access to points and improves ventilation of use. effectiveness, contaminant control, • Zones with high heating loads need heat capture, and reduces loads to consider the use of ducted linear within the space, transferring them bar diffusers that deliver the directly to the return air. conditioned air to the load point • Airflows need to be closely matched (usually exterior walls with glazing) to the loads within the occupied zone without significant thermal decay. so as not to over air the space, losing Insulate ductwork to prevent loss of the partial displacement ventilation cooling capacity. effect. • Perimeter zone size may be restricted • Mixing only occurs within the by ASHRAE 90.1, requirements for occupied zone and happens so exposure zoning, which limits linear efficiently that it is normal for a 3 to wall per exposure to 50 feet. 5º F temperature gradient to occur, • Locating zone-mixing terminals in right at the ASHRAE 55 comfort vertical equipment closets does take standard limit. up more useable floor space Do not include excess safety factors; it will compared to locating the units within lead to over airing the zone spaces, resulting the underfloor plenum, but is a good in many occupant comfort complaints and choice for reasons of ease of loss of the stratification layer. maintenance and minimizing obstructions within the plenum. Zoning Decisions • When high cooling or Because of the underfloor air supply plenum dehumidification loads are present, aspects of a UFAD system, zoning decisions such as conference rooms and rooms have a few complexities: with plants, process or large • Thermal decay, the increase in equipment or solar loads, consider cooling supply air temperature due to conditioning these spaces with heat transfer to the floor slab and traditional VAV terminals using access flooring, limits the zone lower temperature (50 to 55ºF) maximum travel distance from the primary air. This hybrid system terminal discharge into the plenum to design will result in smaller central the furthest diffuser to 50-60 feet.

13 Design Guide to Underfloor Air Distribution

equipment, as well as ductwork and equipment chases strategically mains and branches. located around the floor plate. • When using zone fan powered mixing boxes (FPMB) above the General Layout access floor the chases must provide When beginning the layout of a UFAD adequate room to locate the terminal system it is best to think of it as an upside as well as provide required service down conventional overhead system, with clearances. the following similarities and differences: • UFAD systems use diffusers in the • Central cooling and heating access floor, much closer to the equipment, as well as ductwork occupants, so special care needs to mains and branches to zone mixing be exercised in their layout. Since it terminals, can be nearly identical to is a non-ducted system, simply traditional overhead systems.s. swapping floor panels can make Systems that do not employ the use relocations easy. of zone mixing terminals require • UFAD systems work best with special consideration based on the uniformly loaded thermal areas. In volume and temperature from the high-load and variable-loaded zones, air-handling source. UFAD can be used in conjunction • Zoning occurs in much the same way with traditional overhead systems. as with other systems, but due to the This creates an overall hybrid system duct mains above and zoned that utilizes the best features of both distribution plenum below, there is a types of systems (figure 10b). need for multiple vertical duct drops Figure 10 – Carrier UFAD Design Solutions 84° RA to Air Source 55° SA from Air Source

84°F

RETU RETURN

84°F

75°F 75°F

62°F

Zone Parallel Passive Floor Diffusers Underfloor Fan Coil or Fan Powered mixing Box Fan Powered mixing Box

Figure 10a - UFAD with Zone Mixing Box

14 Design Guide to Underfloor Air Distribution UFAD Hybrid System

84° F Return to Air Source

55° F Supply from Air Source VAV Moduline Terminal

84 ° F RETUR RETUR

84 ° F

75°F 75°F

62°F

Zone Parallel Fan Powered Passive Floor Diffusers Underfloor Fan Coil or Series mixing Box Fan Powered Mixing Box

Figure 10b - UFAD Hybrid System

15 Design Guide to Underfloor Air Distribution

Building Load Considerations • Lighting energy is predominantly radiant; so only assign 1/3rd of total NEED FOR TWO ZONES - With an watts directly to the stratified zone. effective stratification layer, running cooling When return air is taken through the and heating zone loads for use with a UFAD light troffers, a greater portion of the system is complex. The claim that loads total watts used, up to 50 percent, that occur in the upper stratified zone do not can be assigned to the stratified zone. affect the occupied zone have been observed • Exterior wall conductive load is still in tests and completed projects, but to date, transferred into the zone as no computerized load estimating program is predominantly radiant (often over 60 capable of modeling this condition. Using percent), so only 1/3rd of the upper recent project experience, and not being wall area load should be assigned to conservative in assigning loads or safety the stratified zone. factors to the occupied space can reduce • Alternatively, lowering the zone space cfm requirements with a UFAD ceiling height to the midpoint value system. of the stratified zone would allow most computerized load programs to automatically assign the loads addressed above to the return air. • Solar loading through glazing is 100 percent radiant, unless internal shading is used. Since manually adjusted shades cannot be counted on when sizing equipment or setting airflows, do not assign any direct solar load to the stratified zone. • Solar wall loading becomes over 1/3rd convective, so an appreciable portion of this amount can be manually assigned to the upper stratified zone, especially if wall- Figure 11 – UFAD Design Load Concepts washing linear bar supply registers and overhead perimeter returns are used in the room air distribution LOAD ESTIMATING CONCEPTS & design. FACTORS - Manual output adjustment of • People loads should always remain computer program inputs and outputs allow 100 percent assigned to the occupied designers to more closely model UFAD zone. systems and correctly assign loads and set • Equipment loading should not be zone airflows. assigned to the stratified zone unless • For stratification to benefit cooling return register placement near the loads, floor diffuser 50 fpm throw equipment is used to capture a cannot exceed 4 to 5 feet, so as not to portion of the load with the return affect the stratification height, which air, or it is believed that aggressive is typically 6 feet. thermal plumes will develop and

16 Design Guide to Underfloor Air Distribution

carry a substantial portion of the load T AHU Supply is the assumed primary into the stratification zone. air temperature into the zone-mixing • Infiltration loads always remain box. within the occupied zone, and outdoor air ventilation loads always 3. Finally, solve for TReturn from the remain a part of the central stratified zone. This is the return air equipment coil loading. temperature to the zone mixing box, and the air handler before outdoor air When the load estimates have been run with mixing occurs. appropriate UFAD adjustments made, two loads should be compiled by the designer, TReturn = (Qunoccupied /1.10 * CFMroom)+ Troom QOCCUPIED for the lower mixed zone, and QUNOCCUPIED for the stratified zone above the Equation 3 stratification height. With these loads the Where: designer can then calculate the following TRoom is the thermostat set point for the system design parameters: space, and

1. First, solve for airflow in the occupied CFMRoom is the airflow required in the zone. This is the supply air delivered occupied zone. by the underfloor diffusers (the zone mixing box total airflow). DON’T OVER AIR THE ZONE - When working with Equation 1, the space set point CFM room = Qoccupied / (1.10 * (Troom – T plenum supply)) temperature, TRoom, can be raised up closer to the temperature at the stratification height Equation 1 (approximately 75 F) to more accurately determine the actual minimum airflow Where: required to meet the occupied zone load. TRoom is the thermostat set point for Similarly, in Equation 3, TRoom should use the space, and the same higher value, remembering that with UFAD systems and the partial TPlenum Supply is assumed discharge displacement ventilation effect, return air air temperature in the plenum temperatures are quite a bit higher than the space set point temperature.

2. Solve for CFM primary air required. Other load-related concerns come from air (This is the air supply from the air leakage out of the plenum through handler and used in zone box construction gaps, leaks between the floor selection) tiles directly cooling the occupied zone, and radiant and convection cooling by the cooler Equation 2 floor panels and structural sub-floor. The last two lower the room CFM, and if not CFMZone-Box = Qoccupied / (1.10 * (Troom – T AHU supply)) accounted for, will lead to undesirable zone over airing and increased “too cold” complaints from the occupants in the space. T Room is the thermostat set point for the space, and

17 Design Guide to Underfloor Air Distribution

Central Equipment Selection dehumidification of outdoor air. Bypassing While a UFAD system is thought of as a return air around the cooling coil is one predominantly air distribution design method of obtaining the required 61 to 65º F variation, there are many particulars supply air temperature to the underfloor affecting central equipment selection that plenum. This can be done in the mechanical the designer needs to pay attention to, equipment room, but is also effective by particularly: blending in the return air off the suspended ceiling return plenum using Parallel Fan • Central fan energy should be less Power Mixing Boxes (PFPMB). than a traditional system because of the low plenum pressure and Fan Static Requirements substantial reduction in ductwork downstream from the air terminal. Reduced external static pressure on the • Primary cooling air quantities can be central equipment is an advantage of a reduced when potential load credits UFAD system. Supply ductwork runs and are realized from the stratification air pressure drops on the zone terminals zone partial displacement ventilation leading to the plenums are similar to effect. conventional systems. The same is the case • The entering coil psychrometrics with return ducts, but there is always less should be checked to be sure that the than 0.10 in. wg pressure requirement on the cooling coil selected would be able underfloor plenum and diffusers to achieve the required Apparatus downstream from the zone mixing terminals. Dew Point (ADP) and bypass factor. While this savings of 0.25 to 0.50 in. wg In some cases this may require the appears small, add to it the overall reduction use of larger coils, more coil rows or in primary airflow associated with UFAD’s lower face velocities. partial displacement effect, we should see real reductions in system fan motor • Part load loss of humidity control horsepower. can be a significant issue. A part load

analysis should be done to determine if the coil control methodology being used or the refrigeration staging will Terminal Equipment Selection provide sufficient latent heat removal Room Air Distribution at part load operation. • Air economizers should be used whenever possible based on the local OCCUPIED ZONE DELIVERY climate. The use of an enthalpy When a UFAD system is used, room air wheel is recommended for periods distribution occurs within the occupied zone when the 55° F supply air (up to 6 feet above finished floor), making temperature off the coil is not this area a critical design element. Designs required. Use care in setting up the can be either CAV (constant air volume) or economizer changeover temperature VAV (variable air volume), or as is more to ensure that the 60° F maximum often the case, a hybrid using some of both space dewpoint temperature will not to satisfy the varying room requirements. be exceeded. CAV systems usually vary the temperature Traditional cooling coil leaving air of the supply air to meet the space temperatures of 50 to 55º F should be used temperature set point, while VAV systems for the coil selection in climates requiring provide constant temperature and vary the

18 Design Guide to Underfloor Air Distribution quantity of air supplied to the zone. These ventilation effectiveness might cause a concepts are not as clear-cut as with designer to lower overall outdoor air overhead systems because UFAD allows the quantities, it is recommended to retain the occupants to easily adjust the diffusers of values derived using the assumed 100 either system to affect airflow volume and percent ventilation effectiveness found in even pattern or direction in some instances. ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality. Working Since the supply air is introduced into the with either the multiple space equation or occupied mixing zone it has to be warmer the new ventilation rate formulas in than traditional systems, around 61-65º F, or Standard 62-2004, you will find minimal only 8-13º F below the usual space cooling CFM reductions for the increased ventilation set point. To avoid comfort complaints the effectiveness (not addressed yet in air must mix quickly into the room and not ASHRAE 62, but believed to be above 1.0 project outside the occupied zone so that the and below 1.2). stratification zone remains intact. Tests have shown an ideal airflow of 0.6 CFM/sqft CORE vs. PERIMETER for good mixing, while maintaining the UFAD systems work best with spaces like stratification height at 6 feet, and keeping core areas and open perimeters with limited the room temperature gradient below 5º F. glass that have low load variations. These zones tend to be larger and constant volume designs will provide the most cost effective installations with superior IAQ. When load concentrations increase and/or high load variability is present, CFM delivery to the space can increase significantly due to the low delta temperature of UFAD systems, and adding VAV equipment will increase system cost. Perimeter offices with high glazing areas, conference rooms, and areas with large equipment loads are examples of zones where use of dedicated traditional systems make the most sense. The resulting Figure 12 – UFAD Room hybrid design makes the most of both Temperature Gradient traditional mixing ventilation systems with lower cooling supply air temperatures for IAQ BENEFITS limited zones, while retaining UFAD Real benefits from the stratification zone benefits for the base design. and partial displacement ventilation effect include eliminating short-circuiting to the return registers and better heat removal, Diffuser Selection and Layout especially of localized loading when returns are located near the load. The overall NOT LIKE OVERHEAD upward movement of the air within the room Since the occupant is much closer to the has shown reduced total particulate counts diffuser in a UFAD system it must be a high and improved local air change indexes in induction design to quickly even out the recent studies. While such improvements in space temperature in the surrounding area

19 Design Guide to Underfloor Air Distribution and avoid drafts and occupant comfort comprehensive information is available to complaints. The higher supply air learn more about these UFAD refinements. temperature of a UFAD system and the need for quick mixing results in greater numbers Passive type diffusers can be easily of diffusers located in the access floor. rearranged by just moving floor panels; Since the diffusers have low pressure drops active types require moving fan units, power and plenum pressure is very uniform, UFAD and control wiring and potentially ductwork. designs can be considered self-balancing at Active diffusers are found more often in the diffuser. TAC/PEM designs and as part of hybrid solutions to satisfy heavy and/or widely APPLYING DIFFERENT TYPES AND variable loaded spaces. UFAD diffusers STYLES should not be used where liquid spillage There are two basic types of diffusers to potential is high, even though they have a choose from: passive and active. Passive are catch basin for incidental spills, and they used in pressurized plenum systems and should not be placed in traffic areas where active diffusers are used in neutral pressure high rolling loads are expected. or fan-assisted pressurized plenum systems. There are three basic styles of diffusers, To select a passive diffuser, determine the swirl, VAV floor grille and linear bar grille. required room airflow with Equation 1 and Passive styles are able to move required air divide it by the tabulated airflow for the quantities at low plenum pressures (0.05 to diffuser at the pressure maintained in the 0.10 in. wg) and have very low noise plenum. This will be the approximate generation values, usually under NC20. In number of diffusers required. Adjustments fact, a white noise generation system within may be made to be sure all spaces receive the plenum is often recommended for proper adequate coverage. acoustic privacy in open office situations.

The table below summarizes the various #Diffusers = CFM /Diffuser CFM features and application recommendations Room at Plenum Pressure for these products. Use of other products, Equation 4 such as task/ambient conditioning (TAC) and personal environmental modules (PEM), Cooling Terminal Selection are not covered in this guide, but Each zone (thermostat) will require a DIFFUSER DIFFUSE UNIFORM VARIABLE dedicated air terminal to satisfy cooling and TYPE R STYLE LOADING LOADING heating airflows to the areas covered by the Core – open zone. Larger, more uniformly loaded open areas & Swirl offices Base loading office interior zones can be supplied by Perimeter – parallel fan-powered mixing boxes Passive open areas (PFPMB) all the way up to 3,700 CFM Large equip. Floor loads, few Base loading primary airflow (5,800 CFM mixed airflow). grilles people Smaller perimeter zones and partitioned Not Not interior zones can be handled with the same Swirl recommended recommended equipment down to 500 CFM primary Active Linear Bar Spaces with Conference airflow. (fan-forced) or tighter Rooms Floor temperature Perimeter, grilles tolerances Htg & Clg

20 Design Guide to Underfloor Air Distribution

Once cooling loads have been completed, Heating Terminal Selection basic terminal selection can occur, using traditional manual or software methods. Heating loads that are significant along a Keep the following items in mind when perimeter wall should be handled with selecting units for UFAD systems: dedicated ducted fan-coils located under the access floor or from overhead. First stage of • Primary airflow can come from a heat would be recirculated room or ceiling traditional temperature source (50 to plenum air in larger zones, with additional 55º F +) or even from a low- zones of heat provided by either hot water or temperature central system, keeping electric heating coils in the fan-coil. When supply ductwork mains and branches zones have low density uniform heating small. needs, either on the perimeter or the interior, • Primary airflow is the airflow it is possible to provide warm-up and/or determined with equation 2 and is zone heating using a low-profile series fan based on the air handler supply air powered box used for cooling, with either temperature. Total airflow is the hot water or electric heating coils. amount calculated by equation 1 above. Remember that a UFAD system in heating • Care must be exercised in the mode is no different than any other system; selection of the zone mixing unit to there is a heating demand placed on the be sure it will provide the total space by the minimum ventilation airflow mixed airflow based on calculated setting of the terminal, determined by the entering conditions to the zone temperature offset between the room set mixing unit. The use of selection point and the supply air temperature to the software is recommended. terminal. • Since the units are closer to the occupied space, often mounted Load/Design Considerations vertically within an equipment closet, care needs to be taken so that UFAD Systems will create a high degree of acoustic values are acceptable, by personal comfort when designed with floor keeping inlet duct pressures down, diffusers that provide a well-mixed zone in selecting units in the low end of their the lower portion of the room. Warm return range, and using the appropriate air travels upward through natural acoustic attenuation options convection and is carried back to the HVAC available. system. If the diffusers are not designed • Optional filters on the recirculated properly, the distance the air travels from the air opening are advisable, removing diffuser into the space could be excessive, particulate from the return air stream causing stratified air to circulate back into before it mixes with the primary air. the occupied space. Improper system design could result in poor temperature control and Control options that meet the sequences of the ability to manage building loads. operation will be required. Control schemes tend to be more complex with UFAD, The 35BF-R Swirl diffuser throw is a making a DDC system a logical choice. function of both airflow and ΔT. Quite often, space loads are over estimated. UFAD systems were designed around

21 Design Guide to Underfloor Air Distribution

100cfm/person at a 10° FΔT from room to The temperature/velocity profiles are shown discharge air temperature. System analysis on figure 14. proves that with many projects, the cfm was too great causing over cooling, and with the At the perimeter, or in conference rooms, 10° FΔT, difficult to control humidity. As a where higher cooling demand requires result of this analysis, many current project considerably more air than in normal interior designs have elected to reduce the supply zones, there are other considerations to temperature in the underfloor plenum from review. With the typical supply air 66° F to 62° F and the supply airflow rate temperature of 65° F, the quantity of air may from 1cfm/ft2 to 0.8 cfm/ft2 or less. be such that the diffuser throw may create unwanted mixing in the upper part of the The optimal airflow/delivery rate of the room. 35BF-R, swirl diffuser is 15° FΔT and 80 cfm per diffuser. This tested performance Figure 13 shows the performance of a ensures the ideal throw of 6’ within the typical rectangular floor grille in a perimeter occupied zone. Temperature and velocity zone. By lowering the diffuser discharge profiles for the 35BF-R, swirl diffuser, have temperature, we can reduce the throw and been measured with an anemometer avoid affecting the stratification in the /temperature “tree” in accordance with interior zones. ASHRAE Standard 113-90.

Figure 13 Perimeter Throw

22 Design Guide to Underfloor Air Distribution

Figure 14 Temperature/Velocity Profile Charts

23 Design Guide to Underfloor Air Distribution

Distribution System Design detailing is analogous to pressurized stairwell and smoke-barrier With UFAD the major differences over construction in high-rise buildings. more conventional systems become quite Since these are not sheet metal ducts apparent once the designer begins working governed by industry standards like on the air distribution system. The ductwork SMACNA, engineers must work from the air source to the zones is essentially closely with architects to assure the same as any other all-air system, but proper design and close follow-up with air delivery occurring underfloor there during construction to make sure the are routing and design requirements that are work is done and done properly. quite different than one is used to, especially • Sealing concrete floor slabs reduces with air diffusion occurring within the particulate generation, though occupied zone (6 inches to 6 feet above the heavier dirt accumulation within the finished floor). plenum will not be entrained by the low velocity airflows. • Insulate plenum exterior surfaces, Plenum Design Considerations both floor and sidewall, to minimize Plenums for UFAD systems are grouped thermal losses, which can affect the into two types, pressurized and zero- temperature of the supply air, pressure or neutral. Pressurized are more delivered to the furthest diffusers. common because they offer the greatest • Obstructions within the plenum offsetting cost benefits from reduction in should be kept to less than 50% of HVAC system components, but neutral the plenum depth to avoid affecting plenum designs are easier to zone and are airflow. This is more an issue with not affected by plenum air leakage. shallow plenums. Pressurized plenums predominantly use • Plenum dividers, a part of a passive diffusers while neutral plenums manufactured access flooring require the use of active style diffusers. system, should be provided when zoning is critical, as part of an FEATURES/BENEFITS/CAUTIONS FOR architecturally determined smoke PASSIVE PLENUMS barrier, and to separate the perimeter The following guidelines can be used in from the core when a hybrid solution developing a pressurized plenum design for is used. HVAC use with passive and active diffusers. • For good acoustic privacy between • Common plenum depths are 12-18 offices, the floor panels should inches from top of slab to top of weigh at least 10 psf. Placing a floor panel, with most panels being < physical barrier in the plenum along 2 inches thick. Practical minimum the common wall line provides plenum depth is 8 inches and additional attenuation. plenums deeper than 24 inches offer • Floor panel temperature will be no HVAC advantages. midway between room set point and • Airflow leakage must be designed plenum temperature, creating both a out with superior construction radiant cooling effect, and sealing of all joints, including convective heat transfer into the overlapping joints when laying down plenum, lowering the required room carpet squares. The type of required airflow.

24 Design Guide to Underfloor Air Distribution

• Floor slab mass can allow for content during humid summer conditions, unoccupied thermal storage to be off-coil temperatures must be lower than the used later for occupied period load supply air temperatures used with UFAD leveling benefit. diffusers. To warm these temperatures back up, return air is routed back into the supply air stream, either at the zone air terminal or at the supply fan housing, bypassing the cooling coil. For the zone terminal design to work, warm air is usually ducted back from a ceiling return plenum. Placing ceiling return registers along the exterior walls brings an added benefit of pulling in any thermal plumes that develop under high solar loading conditions, reducing zone cooling load in the summer and increasing

the first stage heating effectiveness in the Figure 15 – Carpet used to winter. If active diffusers are used, prevent plenum leakage additional floor registers are provided to allow room air to reenter the plenum, Return Air Design maintaining supply diffuser CAV feature when reduced loads lower the supply airflow entering the plenum. Remember, if room air MAINTAINING DISPLACEMENT is returned back into the zone without BENEFITS flowing through the main cooling coil, The key design elements for the return side cooling load credits indicated in the loads of the air distribution system are to help section discussed above need to be lowered create the partial displacement ventilation or eliminated all together. effect and more directly remove local high cooling loading conditions. Placing return registers on the ceiling or high sidewall, Ductwork Design coupled with the stratification layer created by the short throw supply diffusers in the One of the main benefits of UFAD is floor, creates the desired overall upward deleting ductwork associated with air vertical flow of the air within the room. distribution downstream of the zone When a space has high cooling loads like a terminals and using an airflow plenum copier or a cluster of data terminals, place underneath a raised access flooring system the return directly above the area to for low-pressure distribution to the room effectively capture a large portion of the diffusers. All the ductwork from the air- heat. How much will depend on the height handling unit to the zone terminals is laid of the ceiling and nature of the supply out and sized using the same methods used airflow in the immediate area. on traditional mixing ventilation systems.

This applies to both supply and return ducts, RECIRCULATING RETURN AIR as well as the outdoor air ductwork. There Variations in return airflow occur when is little which is special about UFAD space humidity control is critical or a CAV systems for duct mains and branches. plenum design is used, either pressurized or neutral. To control the supply air moisture

25 Design Guide to Underfloor Air Distribution

VERTICAL DROPS contractor and need to be sized like normal A vertical drop into an access floor plenum ductwork (limit mains to 1,200 – 1,500 fpm could result in excessive thermal decay if due to limitations on placement of vertical the distance from the air source to the drop entries into the plenum, remember that farthest point in the underfloor plenum is too future rearrangement of furniture/partition great. Due to the thermal decay of the zoning above the floor will likely require supply air temperature within the plenum, similar ductwork adjustment under the floor, travel distance needs to be limited to 50-60 negating some of the cost savings. Also, air feet from the plenum entry to the furthest highways almost always cause service utility diffuser. This will require multiple entry re-routings around them when laying out points on all but the smallest system. If power and communications. shallower plenums are used and frequent obstructions are possibilities, more than the SMOKE PARTITIONS AND FIRE usual number of plenum entry points should WALLS be included in the duct design. Whenever plenum areas become too large, they inevitably cross over partitions or walls AIR HIGHWAYS that need to meet life safety code Another popular plenum distribution requirements extend down through the technique is to use air highways within the plenum to the floor slab. HVAC zoning plenum to distribute zone supply air to needs to pay attention to these lines and try multiple points within the plenum. These not to cross over them. If air needs to be are typically provided by the installing transferred over, do so with a hard duct

Figure 16– Floor Plan Chase Locations 26 Design Guide to Underfloor Air Distribution

sleeve with the appropriate smoke and/or HEATING INTERIORS WITH CENTRAL fire damper installed per code. EQUIPMENT Smaller single-story buildings are often able Heating System Design to switchover during the peak heating times to full heating from the central HVAC CENTRAL SYSTEM OR HYBRID equipment. When this happens, normal How we choose to heat the perimeter zones control strategies for the zone terminals will depends primarily on the magnitude of the control mixing of the warmer SAT with load and whether or not simultaneous recirculated plenum air and adjust terminal heating and cooling is required. The need fan speed to control plenum pressure so that for continuous cooling in too many interior the “cooling” diffusers will now operate zones makes it hard to switch over the properly as heating diffusers. central equipment to heating. If the wintertime temperatures drop below 40ºF and the building perimeter has more than 1/4 Control Systems glass area, increased SAT’s will be required to counteract the high heating loads and UFAD systems rely on proper design and downdrafts along the outside walls. In operation of the controls like any other either of these cases it is recommended that system. Listed below are concepts and a separate heating system be designed for concerns that, if incorporated into the the perimeter zones, creating a hybrid design, will result in maintaining the system with UFAD heating being reserved stratification layer and its many benefits: for the interior. • Most UFAD systems use pressurized plenums and CAV passive diffusers, PERIMETER FAN-COILS WITH LINEAR requiring some type of pressurization BAR GRILLES control on either the zone mixing High heating loads along the outside walls box or central equipment. often require higher SAT’s than interior • Zone control with CAV diffusers spaces need for morning warm-up or requires adjustment of the supply air offsetting roof loads, so selecting zone fan- temperature at the air terminal. This coils and locating them in the perimeter is best accomplished using Parallel plenums is an ideal solution. Using either Fan Powered Mixing Boxes hot water or electric heating coils allows (PFPMB) connected to the zone local boosting of the zone SAT above those ceiling plenum return air that mixes obtained by recirculating ceiling plenum air with the primary air. that is traditionally stage one heating for • Resetting Supply Air Temperature light load conditions. Use of a linear bar up under light loads allows grille style diffuser is recommended to additional hours of outdoor air spread the air out along the perimeter wall economizer cooling, especially in and offset the downdrafts that develop. drier climates. During summer cooling this style of grill is • Nighttime pre-cooling, done also beneficial in treating the higher cooling correctly to avoid potential loads that are typical along the perimeter of condensation on the plenum most buildings. surfaces, can offset or delay thermal decay of the zone SAT.

27 Design Guide to Underfloor Air Distribution

• Thermostat mounting heights are • A wall mounted space temperature often being set lower because of sensor located in the zone will sense ADA (Americans with Disabilities load requirements. Act), from 54 to 48 inches above the • As the zone’s cooling load decreases, access floor. This is actually a the fan speed increases, which benefit because if mounted too high increases the amount of recirculated the thermostat would be sensing the air drawn in from the ceiling plenum. higher temperatures near the • Simultaneously, the amount of stratification layer, requiring primary air is reduced to maintain a adjusting the set point upwards to constant plenum pressure. compensate. • The terminal fan will operate • Systems, which use Parallel Fan whenever the primary air source is Powered Mixing Boxes to control operating. the zone primary airflow for System Start-up ventilation requirements, can use • Points 1 to 2 from figure17 indicate Demand Controlled Ventilation, that maximum cooling airflow is DCV, to reduce primary airflow in established by the user-defined lower occupancy periods resulting in maximum cooling plenum pressure additional energy savings. set point until the zone comes under • Control requirements of ASHRAE control at point 2. The fan operates Standard 90.1 should be adhered to. at minimum speed. Including setback requirements and • From point 2, slowing the primary optimum start requirements. airflow until the minimum cooling plenum pressure set point is reached TYPICAL CONTROL SEQUENCES (Up at point 3 reduces the plenum To Two 45X Mixing Boxes Serving A pressure. Common Underfloor Plenum): Typical Cooling Operation Plenum mixing boxes with variable speed fans can provide pressure and temperature • Point 3 indicates that the zone control for plenums serving interior zones, temperature is above the control and exterior zones using a separate (heating) set point; the plenum perimeter heating system. This control pressure is maintained at the user- sequence has the capability to over defined minimum cooling plenum pressurize the plenum at start-up or during pressure setpoint. high load requirements. Typical operating • At point 4 from figure17, the zone sequence is shown in figure 17. temperature decreases; increasing the • The sequence shows basic VAV fan speed to introduce more control of the primary air. recirculated air, while simultaneously reducing the primary • The fan provides plenum airflow, increases the plenum temperature control by varying the temperature. The plenum pressure amount of recirculated air introduced remains constant at the user-defined into the plenum, while the primary minimum cooling plenum pressure air damper simultaneously maintains set point. the plenum pressure by controlling the amount of primary air introduced • As indicated by points 4 to 5, should into the plenum. the zone temperature continue to fall the primary air damper will close,

28 Design Guide to Underfloor Air Distribution

but not below the user defined control may be configured to provide minimum ventilation set point at constant volume heating for a point 5 (may be set to zero), while constant supply of heated air to the the fan provides recirculated air to zone. maintain the plenum pressure set point. Morning Warm-up • Upon receiving a morning warm-up Heating Operation signal generated by the air source • Upon receiving a signal for heating controller, the primary air damper and heated air is being provided from will modulate to maintain the the air source, the system heating- maximum heating plenum pressure mode is automatically in effect. set point if the zone temperature is • If the zone temperature is above the below the occupied heating set point. occupied heating setpoint, the This allows a maximum quantity of primary air damper modulates to warm primary air to be delivered to maintain the minimum heating the zone. As the zone’s temperature plenum pressure set point. The fan rises, the plenum pressure is reduced operates at minimum speed. to the minimum heating plenum • Should the zone temperature fall, the pressure. The terminal fan operates plenum pressure will increase up to at minimum speed. the user-defined maximum heating plenum pressure set point. The

UFAD Control Operating Sequences

Plenum Pressure (Maximum Cooling)

2 1 Maximum Fan Airflow Primary Airflow 5 Warmer 4

Plenum Pressure (Minimum Cooling) Cool Plenum Temperature

Minimum Ventilation 4 5 Min. Fan Airflow

Occupied Heating Setpoint Occupied Cooling Set point (Heating & Morning Warm-up)

LEGEND NTFC – Nighttime Free Cooling ------Air Source Supplying Heated Air Air Source Supplying Cool Air

Figure 17 – Carrier UFAD Control Sequence Equipment Cooling

29 Design Guide to Underfloor Air Distribution

TYPICAL CONTROL SEQUENCES zone comes under control at point 4. (Three or more 45X Mixing Boxes Serving a The fan operates at minimum speed.

Common Underfloor Plenum): For applications requiring multiple mixing Typical Cooling Operation boxes serving a common plenum, it is • Point 3 indicates that the zone necessary to maintain a balance between temperature is above the control each of the mixing units. To accomplish (heating) set point; the terminals this, plenum mixing boxes with variable primary airflow is maximized to speed fans can provide constant cfm and provide cold air to the plenum and temperature control for these large common cool the space. The fan is at a plenums. A Typical operating sequence is minimum airflow and the total shown in figure 18. volume of air is maintained at the • The sequences show basic VAV required constant airflow set point. control of the primary air. • At point 4 from figure 18, as the • The control provides volume airflow zone temperature decreases; the from each terminal into a common discharge access floor plenum, thus air temperature from the terminal maintaining the plenum pressure into the plenum is increased by constant during both cooling and increasing the fan speed to introduce heating. The fan provides plenum more recirculated air, while temperature control by varying the simultaneously reducing the primary amount of recirculated air introduced airflow to maintain the total airflow into the plenum, while the primary into the access underfloor plenum. air damper simultaneously maintains • As indicated by points 4 to 5, should the constant volume airflow set point the zone temperature continue to fall by controlling the amount of primary the primary air damper will close, air introduced into the plenum. but not below the user defined • A wall mounted space temperature minimum ventilation set point at sensor located in the zone will sense point 5 (may be set to zero), while load requirements. the fan provides recirculated air to • As the zone’s cooling load decreases, maintain constant airflow into the the fan speed increases, which underfloor access plenum. increases the amount of recirculated air drawn in from the ceiling plenum. Heating/Morning Warm-up • Simultaneously, the amount of primary air is reduced to maintain • Upon receiving a heating signal constant airflow into the plenum. generated by the air source • The terminal fan operates whenever controller, (or a primary temperature the primary air source is operating. sensor is installed to detect that the air source is heating), the heating mode is automatically in effect and System Start-up the primary air damper is modulated • Points 1 to 4 from figure 18 indicates to maintain constant airflow into the that maximum cooling airflow is plenum. The fan operates at provided by primary air until the minimum speed.

30 Design Guide to Underfloor Air Distribution

UFAD Control Operating Sequences

Warmer 4 3 2 1 Maximum Primary Airflow 5 Maximum Fan Airflow

Resulting Plenum Total Supply Airflow to Plenum Pressure Cooler Plenum Temperature

Minimum Primary Airflow 4 Minimum Fan Airflow Fan Ventilation 5

Occupied Control Setpoint

(Configured Heating)

Figure 18 – Carrier UFAD Control Sequence – Equipment Cooling

31 Design Guide to Underfloor Air Distribution

Installation Costs • Controls costs should be similar to traditional systems when basic Many owners and developers have found zoning is unchanged. that raised floor systems with UFAD • Raised floor systems have the significantly reduce costs associated with potential to reduce slab-to-slab space reconfiguration. With the national heights by as much as 6 to 12 inches churn rate now approaching 50%, it is not per floor. This is particularly unexpected to find that many new office beneficial in mid-rise and high-rise buildings use raised flooring. Buildings markets with zoning code height using UFAD systems have certain cost limitations. components that make them more expensive • Testing and balancing savings can be than traditional systems and compensating realized from CAV pressurized ones that lower the overall price premium plenum designs, which are that currently exist for this application. essentially self-balancing. Recent job costs show that raised floor can represent a cost increase of from $3 to $5 Energy usage per square foot. A few of these cost components are worth mentioning: UFAD systems have the potential to save energy in comparison to traditional designs. • The raised floor system is the The bulk of these savings come from component with the single largest reduced fan energy associated with lower cost increase, which should be cost static pressures and overall CFM reductions justified based on the benefits to the created by the stratification layer partial entire service delivery system displacement ventilation effect. Documented (HVAC, power, voice and data) and energy costs savings remain difficult to not against the UFAD alone. predict because to date there is no energy • Local building and fire inspectors simulation software that accurately models may require sprinkler protection if UFAD system performance. plenum depth exceeds 18 inches and often require smoke detectors within the plenum. O&M Considerations • Generally, HVAC costs for central cooling and heating equipment and Office building UFAD systems are quite ductwork mains are no different for new, making it difficult to get actual owning UFAD systems. and maintenance cost data to compare them • Diffuser costs are dependent on the against traditional HVAC systems. type(s) chosen for the majority of air Operations and maintenance costs are delivered. primarily replacement costs for equipment, • If a relatively open underfloor and labor expenses associated with plenum can be used, significant maintaining the HVAC system and ductwork savings can be realized responding to occupant complaints. While compared to the typical overhead air many engineers believe that UFAD may distribution system. prove to be more costly to service, research suggests that the frequency of occupant

32 complaints will be reduced when they are Tests have shown an ideal airflow of given some individual control of their local 0.6 cfm/sqft for good mixing while environment. maintaining the stratification height at 6 feet and keeping the room RECOMMENDATIONS temperature gradient below 5º F. 6. Carefully run loads for both Q occupied and Q , and determine if load 1. Do not eliminate the suspended unoccupied credits can be taken; equipment size ceiling. Shallow up the ceiling reduction will benefit the job costs. plenum and use it as a return air 7. Select diffusers so that T throws plenum, gaining additional airflow 50 are around 4-5 feet so as not to benefits from assigning greater disturb the stratification zone. Use amounts of cooling load directly to CAV passive swirls in the interior the return air. and uniformly loaded large perimeter 2. Keep plenum depths below 18 inches spaces. to avoid potential requirement for 8. Hybrid designs, using conventional underfloor sprinklers. VAV in high load/variable load 3. Do not use too many zoning barriers situations, maximizes the benefits of in the plenum or you will restrict the the lower AHU supply air ease of future rezoning and running temperatures and keeps summertime of service utilities to various points humidity concerns under control. of use. 9. Placing returns over heat producing 4. Using multiple vertical drops avoids equipment and/or at the outside wall air highways, while meeting the 50- when high solar loads exist will 60 foot guideline for maximum maximize benefit of the partial travel to the farthest diffuser. displacement ventilation effect. 5. Do not include excess safety factors; 10. DDC controls should be considered airflows need to closely match load as a basis of design for realizing true requirements within the occupied UFAD benefits. zone so as not to over air the system.

33 Design Guide to Underfloor Air Distribution

References

Bauman, Fred S., 2003. “Underfloor Air Distribution (UFAD) Design Guide. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Building Owners and Managers Association (BOMA) International and ULI, the Urban Land Institute. 1999. “What do office tenants want”.

IFMA. 1997. Benchmark III International Facility Management Association, Houston Tex.

ASHRAE. 1990. ANSI/ASHRAE Standard 113-1990, Method of testing for room air distribution. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 1992 ANSI/ASHRAE Standard 55-1992, Thermal environmental conditions for human occupancy. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 2001b. ANSI/ASHRAE Standard 62-2001, Ventilation for acceptable indoor air quality. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 2001c. ANSI/ASHRAE Standard 90.1-2001, Energy standard for buildings except low-rise residential buildings. Atlanta: American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc.