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AG-UFAD-02December 5, 2007

Titus

Underfloor Air Distribution (UFAD)Application Guide

Air Distribution Technologies

605 Shiloh RoadPlano, Texas 75074

972.212.4800www.titus-hvac.com

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Titus, the Titus Spiral, The Leader in Air Management is a trademark of Titus.

All other trademarks, noted or not, are the property of their respective owners.

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Contents Page

General..............................................................3

Introduction...................................................... 3

UFAD System Overview.................................. 3

Design Basics .................................................. 3Plenum Design .............................................. 3Perimeter Systems ........................................ 4Conference Rooms & Other Areas of VaryingLoad............................................................... 5Return Air....................................................... 6Humidity Issues ............................................. 6Sizing Jobs..................................................... 6

System Economics.......................................... 6First Costs & Installation Costs...................... 7Higher HVAC Equipment Efficiency .............. 7Lower Horsepower Fans................................ 7Better Heat and Pollutant Removal ............... 7

Easy Installation and Relocation.................... 8Lower Life Cycle Costs.................................. 8

UFAD and LEED............................................... 8Optimize Energy ............................................8Building Reuse...............................................9Controllability of Systems .............................. 9

Abbreviations................................................... 9

GeneralThis document provides application and designhighlights for underfloor air distribution (UFAD)systems.

Additional information may be found at the Tituswebsite, www.titus-hvac.com.

IntroductionThe interest in underfloor air distribution (UFAD)has increased significantly over the last decade.There is currently several million square feet ofaccess floor air distribution systems beingdesigned across the country.

In 1997 Titus introduced the TAF-R diffuser and

the TAF-G grommet, which were installed in theOwens Corning World Headquarters. Since then,Titus, ASHRAE, and the engineering communityhave continued to learn about UFAD systems.

In the decade that Titus has participated in UFADdesigns in the US, we have continued to introducenew products to meet the needs of this uniqueapplication.

UFAD System OverviewUFAD systems utilize the space under an accessfloor as an air plenum. Properly designed UFADsystems take advantage of thermal stratification.

ASHRAE recommend that, for comfort, thetemperature in the occupied zone be between 73

o

and 77oF, relative humidity be between 25 - 60%,

and the maximum velocity in occupied zone be 50fpm in cooling or 30 fpm in heating.

The key to successful access floor systems is theability of the access floor diffuser to rapidly mixroom air into the supply air at low velocities.Because supply air is introduced directly into theoccupied zone, it is important that the supply airreach the ASHRAE recommended temperatureand velocity, mixing of the supply air into the

space should happen rapidly.

The typical application for a UFAD system is theopen plan office. Floor space is at a premium in acubicle so a smaller clear area around the diffuserwill allow more usable space in the cubicle. TheUFAD diffuser manufacturer defines the requiredclear area that their diffuser needs to achieve the

ASHRAE recommended temperature and velocity.

Originally UFAD systems were for computerrooms. The design intent was to cool computerequipment and not to provide comfort. Thecomputer room design concept typically providestoo cold of a space for comfort.

There has been a growth of UFAD systems usedin offices and headquarters in U.S. It is estimatedthat 10% of U.S. construction will utilize accessfloors within few years.

The growing interest in UFAD systems is primarilydue to companies need to easily rearrange officelayouts, information and communications basedoffices, economics of ownership, and greenbuilding programs such as LEED.

Design Basics

Plenum DesignThe raised floor office can be supplied withconditioned air from below the floor in two ways pressurized plenum or neutral plenum.

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Pressurized PlenumsThe pressurized plenum (the area between theslab and the raised floor) is essentially a largeduct maintained at a constant pressure differentialto the room above; typically between 0.05 and0.10 in. pressure (w.g.).

This pressure is maintained through the supply ofconditioned air from a number of supply ductterminations. The spacing and location of theseducts are dependent on the air supplyrequirement and the plenum depth, with shallowplenums and / or high air quantities requiring moreair supply duct outlets under the floor. UFADdiffusers are specially designed grilles with a useradjustable damper to regulate flow.

Sealing the underfloor plenum is critical tooptimizing the operation of a UFAD system. Air

leaking through the floor tiles into the occupiedzone is of minimal concern because it is leakinginto the occupied space. Air leaking into the spacebetween the walls, however, is wasted energy. Allknockouts and holes in the drywall below theraised floor must be sealed during construction.

The advantages of pressurized plenums includelow first cost and easily changed layouts. This isthe most commonly used plenum design.

Neutral Plenums

With the neutral plenum design, the same layoutas the pressurized plenum may be used, but thepressure difference between the plenum and theroom is kept as close as possible to zero.

Floor diffusers either contain integral fans, areducted from a central source, or both. In manycases, these closely resemble conventionalceiling supply systems.

Advantages include the possibility of multiplesmall zones (as with multiple tenants) andinsensitivity to construction details. Disadvantagesinclude higher first costs due to ducting and/or fanconnections under the floor, lower flexibility as thegrilles are individually ducted, and potentiallyhigher noise levels.

This design is rarely used, but may be effective intenant buildings where the utilities will be paid bythe tenant.

Plenum DetailsPlenum heights typically range from 14 to 18,occasionally going as low as 12. The plenumheight is usually determined by the heightrequirements of other equipment that will belocated under the floor. The number of inletsrequired to supply the plenum with sufficient air torun the diffusers is dependent upon the plenumsize and the number of diffusers, which in turn isdetermined by the load of the space.

As a general rule, the longest distance from thesupply air outlet in the plenum and the farthestdiffuser should not exceed50-65 feet. Distanceslonger then this are subject to thermal losses andthe discharge temperature of the diffuser may betoo high.

If zone control is desired from the underfloor

plenum, the plenum can be partitioned intoseparate zones. The zones in the underfloorplenum should correspond to building zoneshaving similar load requirements. However, it isnot necessary to partition the underfloor plenuminto zones and doing so can make future officelayout changes more difficult. If an office layoutmust be changed, the partitioned plenum will needto be changed to match the new layout.

Because of the special heating and coolingrequirements of the perimeter of the building, itmay be necessary to create a perimeter zone in

the underfloor plenum to run a separate perimetersystem. Typically only the perimeter is zoned fromthe core. The perimeter will be discussed in moredetail next.

Perimeter SystemsThe perimeter is typically the most difficult area ofan underfloor system to design. The perimeter isoften handling much larger loads and require themost equipment. In the past, the best way tohandle the perimeter was to use fan poweredterminals with reheat ducted to linear bar grilles.

There are a couple challenges with this designconcept. The throw of a linear bar grille ducted tothe discharge of a fan powered terminal is verylong, possible as long as 15-20 feet. Designinglong throws at the perimeter contradicts theconcept of stratification in a UFAD system. Notonly does the long throw from the grille mix the airabove the stratified layer into the occupied zone,wasting energy, but it also may roll up the glassand across the ceiling, where it drop into the

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occupied zone causing discomfort for theoccupants.

Although the concept of UFAD systems is to bemodular, the function of handling perimeter loadsis not modular. Those loads come with the

building envelope, which is always a line of somesort. The TAF-L Perimeter System was designedto address all of these considerations.

Perimeter CoolingThe perimeter cooling system should combineinduction, buoyancy and displacement ventilationmodels, handles high thermal loads such as 225CFM per 4 length at 0.07 wg, engage theoccupied zone, but not the stratified ceiling layer.This avoids occupant complaints from air rollingback, dumping into interior space and savesenergy by not mixing the warm ceiling air into the

occupied zone.

The TAF-L perimeter system provides this idealair pattern. The TAF-L system consists of amodular cooling plenum, the TAF-L-V, andheating plenum, the TAF-L-W, designed to beintegrated with the CT-TAF-L multi-deflectionlinear bar diffuser.

The TAF-L system provides a continuous lookaround the perimeter. The TAF-L-V coolingplenum used with the CT-TAF-L has anengineered throw pattern that never breaks

through the stratification layer created by theUFAD diffusers in the core. The dual apertureplate design allows the TAF-L-V / CT-TAF-Lassembly to maintain this engineered throwpattern while modulating the airflow volume.

Perimeter HeatingPerimeter heating cannot be accomplished withthe same system as the interior load coolingsystem. Ideally, since ASHRAE 90.1 states thatyou should not simultaneously cool and heat, theperimeter heating system should be completelyindependent of the cooling system. Separateducting of hot or reheated air, hydronic systems,or perimeter fan powered systems are often usedto condition the skin load on the building.

The TAF-L-W, self contained fin tube perimeterheating plenum utilizes room air to heat theperimeter instead of supply air. The TAF-L-Wheating plenum also uses the CT-TAF-L to createa continuous linear look from the occupied space.

The TAF-L-W works by allowing the denser coldair to flow down a window or exterior wall into theTAF-L-W plenum while also inducing room air intothe plenum. A finned tube heater in the plenumreheats this cold air and room air, returning thewarmer air to the window or exterior wall through

natural convection.

Typically the TAF-L-V, cooling plenums, and TAF-L-W, heating plenums are alternated as neededthroughout the perimeter.

The TAF-L system allows you to remove the fanpowered terminals, and their associated energyand maintenance costs, from the UFAD system.However, in the event where the system cannotachieve consistent plenum pressure, a fanpowered terminal may be necessary.

The PFC was designed to be used as a boosterunit for perimeter applications. The PFC fanpowered terminal unit is designed to be installedbetween the pedestals in an underfloor systemand installed in a floor 12 to 18 in height.

The PFC is usually ducted to linear bar diffusers,such as the CT, or diffuser and plenum units, suchas the TAF-D. The airflow of the diffusers isdirected at the glass like a typical ceiling system.

Modulating the fan speed to vary the amount of airsupplied to the zone is the most common control

sequence used for the PFC. The PFC controllerwill determine the speed ofthe fan based on zonetemperature. For example, the fan would run100% when the zone is 75

oF and modulated down

to 30% as the zone approaches 70oF.

Conference Rooms & OtherAreas of Varying LoadMuch like the perimeter, conference rooms mustbe handled separately to adjust for the varyingload conditions. The LHK fan powered terminalwas designed for this application. Like the PFC,the LHK fits within the modular pedestal systemsof the raised floor and is available in variousheights to fit under 12 through 18 raised floors.

With the exception of its unique dimensions, theLHK is like any other series fan powered terminal.The LHK has a supply inlet with a dampermodulated by a controller and actuator. The LHKhas an induced air inlet which pulls air from theunderfloor plenum or from the room depending onhow the LHK is applied.

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The LHK supply inlet would be open to theplenum with the induced air inlet ducted to theroom as the return. The discharge would then beflex ducted to TAF-Rs in the room.

Return AirDue to the upward air flow, returns should belocated at the ceiling or on a high side wall. Thisallows the heat from ceiling lights to be returnedbefore it is able to mix with the conditioned air inthe occupied zone. There will also be a smallamount of free cooling due to the naturalbuoyancy of hot air.

If the system must use 55oF supply air for

humidity reasons, some of the return air can berecirculated from the ceiling to the underfloorplenum to raise the temperature of the air to 63

oF

to 68oF.

Another option is to take the return air back to theair handling unit where it can be filtered anddehumidified before re-entering the underfloorplenum. With this option, you can more accuratelycontrol the air temperature at the diffuser and yougain the cost benefits of the warmer supply airtemperature.

Humidity IssuesA potential problem with the higher supplytemperatures used in underfloor supply systems is

the higher potential moisture content of thewarmer supply air used in these systems.

The supply system must reduce relativehumidities to less than 60% to meet IAQconcerns, and this requires dew points less than65

oF. This implies either reheat or blending of air

to achieve a 65oF supply, 55

oF dew point

condition.

System designs utilizing condenser water reheat,run-around coils, face & bypass, and otherstrategies can be employed to solve these

potential design problems. Other possiblesolutions include the use of a separate system todry outside air or the use of desiccantdehumidification.

Climate and building operation are importantconsiderations when designing a UFAD system.In humid climates, it may be necessary to operatethe HVAC system 24 hours a day to maintainacceptable humidity levels in the building.

Sizing JobsThe optimum design point for the TAF-R is 80 -100 CFM when a 10

oF room / supply differential is

used. At this point, the noise is negligible and thepressure required is less than 0.10.

Throw will be less than 6 ft., preserving thedesired ceiling stratification layer. Our testingshows that there is 100% mixing in the occupiedzone under these conditions.

The stratification in this installation results in a

supply - exhaust T similar to the typical 18oF to

20oF T common in most conventional systems.

For example, with 64oF supply air in a 74

oF room,

the room exhaust, at the ceiling, will probably be

about 82oF, for an 18

oF T.

This means that one diffuser can handle:

18oF T x 100 CFM x 1.08 = 1944 BTUH or

1944 BTUH 3.41 = 570 watts of internal load.

Lights are typically 0.75 W/sq.ft, but with ceilingstratification are probably not a part of the roomload (but are seen by the air handler). Ifcomputers and printers supply about 1W/sq.ft.load and occupants add about 1.2 W/sq.ft, thistranslates to:

1.0 W/sq.ft. (computers and printers)+ 1.2 W/sq.ft. (occupants)= 2.2 W/sq.ft. (room load)

This corresponds to one TAF-R every:

570 Watts of internal heat 2.2 W/sq.ft. room load

= 260 sq.ft. floor space sensed interior zone load.

As a general rule, one TAF-R should be providedfor each occupant.

System EconomicsThe main economic considerations for a UFADsystem are reduced first costs & installation costs,higher HVAC equipment efficiency, lowerhorsepower fans, better heat and pollutantremoval, quick to install and easy to rearrangeoffice layouts, and lower life-cycle building costs.

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First Costs & Installation CostsUFAD systems can be designed with plenumreturns using the same floor to floor height asconventional systems by shifting the occupiedzone up into where the ceiling plenum would

normally be.

The cost of the raised floor, typically $5-7 persq.ft., is offset by the fact that less ductwork isrequired. The installation costs are usually lowerbecause the HVAC and data / power work is doneat floor level.

In a conventional system, ductwork must go toevery diffuser. UFAD systems use a pressurizedplenum to supply each diffuser, so less ductworkis required. In a UFAD system, the only ducting inthe underfloor plenum is the ductwork required tosupply the diffusers that are more than 50-65 feetaway from the dampers or the separation for theperimeter.

Figure 1. Conventional system ductwork

Figure 2. UFAD system plenum

A 1996 study in Building Design & Constructionmagazine showed a $2.13/sq.ft. first cost savingsusing UFAD systems.

Higher HVAC EquipmentEfficiencyIn an UFAD system, supply air enters directly intothe occupied zone at the floor level. In aconventional system, the supply air temperature isusually 55oF because it must mix with the warmair at the ceiling before it enters the occupiedzone.

UFAD systems typically use 63-68oF supply airtemperatures. This warmer supply air can reduceenergy consumption of the HVAC equipment.Some DX equipment cannot supply air at this hightemperature, so this must be considered whenselecting the HVAC equipment.

Lower Horsepower FansUFAD systems move a larger volume of air withoverall lower pressure drops. The diffusers used

in UFAD systems operate with less than 0.1 wg,so lower horsepower fan can be used, reducingenergy costs.

Better Heat and PollutantRemovalHeat from overhead ceiling lights is removedbefore it enters the occupied zone and lateralmixing in the occupied zone is reduced. A

Main Duct Loop

Branch Ducts

Air handler

Core VAV box

Diffusers

Air is

supplied to

the center

of the floor

Fan powered

terminals

handle the

perimeter

Air is

supplied to

the center

of the floor

Fan powered

terminals

handle the

perimeter

Air is

supplied to

the center

of the floor

Fan powered

terminals

handle the

perimeter

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All LEED projects registering after June 26, 2007are required to achieve at least two (2) OptimizeEnergy Performance points

One way to achieve energy optimization credit iswith an underfloor system. An underfloor system

may have higher HVAC equipment efficiency asaccess floor air systems use warmer supply air(63

oto 68

oF) than conventional systems that use

55oF supply air. Raising the discharge

temperature of many system types reducesenergy consumption.

Underfloor systems can move a largervolume of air with overall lower pressure drops.The underfloor plenum needs less than 0.1 wgof water pressure or less for proper diffuserperformance. This results in less fan horsepowerneeded for underfloor systems resulting in lower

energy usage.

The energy savings of an underfloor systemshould be considered as part of the system toreceive an Optimize Energy Performance credit.

ECM motors are another option that should beconsidered for the Optimize Energy Performancecredit. The ECM motor has efficiencies of up to70% across its entire operating range (300-1200rpm) and 80% over 400 rpm. The ECM motor isavailable in the Titus LHK and PFC UFAD fan-powered terminals. See the ECM ApplicationGuide, AG-ECM, for more information.

Building ReuseIf the project is a renovation, Credit 1.1, 1.2, or1.3, Building Reuse, may be applicable.

Credit 1.1 is for maintaining at least 75%, basedon surface area, of existing building structure andenvelope. Credit 1.2 is for maintaining anadditional 20%. Credit 1.3 is for maintaining 50%of existing interior non-structural elements such asinterior walls, doors, floor coverings and ceilingsystems.

Each credit qualifies for one point, meeting thecriteria for all three credits nets a maximum ofthree points for the Material & Resources section.

Utilizing access floors systems can update abuilding for meet current technology needswithout demolishing the current building structure.In situations where the architect wants to leave anornamental ceiling open, access floor airdistribution may be the perfect solution.

Controllability of SystemsCredit 6, Controllability of Systems, has two parts:6.1 Lighting and 6.2 Thermal Comfort. The intentof Credit 6.2 is to provide individual comfortcontrols for 50% of the building occupants to

enable adjustments to suit individual task needsand preferences.

The credit states that, Individual adjustments mayinvolve individual thermostat controls, localdiffusers at floor, as potential strategies toachieve this credit. VAV diffusers, such as theTitus T3SQ would also qualify for this credit aswell as access floor diffusers such as the TAF-R.

AbbreviationsThe following table lists abbreviations used withinthis document.

Abbrev. Term

ASHRAE American Society of Heating,Refrigerating and Air-ConditioningEngineers

UFAD Underfloor Air Distribution

LEED Leadership in Energy & EnvironmentalDesign

USGBC U. S. Green Building Council

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