Manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.New PC 201 Catalog No. 04-51450001-01 Printed in U.S.A. Form 45-1XA Pg 1 605 4-05 Replaces: NewBook 3 3

Tab 6a 7aBook 4 4 4

Tab 4AT1 4AT4 4AT5

Application DataTABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7Benefits of UFAD System . . . . . . . . . . . . . . . . . . . . . . . . 1• REDUCED LIFE CYCLE COST• IMPROVED COMFORTBasic Concepts of Ventilation . . . . . . . . . . . . . . . . . . . . 3• TRADITIONAL OVERHEAD AIR DISTRIBUTION• DISPLACEMENT VENTILATION• UNDERFLOOR AIR DISTRIBUTION• TASK AMBIENT CONDITIONINGUFAD System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3• BENEFITS OF A UFAD SYSTEM WITH ZONE

MIXING BOX• APPLICATION CONSIDERATIONS• UFAD SYSTEM CHARACTERISTICS• SUSTAINABILITY (LEED™ POINTS)PRODUCT OVERVIEW. . . . . . . . . . . . . . . . . . . . . . . . . . 7, 8Carrier UFAD System with Zone Mixing Box. . . . . . 7• FLOOR PLENUM• DIFFUSERS• UNDERFLOOR SERIES FAN-POWERED MIXING

BOX• UNDERFLOOR PERIMETER FAN COIL BOX• FAN-POWERED ZONE-MIXING BOX• CENTRAL EQUIPMENTDESIGN PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-17Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Zoning Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9General Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Building Load Considerations. . . . . . . . . . . . . . . . . . . 10• TWO REGIONS• LOAD ESTIMATING CONCEPTS AND FACTORS• LOAD ESTIMATING CONCERNSCentral Equipment Selection . . . . . . . . . . . . . . . . . . . . 11• FAN STATIC REQUIREMENTS• PRIMARY COOLING AIR QUANTITIES• COOLING COILS• ECONOMIZERSRoom Air Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 11• DIFFUSER SELECTION AND LAYOUT• DIFFUSER TYPES AND STLESZone Mixing Box Selection . . . . . . . . . . . . . . . . . . . . . . 13Distribution System Design . . . . . . . . . . . . . . . . . . . . . 14• PLENUM DESIGN CONSIDERATIONS• RETURN AIR DESIGN• DUCTWORK DESIGNHeating System Design . . . . . . . . . . . . . . . . . . . . . . . . . 15• CENTRAL SYSTEM VERSUS HYBRID SYSTEM

FOR PERIMETER ZONES• PERIMETER FAN-COILS WITH LINEAR BAR

GRILLES• HEATING INTERIORS WITH CENTRAL

EQUIPMENTControl Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15• CONTROL DESIGN CONCEPTS FOR UFAD

SYSTEMS• TYPICAL CONTROL SEQUENCES

Cost Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16• INSTALLATION COSTS• ENERGY USAGE• OPERATION AND MAINTENANCE COSTSSUMMARY OF RECOMMENDATIONS . . . . . . . . . . . . 18

INTRODUCTION

The concept of underfloor air distribution (UFAD) is notnew, however, recent developments in office space usage, sus-tainable design, and indoor air quality issues have sparked newinterest in this design concept.

Underfloor air conditioning had its start in computer rooms,at a time when mainframe computers generated considerableheat and required complex cabling. Access floor systemsallowed plenty of open space to run cabling and provided agenerous pathway to supply large quantities of cooling airbelow the intense heat generated by the electronics. The naturalconvection currents of warm air rising allowed cool air to enterlow, cool the equipment, and remove the warm air near theceiling. While today’s computer rooms no longer require thistype of cooling, the recent interest in access floor systems inother applications is based on the same needs that made theUFAD attractive for the old computer rooms: reduced life cyclecost and improved comfort.

Benefits of UFAD SystemREDUCED LIFE CYCLE COST — In the computer roomsof the past, the large open floor plenum was a convenient spaceto run large amounts of cable for power and communications.Today, the open floor plenum can be used to meet a new need.The greatest change in office requirements over the past15 years has been an ever-growing necessity for voice, data andpower connections to every individual workstation. The openfloor plenum makes it easier to provide these connections. Arecent trend in office design makes use of the open space floorplan to facilitate the relocation of workers into cross-functionalworkgroups. The IFMA (International Facility ManagementAssociation) reported in one of its surveys that, on average, anoffice worker experiences a move every 51/2 months; thismeans that 44% of the workers move within a year. The termoften used for these types of office moves is churn.

The costs associated with these churn moves are some ofthe highest costs an owner or tenant may face, largely becauseof the need for cabling moves and changes to the HVACsystems. A UFAD system could greatly reduce the costsassociated with churn moves and help lower total building lifecycle cost.

AXIS™45XA

Access Floor Terminal Units

2

IMPROVED COMFORT — The UFAD application in com-puter rooms was based on the principle that warm air rises. Inthe computer room, the warm air was generated by hot electri-cal equipment, but the same principle may be applied tothe warm air surrounding people. The Building Owners andManagers Association (BOMA) reported in its “What TenantsWant Survey” that thermal and indoor air quality concerns aretwo of the top concerns and least met expectations of tenants.Traditional overhead air distribution systems are designed to doa good job of mixing the air in the space and preventing stratifi-cation, but they cannot deliver thermal comfort to everyoccupant and provide personal ventilation without becomingcost prohibitive. Overhead systems may require more fanenergy to overcome the static losses that would result from themixing airflow patterns required within the space.

Delivery of ventilation air requires sufficient mixing toassure that the ventilation component is delivered to the occu-pied breathing region. By applying the principle that warm airrises, air can be provided below the occupants and dischargeddirectly into the breathing region at relatively low velocity. Aspeople warm the air, the air will rise toward the ceiling bynatural convection. People only breathe the air in the regionthat extends from the floor to approximately 6 feet above.

Therefore, the space above this region can be treated as a strati-fied air layer and the load components in this region can betreated differently. The result is that air provided underfloor canbe supplied at low pressure and the energy used for space con-ditioning can be reduced. The ventilation air can be provided inthe region where it is needed most, with pollutants moved gen-tly toward the return. Additionally, adjustable diffusers, whichdischarge air to small areas in the space, may be provided asadjustable by the occupant, which would create a perceivedimprovement in thermal comfort. The end result is a systemthat is more energy efficient, comfortable, and provides betterventilation and improved indoor air quality.

The application of a UFAD system requires the systemdesigner to view the project from a different perspective thanother design projects. The UFAD design process requiresspecial consideration from the schematic stage, through loadestimating, to commissioning. This document is intended toprovide the designer with some basic guidelines. Furtherin-depth analysis of all aspects of UFAD design can be foundin the ASHRAE UFAD Design Guide.

A typical UFAD system is shown in Fig. 1.

RETURN AIR AT CEILING

STRUCTURAL SLAB

UNDERFLOOR PLENUM

RAISED FLOOR

SWIRL DIFFUSERS

PRESSURIZED PLENUM AIR PERIMETER GRILLES

PERIMETER FAN COIL

Fig. 1 — Typical Underfloor Air Distribution System

3

Basic Concepts of VentilationTRADITIONAL OVERHEAD AIR DISTRIBUTION — Atraditional overhead air distribution system is shown in Fig. 2.Traditional ducted systems with ceiling level supply haverelatively high discharge velocities to achieve mixing in theconditioned space. The intention is to fully mix the supplyair with existing air in the space. This results in pollutantsbeing recirculated throughout the space. Ideally, this mixingwill dilute pollutants in the entire volume of the space withventilation air, so that the entire room is a uniform mixtureof old space air and ventilation air, with adequate ventilationeffectiveness. Ventilation effectiveness is the ability of a sys-tem to deliver fresh ventilation air into the breathing zone ofthe occupants. In order to achieve ventilation effectiveness, atraditional system often requires that excess ventilation air bedelivered to the space. In addition, heating from overhead andthe location of returns often results in less than the idealmixing.

Supply air velocity and ventilation effectiveness are two ofthe main design features that distinguish underfloor air distri-bution from a conventional overhead ducted system, asdescribed below.

There are three concepts associated with providing floorlevel cooling, each with distinctive characteristics.DISPLACEMENT VENTILATION — A displacement ven-tilation system discharges air horizontally near the floor at verylow velocities and near laminar flow conditions. The goal is touse only the buoyancy effects to create air motion within thespace and to maintain the stratification layer above thecontrolled region that is not mixed. While this provides excel-lent ventilation effectiveness in the occupied region, it is notthe optimum solution for thermal comfort. The temperaturevariance from head to toe is much greater than the 3 to 5 degreedifference that is accepted as the standard for thermal comfort.UNDERFLOOR AIR DISTRIBUTION — A traditional UFADsystem uses the floor space between the structural floor and theaccess floor as a supply air plenum. A UFAD system uses thesame buoyancy principles as displacement; however, the air isdischarged into the space with adequate velocity and air patternrequired to provide mixing only within the occupied region. Inthis system, as well as in the displacement ventilation system, thegoal is to maintain the stagnant region above the breathingregion. The UFAD system provides a more uniform temperaturein the occupied region to stay within the comfort envelope.

TASK AMBIENT CONDITIONING — This system pro-vides airflow from underfloor that is controlled at the worksta-tion. The control is normally accomplished with a small fanthat directs an air jet at the occupant within the workstation.

UFAD System Overview — The use of the underfloorplenum in the UFAD system has two major impacts on systemdesign.

First, since the air is brought in at floor level, a chance existsfor a cold floor or for cold discharge temperatures near theoccupant’s feet. Either condition would be objectionable, so theair must be provided at a much warmer temperature thannormal air conditioning levels. The accepted level of supply airin a UFAD system is between 61 F to 65 F. This requirementaffects equipment selection and operation and also impacts airdistribution devices. The second issue is that the large surfacearea and mass of the structural slab result in a thermal mass thatshifts load patterns and creates significant amounts of thermaldecay to diffusers located far from the point at which supply airenters the plenum.

In a UFAD system, the plenum may be configured usingone of two approaches. Under the first method, air isdischarged into the plenum at zero or neutral pressure and fansnear the outlets draw the supply air from the plenum anddischarge it to the space (Fig. 3). Neutral pressure plenumsmay have less leakage around floor tiles and plenum penetra-tions, but fan wiring and energy costs for this system negatesome of the UFAD system’s inherent benefits. Using thesecond method, the entire underfloor plenum is pressurized to arelatively low pressure of approximately 0.10 in. wg.

In the UFAD system with a pressurized plenum, condi-tioned air is handled using one of two methods. The firstmethod (Fig. 4) removes the space return air near the ceilingand the entire volume is returned to the air-handler. Thismethod requires special treatment by the air-handler to bypassthe required air and condition the required supply amountwithout losing control of temperature and particularly humidityin the space. The Carrier system (Fig. 5) uses a mixing boxlocated near the space to draw a portion of the return air nearthe ceiling and then mix it with the supply air to maintain thefloor plenum pressure and temperature.

Fig. 2 — Overhead Air Distribution System

4

→ Fig. 3 — UFAD System with Zero Pressure Plenum

→ Fig. 4 — UFAD System with Pressurized Plenum and Full Return

→ Fig. 5 — UFAD System with Pressurized Plenum and Zone Mixing Box

605

5

Perimeter zones and spaces such as conference roomspresent some interesting challenges for UFAD systems. Thesespaces have loads that vary greatly with time and that mayhave air requirements that cannot be handled by 65° F air.These spot cooling situations can be handled in two ways. Afan-powered box can be provided under the floor to increaseairflow, or a separate unit can be provided that can use standard55° F air. A fan coil unit located under the floor can also beused to meet cooling and heating needs. When 55° F air issupplied to zone mixing units, the system may also employtraditional methods used with overhead air distribution, to meethigh load requirements. The Carrier approach uses this hybridsystem (Fig. 6).

Heating along the perimeter also presents challenges forUFAD systems. Terminal units are frequently added to providethe necessary heat. One very common method uses a fan coilunit that contains an electric or hot water heating coil. The fancoil unit would be placed under the floor near the perimeter andwould be controlled to overcome transmission losses.

BENEFITS OF UFAD SYSTEM WITH ZONE MIXINGBOX — The Carrier approach employs standard air-handlingequipment and uses duct supply temperatures in the normal airconditioning range, which can best handle the spot coolingrequirements that arise on every project. The air is distributedin the space through a pressurized plenum with swirl stylefloor grilles. The plenum is supplied by a zone parallel fan-powered box with a special DDC (direct digital control) thatmodulates temperature and controls plenum pressure. Theprimary damper provides the minimum ventilation air to thezone and maintains plenum pressure. The ECM (electronicallycommutated motor) style fan motor in the box modulates tomix return air and primary air to maintain the required temper-ature. Heating and cooling requirements for specialized highload spaces and perimeter zones are handled with conventionaloverhead systems, underfloor series fan boxes or fan coil units.

APPLICATION CONSIDERATIONSAccess Flooring Systems — An access floor is a flooring sys-tem that is installed above the standard structural floor of thebuilding. This book describes the Haworth Access FlooringSystem (see Fig 7). This flooring system consists of a matrix of

adjustable pedestals, mounted on two-foot centers, that supportthe weight of the access floor panels and the loads on the floorabove. The concrete floor panels (two-by-two, approximately11/8 in. thick) are laid into this grid system. The structural floorbelow is normally sealed to provide a moisture and dust barrier.The primary use of the flooring system is to provide a serviceand utility space to run cabling for voice, data and power. As aresult, the floors can be as little as 4 in. over the structural flooror they may be much higher, depending on the need. When thefloor space is to be used as a supply plenum for air distribution,floor heights are normally at 12 to 18 inches.

Access floor systems are successfully applied to many typesof buildings in both new construction and renovation. Howev-er, when these floor systems are applied to retrofit situations, afew special considerations are required. Elevator lobbies andbathrooms that are built on the original structural floor must beraised to the new floor elevation or ramps must be provided tomove from the structural floor heights to the raised floorheight.Types of Buildings — Underfloor air systems are not an an-swer for every building, but may represent significant life cyclecost savings when applied to the appropriate buildings.

Fig. 7 — Access Flooring System

→ Fig. 6 — Carrier UFAD Design Solution

605

6

New Construction — In new construction, one architecturalbenefit to be considered is that ceiling space could be reduced.Since large supply ducts are not required, the ceiling space canbe reduced or open ceiling space can be used for lighting andsprinklers, allowing a floor-to-floor height one foot less thanstandard. Then for every 12 stories of building, an additionalstory could be added with minimal additional cost in thebuilding shell cost. In buildings of fewer than 12 stories, thereduction in height would result in a reduction of overallbuilding shell cost.

In new construction spaces, the best applications are officeswith open plan construction and buildings in which frequentoffice changes are anticipated. Owners can benefit from thereduced costs of moving offices and the required communica-tions and wiring. UFAD systems may also be used successfullyin other types of space with mostly internal loads and open planconstruction, as found in some schools.

In general, small spaces within a building are not well suitedto underfloor air systems. Nor does a UFAD system make goodsense in a building that has a very high ratio of perimeter tointerior space, or buildings with many spaces that have highlyvariable loads.Retrofit Buildings — A UFAD system is a good choice in aretrofit situation where the building characteristics makeoverhead air distribution systems difficult to apply. Recenttrends in downtown revitalization have led to the rehabilitationof old warehouses and other existing structures into offices.There may be costs associated with the relocation of elevatorsand plumbing connections, however, the savings and ease ofproviding the cabling services and air distribution under thefloor may well offset those costs. In addition, the benefits ofimproved thermal comfort and indoor air quality may make thespace even more attractive to tenants.Special Applications — Another good fit for a UFAD systemis in a building with very high ceilings and no place to runoverhead air distribution. Some places of assembly (meetingrooms, churches, auditoriums) fit into this category. Again,open plan and internal load concentration are the factors com-mon to successful applications.UFAD SYSTEM CHARACTERISTICS — A UFAD systemoffers several benefits over traditional systems.Thermal Comfort — A UFAD system provides a higher levelof perceived comfort by allowing the occupant to adjust adiffuser and achieve a degree of climate control. The systemalso offers an increase in actual thermal comfort, since air dis-tribution diffusers provide good head-to-toe temperature varia-tions without disrupting the benefits of a stratification region.Ventilation Effectiveness — Fresh air supply is introduced tothe space low. As the air warms, the warm air rises and carriespollutants toward the ceiling (see Fig. 8). Rather than mixingthe air within the entire space, fresh air is provided to thebreathing region. In ASHRAE (American Society of Heating,Refrigeration, and Air Conditioning Engineers) Standard 62,Table 6.2, ventilation efficiency of UFAD systems is listedat 100%. This ventilation method may actually result inefficiencies greater than 100%, although this has not yet beendocumented. In addition, the perceived indoor air quality isbetter. Although it is not recommended, total ventilationairflow potentially could be reduced because of the greaterventilation effectiveness of the system.Energy Usage — The lower fan static requirements of UFADsystems may provide significant energy savings whencompared to overhead air systems. Economizer hours of opera-tion may increase energy savings as well, depending on thelocal climate.

Reduced Life Cycle Costs — Based on the reduction in costsassociated with office churn, UFAD systems may have bettertotal life cycle costs. In addition, the energy efficient designhelps to reduce the life cycle costs.Reduced Floor-to-Floor Height — The ability to reduce thefloor-to-floor height by up to one foot per floor can increaseavailable floor space or reduce shell construction cost. Thereduction in floor-to-floor height is based on reducing or elimi-nating ceiling plenums. Therefore, the need for ceilings to hidelight fixtures or piping for sprinklers or to help improve roomacoustics must be evaluated.Improved Productivity and Health — Recent studies havedrawn a positive correlation between productivity and good in-door air quality and comfort.

There are some characteristics of UFAD systems that mustbe considered when judging the suitability of the technologyfor a particular application.Cold Floor — One of the most sensitive points of thermalcomfort on the body is the ankle. Control of floor temperaturecan be critical in order to avoid complaints resulting from theradiant effects of a cold floor. When considering floor diffuserplacement, sensitivity to cold drafts must also be addressed.Code Issues — Although the concept is not new, UFAD tech-nology as it is applied today is relatively new. Building codeofficials may not know how to interpret the codes as appliedto the technology and may impose unexpected requirementsfor life safety components such as fire resistant materials,sprinklers, and smoke detectors.Condensation — The thermal mass effects of the plenum floorslabs can have a positive effect on sensible load swings, butcare must be exercised when using control schemes thatpre-cool the slab and then allow large quantities of potentiallymoist outdoor air to enter the underfloor plenum. Air suppliedto the underfloor plenum must be kept above the dew pointtemperature of the floor slab.Humidity Control — The impact of local climate and partload control methodologies must be carefully evaluated.Higher supply-air temperatures have the potential to counteractthe capability of the system to remove latent heat. The designanalysis should evaluate the effects of part load latent controland control methodology to be sure that room relative humidityis kept below 60%.

Fig. 8 — Thermal Plume Effect

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SUSTAINABILITY (LEED™ POINTS) — There is a na-tionwide trend toward constructing buildings that are moreenvironmentally friendly to meet green building or sustainabledesign construction standards. The best-known green buildingstandard is the LEED-NC Rating System® issued by the U.S.Green Building council (LEED stands for Leadership in Ener-gy and Environmental Design). In addition, two new standardshave been developed that will have an impact on HVACdesign: LEED-CI Rating System® (for corporate interiors) andLEED-EB Rating System® (for existing buildings). TheLEED (Leadership in Energy and Environmental Design) vol-untary rating system provides standards that rate the buildingdesign in 5 categories: Indoor Environmental Quality, Energyand Atmosphere, Materials and Resources, Sustainable Site,and Water Efficiency.

This standard sets green building objectives in the 5 catego-ries listed above for which a design can accumulate up to69 points. The rating system is based on the number of pointsawarded and goes from a certified level at 29 to platinum levelat 52. Some prerequisites must be met for certification, but be-yond that, the points may come from any of the 5 categories. Ofthe total points possible, HVAC related systems account for40%. The points are based on the entire building as a system,and it is the entire building that is certified, not a particular prod-uct or system. Underfloor air systems have the potential to earnpoints in at least seven areas, primarily in the Indoor Environ-mental Quality and Energy and Atmosphere categories.

The largest points area in the program is for optimizingenergy performance in the category of Energy andAtmosphere. This credit varies from 1 point for exceeding theASHRAE 90.1 energy cost budget (ECB) model by 15% to upto 10 points for a 60% reduction. In most UFAD projects theanalysis would involve using the ASHRAE energy cost budgetto compare the underfloor system to the energy costs of a VAV(variable air volume) reheat system.

The UFAD systems may have a distinct advantage in thisarea since they incorporate many potential energy savingsfeatures. Lower fan static requirements as the result of lowplenum pressure distribution will reduce required fan energy.Mechanical cooling requirements and hence equipment sizeand part load efficiency may be improved due to the use ofhigher discharge temperatures. Economizer usage may beextended to get more free cooling hours and help reduce partload energy requirements. In general, UFAD systems have thepotential to show significant energy savings over the requiredbase system used in the energy budget model.

The UFAD system may also qualify for points under theMaterial and Resources category. Within this category 3 pointscan be earned for building reuse. If the project involves an exist-ing building, the percentage of building shell reuse would helpobtain these credits. UFAD systems lend themselves very well tocertain retrofit situations where the building is being renovatedto meet the requirements of new technology without destroyingthe current building structure. UFAD gives the architect a rangeof options to achieve these goals. Also, depending on the loca-tion, it is possible that the floor system used would qualify forthe regional credit that requires that the project be within 200 or500 miles of the equipment manufacturing location.

In the Indoor Environmental Quality category, UFADsystems have the potential to qualify for points in 3 areas. Thiscategory is the one in which many of the existing certifiedprojects have relied on points from UFAD systems to achievecertification. The first set of points associated with this catego-ry is in the area of ventilation effectiveness. As noted earlier,UFAD systems are not rated by ASHRAE as having anyadvantage in ventilation effectiveness. However, in the currentLEED documentation, the UFAD system is implied to haveventilation effectiveness greater than one. As a result, the useof UFAD systems may be a qualifying point. The second set ofpoints addresses system controllability, where the LEED

requirement is for interior zones to provide individual controlto at least 50% of the occupants. Since UFAD diffusers provideindividual control inherently, the system again is nearly anautomatic qualification for this point. The third set of pointsrelates to thermal comfort. Since UFAD systems have beenshown to provide a high degree of personal comfort with spacecontrol parameters that fall within the guidelines of theASHRAE standard 55 comfort envelope, the system canqualify for this credit as well.

The UFAD system has proven to be an excellent system toconsider on projects seeking LEED certification. Remember,however, that certification is based on the total building systemand the points and overall rating for a specific project are de-pendent on the design of that project.

PRODUCT OVERVIEWCarrier UFAD System with Zone Mixing BoxFLOOR PLENUM — The space between the access floor andthe structural floor is the floor plenum. Using this space as anair distribution plenum requires some special considerations.First, the floor panel seams must provide a tight seal to preventair leakage from the plenum into the space. In some casesductwork is run through the space to special variable loadareas. Also, the space may need partitioning for life safety orthermal zoning. A typical underfloor plenum is between 12 and18 inches in height.DIFFUSERS — Room air distribution is accomplished throughone of two types of floor diffusers. Interior spaces are controlledthrough a system of adjustable passive floor mounted diffusers.The Carrier 35BF-R diffuser consists of an adjustable swirlplate located in a mounting basket (Fig. 9). The diffuser ismounted through a hole cut in the concrete floor panel. Thebasket catches any dirt or spilled liquids and prevents contami-nation of the space below the floor. The adjustable swirl plateallows the occupants to adjust their diffusers to a comfortablelevel in their space. The diffusers are designed to achievemixing of the approximately 65 F degree supply air within avery tight radius of the diffuser and at very low velocity. Typicaldiffuser airflow is about 60 to 100 cfm per diffuser.

A second type of diffuser is used with underfloor series fan-powered mixing boxes and perimeter fan coil units. TheCarrier 35BF-D rectangular grille is used to provide an air cur-tain that washes exterior surfaces with warm or cold air(Fig. 10). These grilles depend on much higher velocities andcan be used in various lengths to match the airflow require-ments of the mixing box or fan coil unit. A deflection blade ofeither 0 or 15 degrees can be used to provide the required throwpattern. A version of the rectangular grille can also be installedin a plenum that has a control damper to provide variable vol-ume control to selected spaces. The rectangular grilles can alsobe used to allow return air from above the floor back down tothe underfloor mixing boxes if desired. If used directly in thepressurized plenum, a standard balancing damper can be addedto adjust airflow.

Fig. 9 — Swirl Plate Style Diffuser — 35BF-R

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UNDERFLOOR SERIES FAN-POWERED MIXING BOX(45UC) — The 45UC unit is designed to fit underfloor in thespace between pedestals and the grids of the access flooringsystem (Fig. 11). Sizes range from 280 cfm to 1200 cfm. Theseunits are used for perimeter spaces, conference rooms andother spaces with a highly variable load pattern. Typically,these fan-powered boxes use supply air from the floor plenumand increase the volume delivered to the room to meet thevariable needs. The same style box can be used when ductedprimary air is required to meet high thermal load requirements.Heating can be provided with either hot water coils or electricheaters.UNDERFLOOR PERIMETER FAN COIL BOX (42KC) —The 42KC fan coil box can be used for spaces with variableload patterns and are well suited to addressing heating require-ments of perimeter zones (Fig. 12). Available in sizes from 325to 2800 cfm, these fan coil units also fit underfloor in the spacebetween pedestals and the grids of the access flooring system.Heating can be provided with either hot water or electric heatcoils.

FAN-POWERED ZONE MIXING BOX (45XC) — The 45XCis a parallel fan powered box with a standard Carrier DDC con-troller (Fig. 13). One box is used for each zone and is mounted ina closet or utility space to control the airflow to the underfloorplenum. The controller is configured for UFAD to control thetemperature and pressure of the zone. The box uses standard55 F supply air from the air handler and blends the air with returnair from the zone to provide the 63 F discharge air required in theunderfloor plenum. The ECM motor in the box varies fan speedas required to control plenum temperature by varying the amountof recirculated air introduced into the plenum, while the primaryair damper simultaneously maintains the plenum pressure bycontrolling the amount of primary air introduced into theplenum. Minimum damper position stops assure that ventilationair is always provided to the underfloor plenum. Plenum pres-sure is maintained at approximately 0.1 in. wg.CENTRAL EQUIPMENT — Since Carrier’s UFAD systemuses a zone mixing concept and air is supplied to the zone mix-ing box at normal unit design conditions, special air-handlingequipment is not required. Mixing boxes may be used with airhandlers in chilled water or direct expansion type systems orwith many types of packaged VAV equipment as well. Carefulpsychrometric analysis should be made of coil entering condi-tions to be sure the coil will meet the specific leaving airconditions for entering sensible heat factors that may be higherthan normal. Part load operating conditions in terms of coilcontrol or staging should be evaluated to be sure room humiditylevels can be maintained below 60% relative humidity. Econo-mizers should be used with all system types where local climatepermits and should use an integrated differential enthalpy typecontrol. Because of low static pressure requirements, fan sizesmay be lower than conventional overhead systems. Due tovarying air volumes to the zones, the use of variable frequencydrive (VFD) is recommended; the VFD will also improve sys-tem efficiency. The use of high-quality filtration is recommend-ed since a prime consideration of the UFAD system is goodindoor air quality.

Fig. 11 — Underfloor Series Fan-PoweredMixing Box — 45UC

Fig. 12 — Underfloor Perimeter Fan Coils — 42KC

Fig. 10 — Rectangular Grillewith Plenum — 35BF-D

Fig. 13 — Fan-Powered Zone Mixing Box — 45XC

9

DESIGN PROCESS

Design Considerations — When designing a UFADsystem, it is important to keep in mind certain differencesbetween the UFAD system and a traditional overhead mixingsystem. Following are key concepts to be considered to avoidincorporating elements of a traditional system that are notdesirable in a UFAD system.• Underfloor systems work best when a stratification layer

is established at 6 feet above the access floor. Thiscreates a partial displacement ventilation effect withinthe room and improves ventilation effectiveness, con-taminant control, heat capture, and reduces loads withinthe space, transferring them directly to the return air.

• Airflows must be closely matched to the loads within theoccupied region so that design requirements do not “overair” the space, which would disrupt the partial displace-ment ventilation effect.

• Mixing occurs only within the occupied region andoccurs so efficiently that a 3º to 5º F temperature gradientis typical, meeting ASHRAE 55 comfort limit.

• Supply airflow should not include excessive safety fac-tors; this will lead to “over airing” the system, resultingin many occupant discomfort complaints and the loss ofthe stratification layer.

Zoning Decisions — The underfloor air supply plenumused in a UFAD system requires that the following factors beconsidered when making zoning decisions.• Thermal decay refers to the increase in cooling supply air

temperature resulting from heat transfer to the floor slaband access flooring. Zone maximum travel distance ismeasured from the terminal discharge into the plenum tothe farthest diffuser. Thermal decay will limit the zonemaximum travel distance to 50 to 60 ft. This means thatfrom a typical core location, the maximum zone sizewould be 120 ft by 60 ft, or 7200 sq ft or less.

• Open area zones that have loading differences are hard to“isolate” from one another without the use of plenum bar-riers beneath the access floor. However, too many barrierswill limit accessibility and reduce the ease with whichrezoning and utility changes may be made in the future.

• Zones with high heating loads, such as exterior wallswith glazing, may benefit from the use of ducted linearbar diffusers that deliver the conditioned air to the loadpoint without significant thermal decay. Ductworkshould be insulated to prevent loss of cooling capacity.

• Perimeter zone size may be restricted by ASHRAE 90.1requirements for exposure zoning and limits of 50 ft oflinear wall per exposure.

• Zone-mixing terminals should be located in verticalequipment closets. This approach will occupy someuseable floor space, but is a better choice than locatingthe units within the plenum. Locating the terminals out-side of the plenum makes maintenance easier and mini-mizes obstructions within the plenum.

• Zones that include conference rooms and rooms withplants, process or large equipment or solar loads mayhave high cooling or humidity loads. The designershould condition these spaces with traditional VAV ter-minals using lower temperature primary air (50 to 55 F).The Carrier hybrid system design will require smaller airhandling equipment and ductwork.

General Layout — In terms of general layout, a UFADsystem may be thought of as an inverted conventional over-head system. A UFAD system is similar to a traditional systemin the following ways:• Central cooling and heating equipment, as well as duct-

work mains and branches to zone mixing terminals, canbe nearly identical to traditional overhead systems.

• Zoning occurs in much the same way as with othersystems, but because of the duct mains above and zoneddistribution below, multiple vertical duct drops andequipment chases must be strategically located aroundthe floor plate.

The following layout considerations are unique to the UFADsystem design:• When using zone fan powered mixing boxes above the

access floor, the chases must provide adequate room tolocate the terminal as well as provide required serviceclearances (Fig. 14).

• Diffusers are located in the access floor, much closer tothe occupants, so special care must be used in theirlayout. Since the UFAD system is a non-ducted system,relocation of diffusers can be accomplished simply byswapping floor panels.Since UFAD systems work best with uniformly loaded ther-

mal areas, high load zones and variable load zones should bemanaged with traditional systems, creating an overall hybridsystem that utilizes the best features of both types of systems(Fig. 15.)

Fig. 14 — UFAD with Zone Mixing Box

10

Building Load ConsiderationsTWO REGIONS — The UFAD system depends on establish-ing two regions: The occupied region and the stratificationregion (Fig. 16). Tests have shown that loads that occur in theupper stratified layer do not affect the occupied region, how-ever, there is no computerized load-estimating program thatcan model this condition. The designer must take into accountthese two regions when assigning loads. Using too many safetyfactors in the occupied region or being overly conservative inassigning loads may prevent the system from achieving thebenefit of a reduction in airflow requirements for the space.LOAD ESTIMATING CONCEPTS AND FACTORS — Thefollowing items address those areas that require either manualoutput adjustment or computer program input modifications toallow designers to more closely model UFAD systems andcorrectly assign loads and set zone airflows.• For stratification to benefit cooling loads, floor diffusers

should mix the air within a tight radius and the throwshould not exceed 4 to 5 feet, so that that it will not affectthe height of the stratification layer, which typicallystarts at 6 feet.

• Lighting energy is predominantly radiant energy, so only1/3 of total watts should be assigned directly to the upper,unoccupied region. When return air is taken through thelight troffers, a greater portion of the total watts used (upto 50%) can be assigned to the unoccupied region.

• Exterior wall conductive load is still transferred into thezone as predominantly radiant (often over 60%), so only1/3 of the unoccupied wall area load should be assignedto the unoccupied region.

NOTE: Lowering the zone ceiling height to the midpoint valueof the unoccupied region would allow most computerized loadprograms to automatically assign the loads addressed above tothe return air.• Transmitted solar loading comes into the space as 100%

radiant, unless inside shading is used. Since manuallyadjusted shades cannot be counted on when sizing equip-ment or setting airflows, do not assign any transmittedsolar load to the unoccupied region.

• Absorbed solar loading becomes over 1/3 convective, soan appreciable portion of this amount can be manuallyassigned to the upper stratified layer, especially ifwall-washing linear bar supply registers and overheadperimeter return are used in the room air distributiondesign.

• People load should always be assigned 100% to theoccupied region.

• Equipment loading should not be assigned to the unoccu-pied region unless specific return register placement isused to capture a portion of that load with the return air,or it is believed that aggressive thermal plumes willdevelop and carry a substantial portion of the loadthrough the stratification layer.

• Infiltration loads always remain within the occupiedregion and ventilation loads always remain a part of thecentral equipment coil loading.

Fig. 15 — Carrier UFAD Design Solution

Fig. 16 — UFAD Design Load Concepts

11

When the loads have been run and certain manual adjust-ments made, the designer should be able to define two regions:

QOCCUPIED for the lower mixed region, and QUNOCCUPIED forthe region above, which is the stratification layer.

With these loads the designer can then calculate the follow-ing required design parameters:

1. Solve for CFM required in the occupied region. (This isthe air supply that must be supplied by the underfloorsupply diffusers and zone mixing box total air flow.)CFMRoom = QOccupied/(1.1(TRoom –TPlenum–Supply)) Equation 1TRoom is the thermostat set point for the spaceTPlenum-Supply is assumed discharge air temperature in theplenum

2. Solve for CFM primary air required. (This is the airsupply from the air handler and is used in zone boxselection.)CFMzone–Box = QOccupied/(1.1(TRoom –TAHU–Supply)) Equation 2TRoom is the thermostat set point for the spaceTAHU-Supply is assumed primary air temperature into thezone-mixing box

3. Solve for TRETURN in the stratified, or unoccupied region.(This is the return air temperature to the zone mixing unitand the AHU before outdoor air mixing occurs.)TReturn = QUnoccupied/(1.1xCFMRoom) + TRoom Equation 3TRoom is the thermostat set point for the space

LOAD ESTIMATING CONCERNSAirflow — Be careful not to “over air” the space. Whenworking with Equation 1, the space set point temperature,TRoom, can be increased to be closer to the temperature at thestratification layer; this will help to more accurately determinethe actual minimum airflow required to meet the occupiedregion load. Similarly, in Equation 3, TRoom can be adjusted tothe higher value at the stratification layer, remembering thatwith UFAD systems and the partial displacement effect, returnair temperatures are quite a bit higher than the zone set pointtemperature.

Floor panel temperature will be midway between room setpoint and plenum temperature, creating both a radiant coolingeffect and convective heat transfer into the plenum, loweringthe required room airflow.Leakage — Leakage is another concern that should be ad-dressed during the design process. Following are leakagesources that should be considered:• Supply air leakage out of the pressurized plenum into

unconditioned spaces through construction gaps• Supply air leakage between the floor tiles into the occu-

pied region• Radiant and convective cooling by the lower-tempera-

ture floor panelsLeakage into the occupied region and radiant and convec-

tive cooling from floor panels may result in overcooling theconditioned space.Core vs Perimeter — UFAD systems work best with coreareas, open perimeters with limited glass, and similar spacesthat have low load variations. These zones tend to be larger andconstant volume designs will provide the most cost effectiveinstallations with superior IAQ.

When load concentrations increase and/or high loadvariability is present, the requirements for airflow delivery tothe space can increase significantly, since UFAD systems aretypicaly designed to provide supply air at a higher than usualtemperature (65 F) in order to provide a more uniform temper-ature for the occupants. Adding variable volume equipmentwould increase system cost. Therefore, perimeter offices with

high glazing areas, conference rooms, and areas with largeequipment loads are examples of zones where use of dedicatedtraditional systems make the most sense. The resulting hybriddesign makes the most of both traditional mixing ventilationsystems with lower cooling supply air temperatures and UFADbenefits of the base design.

Central Equipment Selection — While a UFAD sys-tem is thought of primarily as a variation in air distributiondesign, there are many particulars that may affect centralequipment selection:FAN STATIC REQUIREMENTS — Central fan energy shouldbe less than required in a traditional system because of the lowplenum pressure and reduction in ductwork downstream fromthe air terminal. Reduced external static pressure on the centralequipment is an advantage of a UFAD system. Supply ductworkruns and air pressure drops on the zone terminals leading tothe plenums are little different than conventional systems. Thesame is the case with return ducts, but there is always lessthan 0.10 in. wg pressure requirement on the plenum anddiffusers downstream from the zone terminals. While thissavings of 0.25 to 0.50 in. wg appears small, when added to theoverall reduction in CFM associated with UFAD’s partialdisplacement effect, the result is a significant reduction in fanmotor horsepower.PRIMARY COOLING AIR QUANTITIES — These quan-tities can be reduced when potential load credits are realizedfrom the stratification layer partial displacement ventilationeffect.COOLING COILS — Temperature and humidity conditionsentering the cooling coil should be checked to be sure that thecooling coil selected will be able to achieve the required ADPand bypass factor. In some cases, this may require the use oflarger coils, more coil rows, or lower face velocities.

Part load loss of humidity control can be a significant issue.A part load analysis should be done to determine if thecoil control methodology being used or the refrigerationstaging will provide sufficient latent heat removal at part loadoperation.ECONOMIZERS — Air or water economizers should beused whenever possible for the local climate. The use of anintegrated differential enthalpy type is recommend. Use care insetting up the economizer changeover temperature to be surethat the 60% maximum space dew point will not be exceeded.At 75 F room design, this would be an upper limit of 59 F dewpoint temperature.

Room Air DistributionDIFFUSER SELECTION AND LAYOUT — Since the sup-ply air is introduced into the occupied region, it must bewarmer than traditional systems. The temperature of the supplyair should be approximately 61 to 65 F, or only 8º to 13º Fbelow the usual room cooling set point. In addition, since theoccupant is much closer to the diffuser in a UFAD system, thediffuser must be a high induction design to quickly even out thesurrounding space temperature and avoid drafts and occupantcomplaints. The air must mix quickly into the room and notproject outside the occupied region so that the stratificationlayer remains intact. Tests have shown an ideal airflow of0.6 cfm/sf for good mixing while maintaining the stratificationheight at 6 feet and keeping the room temperature gradientbelow 5º F (see Fig. 17).

Assuming similar zone loads, the higher supply air tempera-ture of a UFAD system means that larger quantities of air areneeded, resulting in greater numbers of diffusers located in theaccess floor. Since the diffusers have low pressure drops andplenum pressure is very uniform, UFAD designs can beconsidered self-balancing at the diffuser.

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DIFFUSER TYPES AND STYLES — The Carrier UFADsystem uses passive type diffusers that are designed for use inpressurized plenum systems. The two styles, described in theProduct Overview section, page 7 are the swirl diffuser(35BF-R) and the floor grille (35BF-D). Each passive style isable to move required air quantities at low plenum pressures(0.05 to 0.10 in. wg) and has very low noise generation values,usually under NC 20. In fact, a white noise generation systemwithin the plenum is often recommended for proper acousticprivacy in open office situations.

Table 1 summarizes the application recommendations forpassive diffusers. Passive type diffusers have an advantageover active type diffusers in that passive diffusers can berearranged easily by just moving floor panels; active typesrequire moving fan units, power and control wiring and possi-bly ductwork. The UFAD diffusers should not be used whereliquid spillage potential is high, even though they have a catchbasin for incidental spills, and they should not be placed intraffic areas where high rolling loads are expected.

To select a passive diffuser, determine the required roomCFM with Equation 1 above and divide it by the tabulatedairflow for the diffuser at the pressure maintained in theplenum. This will be the approximate number of diffusersrequired. Adjustments may be made to be sure all spacesreceive adequate coverage.# Diffusers=CFMRoom/(Diffuser_CFMat_Plenum_Pressure) Equation 4

Table 1 — Application of Passive Diffusers

UFAD Systems will create a high degree of personalcomfort when designed with floor diffusers that provide awell-mixed region in the lower portion of the room. Warmreturn air travels upward through natural convection and iscarried back to the HVAC system. If the diffusers are notdesigned properly, the distance the air travels from the diffuserinto the space could be excessive, causing stratified air to circu-late back into the occupied space. Improper system designcould result in poor temperature control and the inability tomanage building loads.

The 35BF-R Swirl diffuser throw is a function of bothairflow and ∆T. Quite often, space loads are over estimated.The difference in estimated and actual space load results from acombination of factors. The first factor is that the listed ampdraw of equipment located in the space (such as computers andcopy machines) is used to estimate the load. This estimate mayvary from the actual heat generated by this equipment whenused. The second factor is variable occupancy. Early UFADsystems were designed around 100 cfm/person at a 10° F ∆Tfrom room to discharge air temperature. System analysisproved that with many projects, the airflow (cfm) was toogreat, which caused over-cooling. This overcooling, combinedwith the 10° F ∆T, made it difficult to control humidity. As a re-sult of this analysis, many current project designs have electedto reduce the supply temperature in the underfloor plenumfrom 66° F to 62° F and the supply airflow rate from 1cfm/ft2to 0.8 cfm/ft2 or less.

The optimal airflow/delivery rate of the 35BF-R is 15° F ∆Tand 80 cfm per diffuser. This tested performance ensures theideal throw of 4 to 6 ft within the occupied region. Temperatureand velocity profiles for the 35BF-R are shown in Fig. 18.

PASSIVEDIFFUSER

STYLE

UNIFORMLOADING

VARIABLELOADING

Swirl Core — open areas & officesPerimeter — open areas Base loading

Floor grilles Large equip. loads, few people Base loading

Fig. 17 — UFAD Room Temperature Gradient

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Zone Mixing Box Selection — Each zone (thermo-stat) will require a dedicated air terminal to satisfy that zoneand supply cooling and heating airflow to those areas coveredby the zone. Larger, more uniformly loaded open office interiorzones can be supplied by parallel fan-powered mixingboxes (PFPMB) all the way up to 3,700 cfm primary airflow(5,800 cfm mixed airflow). Smaller perimeter zones andpartitioned interior zones can be handled with the same equip-ment down to 500 cfm primary airflow.

Once cooling loads have been determined, basic terminalselection can occur, using traditional manual or software meth-ods. The designer should keep in mind the following itemswhen selecting units for UFAD systems:• Primary airflow can be supplied by a traditional tempera-

ture source (approximately 50 to 55 F) or even from alow-temperature central system, keeping supply duct-work mains and branches small.

• Primary airflow is the airflow determined with equation2 (page 11) and is based on the AHU supply-air tempera-ture. Total airflow is the amount calculated by equation 1(page 11).

• Care must be exercised in the selection of the zone mix-ing unit to be sure it will provide the total mixed airflowbased on calculated entering conditions to the zone mix-ing unit. The use of selection software is recommended.

• Although the units are installed closer to the occupiedspace, acoustic values should not be a problem if inletduct pressure levels are kept low. Acoustic attenuationoptions are also available.

• Optional filters on the recirculated air opening are advis-able, removing particulate from the return air streambefore it mixes with the primary air.Control options that meet the sequences of operation will be

required. Control schemes tend to be more complex withUFAD, making a DDC system a logical choice.

8’

7’

6’

5’

4’

3’

2’

1’

36"

left

18"

left

12"

left

6"le

ft

6"rig

ht

12"

right

18"

right

36"r

ight

Cen

ter

Distance From Diffuser Centerline

36"

left

12"

left

6"le

ft

Cen

ter

8’

7’

6’

5’

4’

3’

2’

1’

18"

left

6"rig

ht

12"

right

18"

right

36"

right

Distance From Diffuser Center

DistanceAboveFloor

77.0-78.0

76.0-77.0

75.0-76.0

74.0-75.0

73.0-74.0

72.0-73.0

71.0-72.0

70.0-71.0

69.0-70.0

68.0-69.0

8’

7’

6’

5’

4’

3’

2’

1’

8’

7’

6’

5’

4’

3’

2’

1’

36"le

ft

18"

left

12"

left

6"le

ft

9"le

ft

3"le

ft

6"rig

ht

9"rig

ht

3"rig

ht

12"

right

18"

right

36"

right

Cen

ter

Distance From Diffuser Center

18"

left

12"

left

6"le

ft

9"le

ft

3"le

ft

6"rig

ht

9"rig

ht

3"rig

ht

12"

right

18"

right

36"

right

Cen

ter

Distance From Diffuser Center

DistanceAboveFloor

36"le

ft

AirSpeed,FPM

175-200

150-175

125-150

100-125

75-100

50-75

25-50

0-25

DistanceAboveFloor

Fig. 18 — 35BF-R Diffuser Temperature and Velocity Profiles

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Distribution System Design — As stated earlier inthe General layout section, in a UFAD system the ductworkfrom the air handler to the zones is essentially the same as anyother all-air system. However, since the air delivery occursunderfloor, there are routing and design requirements that arequite different than in a conventional system. One of the majordifferences to take into account when designing the air distribu-tion system is that air diffusion occurs within the occupiedregion (up to 6 feet above the finished floor).PLENUM DESIGN CONSIDERATIONS — The followingguidelines can be used in developing a pressurized plenumdesign for HVAC use with passive and active diffusers.• Common plenum depths are 12 to 18 in. from top of slab

to top of floor panel, with most panels being ≤ 2 in. thick.Practical minimum plenum depth is 8 in. and plenumsdeeper than 24 in. offer no HVAC advantages.

• Airflow leakage must be minimized with superior con-struction sealing of all joints, including overlappingjoints when laying down carpet squares. (Fig. 19.) Ifcarpet is not used, the floor manufacturer can provide asealing strip to use between the floor panels.

• Sealing of concrete floor slabs reduces particulategeneration, although heavier dirt accumulation withinthe plenum will not be entrained by the low velocityairflows.

• Plenum exterior surfaces, both floor and sidewall, shouldbe insulated to minimize thermal losses, which can affectthe temperature of the supply air delivered to the furthestdiffusers.

• Obstructions within the plenum should be kept to lessthan 50% of the plenum depth to avoid affecting airflow.This is more important with shallow plenums.

• Plenum dividers (available as part of a manufacturedaccess flooring system) should be provided when zoningis critical, as part of an architecturally determined smokebarrier, and to separate the perimeter from the core whena hybrid solution is used.

• For good acoustic privacy between offices, the floorpanels should weigh at least 10 lb/ft2. Placing a physicalbarrier in the plenum along the common wall lineprovides additional attenuation.

RETURN AIR DESIGNMaintaining Displacement Benefits — When designing thereturn side of the air distribution system, keep in mind thatthere are two key elements of the UFAD system that the returnair design should support. The first is to help create the partialdisplacement ventilation effect and the second is to moredirectly remove local high cooling loading conditions. Placingthe return registers on the ceiling or high sidewall will combinewith the stratification layer created by the short throw supplydiffusers in the floor to achieve the desired overall upwardvertical flow of the air within the room. When a space has highcooling loads, such as a copier or a cluster of data terminals,placing the return directly above the area may capture a largeportion of the heat, depending on the height of the ceiling andnature of the supply airflow in the immediate area.Recirculating Return Air — Variations in return airflow occurwhen space humidity control is critical. To control the supplyair moisture content during humid summer conditions, coolingcoil leaving air temperatures must be lower than the supply airtemperatures used with UFAD diffusers. To reheat the airleaving the cooling coil, return air is routed back into thesupply airstream, at the zone air terminal. For the zone terminaldesign to work, warm air is usually ducted back from aceiling return plenum. Placing ceiling return registers along theexterior walls brings an added benefit of pulling in any thermalplumes that develop under high solar loading conditions,reducing zone cooling load in the summer, and increasing thefirst stage heating effectiveness in the winter.

DUCTWORK DESIGN — One of the main benefits of aUFAD system is the elimination of the requirement forductwork downstream of the zone terminals. An airflowplenum underneath a raised access flooring system provideslow-pressure distribution to the room diffusers. All theductwork from the air-handling unit to the zone terminals islaid out and sized using the same methods used on traditionalmixing ventilation systems. This applies to both supply andreturn ducts, as well as the outdoor air ductwork.Vertical Drops — One way in which a UFAD system differsfrom a traditional system is in the vertical drop from the ceilingplenum down into the access floor plenum. Due to the thermaldecay of the supply-air temperature within the plenum, traveldistance must be limited to 50 to 60 ft from the plenum entry tothe furthest diffuser. This will require multiple entry points onall but the smallest system. If shallower plenums are used andfrequent obstructions are possibilities, more than the usualnumber of plenum entry points should be included in the ductdesign. Figure 20 shows a floor plate of a typical large officebuilding with multiple drops supplying each zone mixing box.Air Highways — One plenum distribution technique usedwith UFAD systems is an air highway. Air highways areinstalled within the plenum to distribute zone supply air tomultiple points within the plenum. These are typicallyprovided by the access floor installing contractor and must besized in the same way as normal ductwork. Airflow velocityshould be limited to a range of 1,200 to 1,500 fpm in the mainduct and to a range of 800 to 1,000 fpm in dampered branchoutlets. Diffusers should not be installed in the floor panelsabove an air highway.

Although air highways may serve a useful purpose, they ob-struct the placement of electric, communication and data cablesunder the raised access floors, and may also interfere with othersupply air entry points into the plenum. Future rearrangementof furniture and partitions above the floor may require air high-way adjustments under the floor, which negates the efficiencyand savings possible with a raised floor system.Smoke Partitions and Fire Walls — Whenever plenum areasbecome too large, they inevitably cross over partitions or wallsthat, in order to meet life safety code requirements, mustextend down through the plenum to the floor slab. The HVACzoning design should take into account these lines and avoidcrossing them. If air must be transferred over the partitions, ahard duct sleeve with the appropriate smoke and/or fire damperinstalled per code should be used.

Fig. 19 — Carpet Used toPrevent Plenum Leakage

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Heating System DesignCENTRAL SYSTEM VERSUS HYBRID SYSTEM FORPERIMETER ZONES — The heating method chosen to heatthe perimeter zones depends primarily on the magnitude of theload and whether or not simultaneous heating and cooling isrequired. The need for continuous cooling in many interiorzones makes it difficult to switch the central equipment over toheating. If the wintertime temperatures drop below 40 F andthe building perimeter has more than 25% glass area, increasedsupply-air temperatures will be required to counteract the highheating loads and downdrafts along the outside walls. In eitherof these cases, it is recommended that a separate heatingsystem be designed for the perimeter zones, creating a hybridsystem with UFAD heating being reserved for the interior.PERIMETER FAN-COILS WITH LINEAR BARGRILLES — High heating loads along the outside walls oftenrequire higher supply-air temperatures than interior spacesneed for morning warm-up or offsetting roof loads. The use ofzone fan-coils located in the perimeter plenums is an ideal so-lution. Using either hot water or electric heating coils allows lo-cal boosting of the zone supply air temperatures above thosetemperatures obtained by recirculating ceiling plenum air thatis traditionally single-stage heating for light load conditions.Use of a linear bar grille style diffuser (such as Carrier’s35BF-CT) is recommended to spread the air out along theperimeter wall and offset the downdrafts that develop. Duringsummer cooling this style of grille is also beneficial in treatingthe higher cooling loads that are typical along the perimeter ofmost buildings.HEATING INTERIORS WITH CENTRAL EQUIP-MENT — Smaller single-story buildings that do not requiresimultaneous heating and cooling are often able to use a system

that switches to full heating from the central HVAC equipmentduring the peak heating times. When the system switchesfrom cooling to heating, normal control strategies for the zoneterminals will control mixing of the warmer primary air withrecirculated plenum air and adjust terminal fan speed to controlplenum pressure. Passive diffusers that had been operatingin the cooling mode will operate properly as heating diffusers.

Control SystemsCONTROL DESIGN CONCEPTS FOR UFAD SYS-TEMS — UFAD systems rely on proper design and operationof the controls, as do all systems. Listed below are conceptsthat should be incorporated into the design to maintain thestratification layer and its many benefits:• Most UFAD systems use pressurized plenums and

passive diffusers, requiring some type of pressurizationcontrol on either the zone mixing box or centralequipment.

• Perimeter zone control with passive diffusers requiresadjustment of the supply air temperature at the airterminal. This is best accomplished using Perimeter FanPowered Mixing Boxes (PFPMB) connected to the zoneceiling plenum to mix return air with primary air.

• Resetting supply air temperature higher when loads arelight allows additional hours of outdoor air economizercooling, especially in drier climates. However, wheneconomizer cooling is used, special care must be takennot to exceed the relative humidity level of 60% in thespace. Also keep in mind that economizer cooling maynot meet the spot cooling requirements of high loadspaces.

Fig. 20 — Floor Plan Chase Locations

16

• Nighttime pre-cooling, when performed correctly so thatcondensation does not form on the plenum surfaces, canoffset or delay thermal decay of the zone supply airtemperature.

• Thermostat mounting heights are often being set lowerbecause of ADA (Americans with Disabilities Act),from 48 to 54 inches above the access floor. The lowerheight is actually a benefit for UFAD systems, becausethermostats that are mounted too high will sensethe higher temperatures near the stratification layer,requiring that the set point be adjusted upwards tocompensate.

• Systems that use Parallel Fan Powered Mixing Boxes tocontrol the zone primary airflow for ventilation require-ments can use Demand Controlled Ventilation (DCV) toreduce primary airflow in lower occupancy periods,resulting in additional energy savings.

• Control requirements of ASHRAE Standard 90.1 shouldbe followed, including setback requirements and opti-mum start requirements.

TYPICAL CONTROL SEQUENCES — Plenum mixing boxeswith variable speed fans can provide pressure and temperaturecontrol for plenums that serve interior zones and for exteriorzones that use a separate perimeter heating system. Typical oper-ating sequences are shown in Fig. 21 and 22. The sequencesshow basic VAV control of the primary air.

The fan provides plenum temperature control by varyingthe amount of recirculated air introduced into the plenum,while the primary air damper simultaneously maintains theplenum pressure by controlling the amount of primary airintroduced into the plenum. A wall-mounted space temperaturesensor located in the zone will sense load requirements.

As the zone’s cooling load decreases, the fan speed increases,which increases the amount of recirculated air drawn in from theceiling plenum. Simultaneously, the amount of primary air isreduced to maintain a constant plenum pressure.

The terminal fan will operate whenever the primary airsource is operating.System Start-up

1. Points 1 to 2 from Fig. 21 indicate that maximum coolingairflow is established by the user-defined maximum cool-ing plenum pressure set point until the zone comes undercontrol at Point 2. The fan operates at minimum speed.

2. From Point 2, the primary airflow is slowed to reduceplenum pressure until the minimum cooling plenumpressure set point is reached at Point 3.

Normal Operation (Cooling — Fig.21)1. Point 3 indicates that the zone temperature is above the

control (heating) set point; the plenum pressure is main-tained at the user-defined minimum cooling plenum pres-sure set point.

2. At Point 4 in Fig. 21, the zone temperature decreases.Increasing the fan speed to introduce more recirculatedair, while simultaneously reducing the primary airflow,will increase the plenum temperature. The plenumpressure remains constant at the user-defined minimumcooling plenum pressure set point.

3. As indicated by Points 4 to 5, if the zone temperaturecontinues to fall, the primary air damper will close.Primary airflow will not fall below the user definedminimum ventilation set point at 5. (The minimum venti-lation set point may be set to zero.) The fan will providerecirculated air to maintain the plenum pressure set point.

Normal Operation (Heating — Fig. 22)1. Upon receiving a heating signal from the air handler con-

troller, the heating mode will be in effect automatically.

2. If the zone temperature is above the occupied heating setpoint, the primary air damper modulates to maintain theminimum heating plenum pressure set point at Point 4′.The fan operates at minimum speed.

3. If the zone temperature falls, the plenum pressure willincrease until it reaches the user-defined maximum heat-ing plenum pressure set point at Point 5′. The control maybe configured to provide constant volume heating for aconstant supply of heated air to the zone.

Cost ConsiderationsINSTALLATION COSTS — Many owners and developershave found that raised floor systems with UFAD significantlyreduce costs associated with space reconfiguration. With thenational churn rate now approaching 50%, it is not unexpectedto find that many new office buildings use raised flooring.Buildings using UFAD systems have certain cost componentsthat make them more expensive than buildings using tradition-al systems. There are compensating cost components thatlower the overall price premium that currently exists for thisapplication. Since recent job costs show that raised floor canrepresent a cost increase of from $3 to $5 per square foot, a fewof these other cost components should be examined.• The raised floor system is the component with the single

largest cost increase. The cost should be justified basedon the benefits to the entire service delivery system(HVAC, power, voice and data) and should not be evalu-ated based on the UFAD system alone.

• Local building and fire inspectors may require sprinklerprotection if plenum depth exceeds 18 in. and oftenrequire smoke detectors within the plenum.

• Generally, HVAC costs for central cooling and heatingequipment and ductwork mains are no different forUFAD systems than for traditional overhead systems.

• Diffuser costs are dependent on the types of diffuserchosen for the majority of air delivery requirements.

• If a relatively open underfloor plenum can be used,significant ductwork savings can be realized compared tothe typical overhead air distribution system.

• Controls costs should be similar to traditional systemswith the same zoning requirements.

• Raised floor systems have the potential to reduce slab-to-slab heights by as much as 6 to 12 in. per floor. Thisreduction is particularly beneficial in mid-rise and high-rise markets with zoning code height limitations.

• Testing and balancing savings can be realized frompressurized plenum designs, which are essentially self-balancing.

ENERGY USAGE — UFAD systems have the potential tosave energy in comparison to traditional designs. The bulk ofthese savings come from reduced fan energy associated withlower static pressures and overall cfm reductions created by thestratification layer partial displacement ventilation effect.Documented energy costs savings remain difficult to predictbecause to date there is no energy simulation software thataccurately models UFAD system performance.OPERATION AND MAINTENANCE COSTS — Office build-ing UFAD systems are quite new, making it difficult to get actualowning and maintenance cost data to compare against traditionalHVAC systems. Operations and maintenance costs are primarilyreplacement costs for equipment and labor expenses associatedwith maintaining the HVAC system and responding to occupantcomplaints. While many engineers believe that UFAD mayprove to be more costly to service, research suggests that thefrequency of occupant complaints will be reduced whenoccupants are given some individual control of their localenvironment.

17

OCCUPIEDCOOLINGSETPOINT

OCCUPIEDCONTROL SETPOINT

(HEATING &MORNING WARM-UP)

PLENUM PRESSURE

Plenum Pressure(Minimum Cooling)

Plenum Pressure(Maximum Cooling)

Maximum Fan Airflow

Minimum Fan AirflowFAN

PRIMARY AIRFLOW

Minimum Ventilation

12

3

4

5

5

PLENUM TEMPERATURE

Warmer

Cool

4

OCCUPIEDCOOLINGSETPOINT

OCCUPIEDCONTROL SETPOINT

(HEATING &MORNING WARM-UP)

PLENUM PRESSUREPlenum Pressure(Minimum Heating)

Plenum Pressure(Maximum Heating)

Minimum Fan AirflowFAN

PRIMARY AIRFLOW

5’

PLENUM TEMPERATURE

4’

Warm

Maximum Heating Airflow

Minimum Heating Airflow

LEGEND

Fig. 21 — Carrier UFAD Control Sequence — Equipment Cooling

----------- Air Source Supplying Heated AirAir Source Supplying Cool Air

LEGEND

Fig. 22 — Carrier UFAD Control Sequence — Equipment Heating

----------- Air Source Supplying Heated AirAir Source Supplying Cool Air

18

SUMMARY OF RECOMMENDATIONS

1. Do not eliminate the suspended ceiling. Reduce the depthof the ceiling plenum and use it as a return air plenum,gaining additional airflow benefits from assigning greateramounts of cooling load directly to the return air.

2. Keep plenum depths below 18 in. to avoid potentialrequirement for sprinklers.

3. Do not use too many zoning barriers in the plenum;zoning barriers will restrict the ease of future rezoningand running of service utilities to various points of use.

4. Use multiple vertical drops to avoid the need for airhighways.

5. Limit the underfloor air travel distance from the supplyair discharge to the most remote diffuser to a maximumof 60 ft.

6. Do not include excess safety factors; airflow rates shouldclosely match load requirements within the occupiedregion to avoid “over airing” the system. The idealairflow 0.6 cfm/sq ft. This will provide good mixing in

the occupied regions, maintain the stratification height at6 ft and keep the room temperature gradient below 5° F.

7. Carefully run loads for both Qoccupied and Qunoccupied, anddetermine if load credits can be taken; equipment sizereduction will benefit the job costs.

8. Select diffusers so that throws are 4 to 5 ft so that thestratification layer is preserved. Use passive swirls in theinterior and uniformly loaded large perimeter spaces.

9. Use a hybrid designs in high load/variable load situations.Combining the UFAD system with a conventional VAVsystem will maximize the benefits of the lower supply airtemperatures of the air-handling unit and keep summer-time humidity concerns under control.

10. Place return air inlets over heat producing equipment and/or at the outside wall when high solar loads exist. Thisstrategy will maximize benefit of the partial displacementventilation effect.

11. DDC controls should be considered basis of design forrealizing true UFAD benefits.

Manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.New PC 201 Catalog No. 04-51450001-01 Printed in U.S.A. Form 45-1XA Pg 20 605 4-05 Replaces: New

Copyright 2005 Carrier Corporation

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Tab 6a 7aBook 4 4 4

Tab 4AT1 4AT4 4AT5