TYPES OF SUPPLY DUCT SYSTEMS

There are several basic types of supply and returnduct systems. Any one of the system types, or a com-bination of different types, can be utilized to fit theneeds of a particular structure. The general types ofsupply duct systems include:

ª radial system

ª extended plenum system

ª reducing plenum system

ª reducing trunk system

ª perimeter loop system.

Radial system

The radial duct system in its simplest form consists ofa central supply plenum that feeds a number of indi-vidual branch ducts arranged in a generally radialpattern (see Figure 1). It also can be designed andsized so that each individual run leaving the plenumcan feed two or more supply outlets. This is fre-quently the case because of the number of supplyoutlets required to condition the structure success-fully and the amount of space at the plenum availablefor takeoffs.The radial system commonly is applied inattics, crawl spaces, and in slab on grade installa-tions (with the ducts embedded in the slab). It can beused with upflow, downflow, or horizontal air handlersand furnaces.

Extended plenum system

The extended plenum duct system (see Figure 2 onthe next page) generally consists of one or two box-like pieces of ductwork extending from the main

plenum at the indoor unit. This extended plenum hasthe same dimensions (height and width) from thestarting collar to the end of the run. Branch runs tofeed the supply outlets are tapped into the extendedplenum(s). The best results are achieved when themaximum length of the extended plenum is notgreater than 24 ft from the air handler or furnace. Iftwo plenums are used, this total length can beextended to 48 ft (see Figure 3 on the next page). Iflonger distances are required based on the physicallayout of the structure, consideration should be givento using one of the other designs discussed below(such as the reducing plenum or the reducing trunkduct system). There is another area of concern withthe extended plenum system—because of the highervelocities in the plenum, it is possible that thebranches closest to the indoor blower may not feedthe desired amount of air (cfm).

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Refrigeration ServiceEngineers Society1666 Rand RoadDes Plaines, Illinois 60016

DUCT SYSTEM DESIGN CONSIDERATIONSPart 1

by Roger M Hensley, CMS

© 2005 by the Refrigeration Service Engineers Society, Des Plaines, ILSupplement to the Refrigeration Service Engineers Society.

630-148Section 11A

Figure 1. Radial duct system

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Figure 2. Extended plenum duct system (single plenum)

Figure 3. Extended plenum duct system (double plenum)

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Never start a branch run from the end cap of an ex-tended plenum. For best results, the starting collar ofa branch run should never be any closer than 24 in.from the end cap. To sum up, observe the followinggeneral rules for the extended plenum system:

ª Single plenums should not exceed 24 ft in length.

ª Double plenums should not exceed 48 ft in totallength.

ª Keep branch run starting collars 24 in. from theend caps.

ª Never locate a takeoff in the end cap.

Reducing plenum system

The reducing plenum duct system (see Figure 4) canbe used when the physical size or layout of the struc-ture calls for greater distances than the length con-straints imposed on the extended plenum (24 ft). Theconcept of the reducing plenum system is simple—when the air velocity lost to the branch runs reachesapproximately 50%, the plenum size is reduced to

regain the velocity in the remaining portion of theplenum. This reduction also improves the air flowcharacteristics at the branch ducts that are closest tothe air-handling unit.The 50% rule is demonstrated inFigure 5 on the next page. Note that at the start of theplenum, there is an available air volume of 1,200 cfmand an available velocity of 900 ft/min. After the thirdbranch run, a total of 600 cfm has been distributed tothe branches and the velocity in the plenum has beenreduced to 450 ft/min. These conditions indicate thatthe proper location for the reduction in the plenum isafter the third branch. The outlet side of the reductionis sized to restore the velocity in the plenum toapproximately 900 ft/min.

This system is relatively easy to fabricate and install.Additional sheet metal sometimes is required to buildthe system, but if done correctly it can deliver goodresults. It may be necessary to balance the systembranch dampers properly.

Reducing trunk system

The reducing trunk duct system (see Figure 6 on the next page) is very similar to the reducing plenum

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Figure 4. Reducing plenum duct system

system, with the exception that the trunk run isreduced in size after each branch takeoff. These mul-tiple reductions make it possible to maintain a con-stant velocity (ft/min) in the trunk even though thetotal air volume is reduced as each branch is sup-plied. This type of system generally takes more sheetmetal to build and requires more labor to fabricateand install. Another major concern isthat there are more joints to seal (to pre-vent air leakage). The reducing trunksystem also can be applied usinglengths of round duct and manufacturedfittings. Round duct systems can signifi-cantly reduce the cost of labor for fabri-cation and installation, and produce verysatisfactory results if properly applied.

Another configuration that may be usedin some cases is known as the primary-secondary trunk system (see Figure 7).This type of system has a primary trunkand two or more secondary trunks. The“tee” fitting located at the end of the pri-mary trunk in this system performs thesame function as the reduction in the

reducing trunk system. Each secondary trunk has across-sectional area that is smaller than that of theprimary trunk. The secondary trunks are sized todeliver the proper air volume to each branch at theproper velocity. This type of system can be used verysuccessfully in a structure that spreads out in two ormore directions.

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Figure 5. Reducing plenum “50% rule”

Figure 6. Reducing trunk duct system

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Perimeter loop system

The perimeter loop duct system (seeFigure 8 on the next page) is well-suitedfor buildings that are constructed usingconcrete slab on grade. It generally per-forms better than the radial system insuch applications, especially in cold cli-mates. However, the perimeter loop sys-tem does have the disadvantage ofbeing a little more difficult to design andmore expensive to install. It is basicallylaid out around the perimeter of thestructure next to the edge of the slab.The entire perimeter loop is the samesize duct. The loop is fed by four or moreducts radiating out from the centralplenum. They are usually the same sizeas the loop duct. The boot boxes aresized to deliver the proper cfm to eachroom of the structure.

SUPPLY DUCT SYSTEM LOCATIONS

Decisions regarding the location of a supply air dis-tribution system should be made based on the winterdesign temperature for the structure’s geographiclocation.Table 1 in ACCA’s Manual J lists design con-ditions for locations in the U.S. and Canada. Thisinformation should be consulted to ensure that theproper type and location of duct system is selectedfor the structure in question. The ASHRAE Funda-mentals Handbook contains HVAC design criteria formost countries around the world.

The general guidelines state that if the winter designtemperature for the location of the structure is above35°F, then both perimeter floor and ceiling distribu-tion systems will provide satisfactory results. If thewinter design temperature for the location of thestructure is below 35°F, the ceiling distribution sys-tem is not recommended and the floor distributionsystem should be considered. A modified type of ceil-ing distribution system can be used if the registersare moved closer to the outside walls and the primaryair is directed out of the occupied zone and towardthe window and door openings.

There are six basic locations for supply duct systemsin residential structures. Most residential structures

can accommodate one or more of these configura-tions. One of the most important jobs of the designeris to select the type of installation that best suits theair distribution requirements of the structure and theneeds and desires of the customer. This must be bal-anced with the cost of the installation and the comfortconditions within the structure. The six basic locationsfor supply duct systems are as follows:

ª attic installations

ª basement installations

ª between floors of multistory structures

ª crawl space installations

ª conditioned space installations

ª embedded in concrete slab.

Attic installations

Attic installations lend themselves readily to all of theduct system types. A duct system located in an atticmust be insulated and must have a vapor barrierinstalled to prevent condensation on the exterior of

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Figure 7. Primary-secondary trunk system

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the ductwork. Condensation can cause corrosion andrusting of the duct system and possible structuraldamage to the ceilings. All joints and side seamsmust be sealed to prevent duct leakage. Some localcodes do not permit the use of duct tape as asealant. In these cases, waterproof mastic must beused. Depending on the type of equipment beingused, the air handler or furnace may be installed inthe attic space, in the garage area, or in an alcove orcloset in the interior of the structure. A packaged unitlocated outside the structure can be installed on theroof, on the ground, or on a stand raised aboveground level. Special insulation and waterproofingmust be applied to all ductwork that is exposed tooutdoor weather conditions.

The air handler should be located where the shortestduct runs possible are attained. The shorter the ductruns are, the lower the resistance to air flow and thelower the heat gains and heat losses will be. One dis-advantage to locating the air handler in the attic spaceis serviceability. Provisions for service access must beprovided. Most local codes require a floored walkwayfrom the attic entry to the unit. A floored area mustextend at least 3 ft on all sides of the unit to provide aplatform for service work. Another consideration totake into account when the furnace or air handler is

installed in an attic is the requirement for an auxiliarydrain pan, along with a condensate line and/or emer-gency float switch to shut down the system in case ofa condensate overflow. Locating the return air filtergrilles in the conditioned space is recommended withattic installations. This allows the homeowner tochange the filters without having to enter the attic.

The duct system types that lend themselves to atticinstallations include the extended plenum, the reduc-ing trunk, and the radial arrangements. A wide vari-ety of duct materials can be used with atticinstallations. However, great care must be takenwhen installing a flexible duct system. Improperinstallation that allows sagging, sharp bends, kinks,and crimping of flexible duct will increase the frictionloss of the system and increase the total amount ofstatic pressure that the indoor blower must over-come. This can result in service problems and possi-ble equipment failure. It is always necessary to followthe recommendations of the manufacturer wheninstalling a system utilizing flexible duct products.

Basement installations

Basement installations also lend themselves to all ofthe duct system types. A basement system must beinsulated and must have a vapor barrier installed toprevent condensation on the exterior of the ductworkif the basement is to be unconditioned. If the base-ment is to be conditioned, then the ductwork is con-sidered to be in a conditioned space and insulationmay not be required. However, it is recommendedthat a duct liner be installed for sound attenuation. Alljoints and side seams must be sealed to prevent ductleakage. Again, be aware that some local codes donot permit the use of duct tape as a sealant. In thesecases, waterproof mastic must be used. The air han-dler or furnace may be installed in the basement, oroutside the structure if a packaged unit is to beinstalled. Any ductwork exposed to outdoor weatherconditions must be specially treated with insulationand waterproofing.

As in an attic installation, the air handler should belocated where the shortest duct runs possible areattained. One advantage to locating the air handler inthe basement is serviceability. Return air filters maybe located at the unit in a basement installation, orfilter grilles in the conditioned space may be used.

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Figure 8. Perimeter loop duct system

The duct system types best-suited to basementinstallations are the extended plenum and the reduc-ing trunk arrangements. Due to headroom andappearance considerations, the radial duct systemdoes not always lend itself to basement installations,although it can be used in some cases.

All types of duct materials can be used with base-ment installations. However, flexible duct materialsare discouraged, largely because of the appearanceand the problem of providing proper support. Sag-ging ductwork will increase the friction loss of thesystem and increase the total amount of static pres-sure that the indoor blower must overcome, resultingin service problems and even equipment failure.

Between floors of multistory structures

Between-floor installations usually are installed in fir-down areas, with branch ducts running between thecombination ceiling/floor joists. These systems gener-ally are constructed of lined sheet metal. Sometimesduct board is used for sound and noise control.Between-floor ductwork normally does not requireinsulation, since it is located within the conditionedspace. Ductwork that passes through an uncondi-tioned space must be insulated and must have avapor barrier installed to prevent condensation fromaccumulating on the exterior of the ductwork. All jointsand side seams must be sealed to prevent duct leak-age. If local codes do not permit the use of duct tapeas a sealant, waterproof mastic must be used. The airhandler or furnace may be installed in a garage, in aninterior alcove or closet as permitted by local codes,or, if a packaged unit is to be installed, outside thestructure. Any ductwork located outside the structuremust be specially treated with insulation and water-proofing if exposed to outdoor weather conditions.

The air handler should be located where the shortestduct runs possible are attained. One of the majoradvantages of a between-floor installation is that theheat gains and losses often associated with ductworkare negated because the ductwork is in the condi-tioned space. With this type of installation, return airfilters may be located at the unit, or filter grilles in theconditioned space may be used.

The reducing trunk and the extended plenum config-urations are the most common types of duct systems

installed between the floors of the multistory struc-tures. Many types of duct materials can be used withbetween-floor installations. However, flexible ductmaterials generally are discouraged because theyare not as durable as metal ductwork. Once the ductsystem is installed, it is a major project to makerepairs if needed.

Crawl space installations

Crawl space installations are adaptable to all of theduct system types. A crawl space system must beinsulated and must have a vapor barrier installed toprevent condensation on the exterior of the ductwork.All joints and side seams must be sealed to preventduct leakage. If local codes do not permit the use ofduct tape as a sealant, waterproof mastic must beused. The air handler or furnace may be installed inthe crawl space, in the garage area, in the interior ofthe structure as permitted by local codes, or, if apackaged unit is to be installed, outside the structure.

The air handler should be located where the shortestduct runs possible are attained. The main disadvan-tage to locating the air handler or furnace in the crawlspace is serviceability. Provisions for service accessmust be made. With crawl space installations, returnair filter grilles in the conditioned space should beused.

The duct system types that lend themselves to crawlspace installations include the extended plenum, thereducing trunk, the radial, and the perimeter looparrangements. Although many types of duct materi-als can be used in crawl space installations, flexibleduct materials are discouraged due to the problem ofproviding proper support for the ductwork.

Conditioned space installations

Some basement installations, installations betweenfloors, and fir-down duct systems can be considered“conditioned space” installations. Each of these typesof systems has its own considerations, previously dis-cussed. Generally speaking, duct systems that areinstalled within a conditioned space do not requirethermal insulation to prevent heat loss and heat gain.It is desirable, however, to use duct materials such asduct liners or duct board systems constructed prop-erly for sound attenuation. In warm, moist climates,

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duct systems installed in conditioned spaces mayneed to be insulated to prevent condensation fromforming on the exterior surfaces of the ductwork andcausing mold, mildew, and structural damage. Themost common types of duct systems applied to con-ditioned space installations are the extended plenumand the reducing trunk arrangements. Both theextended plenum and the reducing trunk systems fre-quently are installed in fir-down areas above hallways,cabinets, and closets. They typically are associatedwith the high inside wall type of supply outlets.

Embedded in concrete slab

Different types of construction present different prob-lems for system designers and installers. In climateswhere the average winter temperature is below 35°F,slab on grade construction is used. The floor distrib-ution duct system must be embedded in the slab,which can create several challenges for thedesigner/installer. Most codes require that the ductsystem in such cases be installed above the final lotgrade. If metal duct material is to be used, it must betreated to prevent rust and corrosion and completelyencased in a minimum of 2 in. of concrete grout. Fail-ure to treat the metal duct properly can lead to thefailure of the duct system due to rust and corrosion.In areas where the ground water table is high orproper drainage is not ensured, the collapse of theduct system can occur in as little as five years.

When this happens, the system usually must beabandoned and the supply duct system must beinstalled in some other location (e.g., in the ceiling).It also means filling the outlet boots with concreteand most likely replacing the flooring materials in thestructure. Needless to say, this is a very expensiveand time-consuming undertaking—one that is notpossible in some multistory structures without majorremodeling work. Sometimes the floor must beremoved completely so that repairs can be made.The duct system must be graded back toward thesupply air plenum for drainage and removal of anyground water that may enter the duct system. Thebest way to avoid such problems is to make sure thatthe design and installation are right the first time.

PVC duct materials offer a large advantage overmetal in this type of system installation. PVC ductsystems do not need to be encased in concrete grout

or treated for corrosion, but they still must be gradedback toward the plenum for ground water removaland installed above the final lot grade. A wide selec-tion of fittings, boots, and plenums constructed fromPVC materials is available today. When PVC ductmaterials are used, all joints are glued, thus creatinga water-resistant duct system. (Some codes do notallow for the use of screws in the assembly of PVCduct systems.)

Another factor that is sometimes a detractor to theembedded slab system is the code requirement stip-ulating that the duct system must be installed abovethe final grade. The builder may be required toincrease the foundation stem wall height from the nor-mal 16 in. to 20 to 24 in. to accommodate the ductsystem installation. In some areas, builders may resistthis additional expense in the cost of the structure.

The boot boxes and terminal devices used with anembedded concrete slab system should be locatedunder or near doors and windows. They must dis-charge into the unoccupied space of the room to pre-vent the primary airstream from coming in contactwith the room’s occupants. The number of outlets foreach room depends on the room’s usage, its physi-cal layout, cfm requirements, and the heating andcooling loads as determined by a room-by-room loadcalculation.

When a floor distribution system is used, it is alwaysa good idea to be mindful of furniture placement inthe room. The main goal of good system design is tohave the outlets discharging into the unoccupiedzone of the room. The “occupied zone” of a room isgenerally defined as the volume of space that existsbetween the floor and 6 ft above the floor in the ver-tical direction, and is 2 ft or more from the walls in thehorizontal direction (see Figure 9). Outlets shouldnot be placed where room furnishings will coverthem. This may require having multiple outlets insome rooms to ensure that the distribution air beingdelivered matches the load.

SUPPLY AIR OUTLET LOCATIONS

One of the most critical tasks in the design of an airdistribution system is the selection of the proper typeand proper placement of the supply outlets. Thedesigner must select locations that will deliver the

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conditioned air into the space to be conditioned insuch a way that the high heat gain/heat loss loadareas (doors and windows) are covered by the pri-mary airstream. In addition, the secondary air (roomair) must mix with the primary airstream to ensureeven temperatures throughout the room. The designshould accomplish these objectives while keepingthe primary airstream from entering the occupiedzone within the room.

By delivering the primary air into the unoccupiedzone of the room and mixing it with the secondary air,the designer ensures that the primary airstream doesnot come in direct contact with the room occupants.If the primary airstream is allowed to come in contactwith the room occupants, the result will be complaintsof drafts and the overall comfort of the occupants willsuffer. In most cases (depending on the location ofthe duct system and the location of the outlet), thisproblem is next to impossible to solve without theinvestment of major time and expense. The best timeto prevent this potential problem is when the systemis being designed and installed.

There are several specifications listed in the engi-neering tables for outlet devices that are very impor-tant to the designer. They include:

ª face velocity—the velocity of the air leaving theoutlet, measured in ft/min

ª cfm—the volume of air that the outlet devicedelivers as a result of the face velocity (“cfm”stands for cubic feet per minute)

ª noise criteria (NC)—a measure of the air noiseassociated with the outlet device

ª pressure loss—the amount of pressure lossassociated with the outlet device, measured in in. w.g.)

ª spread—the width of the primary air envelopemeasured at the point of terminal velocity

ª terminal velocity—the velocity of an airstream atthe end of its throw, measured in ft/min

ª throw—the distance from the face of the outletthat the air travels before reaching terminalvelocity

ª AK factor (effective area)—the calculated area ofthe outlet device based on the average measuredvelocity between the fins

ª drop—the vertical distance between the base ofthe outlet and the bottom of the airstream at theend of the horizontal throw.

In addition, you should be familiar with the followingterms:

ª diffuser—an outlet that discharges supply air in aspreading pattern

ª grille—a louvered covering for an openingthrough which air passes

ª register—a grille equipped with a damper or acontrol valve that directs air in a nonspreadingjet.

Floor locations

Grilles and registers installed in the floor should belocated in the unoccupied zone of the room. Theyshould be positioned to cover high load areas, suchas door and window locations. As mentioned previ-ously, consideration also must be given to the place-ment of furniture within the room. The selection of the

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Figure 9. “Occupied zone” of a room

proper grille or diffuser (outlet device) is one of thedesigner’s more critical tasks. The outlet device mustbe selected to deliver the required air volume (cfm) atthe correct velocity (ft/min) to condition the space.

Ceiling location (center of room)

There are basically two different types of ceiling out-let locations. The first is in the center of the room,with the directional pattern of the device being essen-tially circular. This type of installation may work wellfor cooling-only applications—however, it is one ofthe poorest selections for heating systems. In theheating season, higher velocities of the conditionedair must be attained in order to force the heated airdown into the conditioned space. Higher noise levelswill accompany the higher discharge velocitiesrequired from the outlet. If the winter design temper-ature is below 35°F, this type of delivery outlet deviceis not recommended for systems that will be used inthe heating season.

Ceiling location (modified)

The second type of ceiling outlet is one that is movedaway from the center of the room and out toward theexterior wall. The discharge is directed toward thehigh load areas of the room (doors and windows).Thelocation of the outlet must be determined by studyingthe manufacturer’s engineering data supplied for theselected outlet device.

The outlet device should be selected to deliver therequired amount of air (cfm) at the proper velocity(ft/min), with an acceptable throw to match the roomheat loss/heat gain. It is recommended that the pri-mary air envelope (air moving faster that 35 ft/min forheating and faster than 50 ft/min for cooling) from theoutlet not be allowed to come in contact with theroom occupants.

For example, let’s assume that the room cfm require-ment is 200 cfm. Due to the physical characteristicsof the room, it is determined that the best solution isto install two outlets to cover the high load areas(there is a large amount of glass in the room). Thedesigner selects two outlet devices, each of which willdeliver 100 cfm at a face velocity that meets the noisecriteria (for residential structures, normally NC35 orbelow). Depending on the outlet device selected, a

face velocity of 500 to 750 ft/min generally is accept-able for residential structures. The outlet devicesselected will deliver 100 cfm at 600 ft/min, with anoise level of NC30. The throw at these conditions isshown in engineering data to be 6.5 ft at a terminalvelocity of 35 ft/min for heating and 8.5 ft at 50 ft/minfor cooling. The terminal velocity listed for heating isthe most critical. Most people are more sensitive todrafts during the heating season, and more tolerantof air movement during the cooling season.

To locate the position of the outlet so that the primaryairstream does not come in contact with the roomoccupants, the designer’s first task is to make surethat the primary airstream is directed toward theunoccupied zone of the room. Chairs and other roomfurnishings sometimes can be placed in the unoccu-pied zone. The primary air envelope, therefore,should not move down the wall past the 4 to 5-ft level.For example, assume that the ceiling height is 8 ft.The terminal velocity for the selected outlet device is6.5 ft for heating. If you start at the 5-ft level on theexterior wall and measure up the wall and across theceiling 6.5 ft, the location of the outlet will be 3.5 ftfrom the wall to the center of the outlet. The primaryair envelope (air moving faster than 35 ft/min) will terminate at the 5-ft level on the exterior wall.

A ceiling supply system designed and installed in thismanner can be used successfully in areas where thewinter design temperatures are below 35°F. This out-let design approach can be utilized with all of the ductsystem types used in ceiling distribution systems.

High inside wall locations

A high inside wall outlet system delivers air into theconditioned space from the interior walls. The selec-tion of the outlet device, which is very similar to theprocedure described above for modified ceiling loca-tions, is critical to a successful system. This type ofdelivery normally is associated with fir-down ductsystems.

Again, the designer must select the outlet device thatwill deliver the required volume of air (cfm) at theproper velocity (ft/min) to condition the space. Thevelocity becomes even more critical with this type ofdelivery system, because the airstream must bedirected across the ceiling all the way to the exterior

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wall and part way down the wall to provide coverageof the high load areas in the room. If the room is 20 ft wide, the outlet device must be selected with athrow of more than 20 ft in order to guarantee that theprimary airstream will remain in the unoccupied por-tion of the room. If the selected device does not havesufficient throw, the primary airstream will drop intothe occupied zone of the room and comfort condi-tions will suffer. The throw and drop characteristicsare very important to the successful application ofthis type of supply outlet.

Unless a high-quality outlet device is used, noiseproblems with this type of delivery system can bevery common. The outlet device selected shouldhave adjustable deflection vanes and should beequipped with a damper for volume control and bal-ancing (a register). The noise criteria specificationsfor the register should be in the range of NC30 to 35.

RETURN AIR DUCT SYSTEMS

The return air duct system is just as important as thesupply system, and too often it is largely ignored. Inmany cases, the return system consists of one or twoimproperly sized openings connected back to the airhandler through improperly sized return ducts orthrough wall, floor, and ceiling framing cavities.Although panned framing members can be used forreturn systems, they generally should be avoided.These types of installations are next to impossible toseal. Air leaking into the system can cause additionalsensible and latent loads on the HVAC system andlead to comfort complaints from the occupants of thestructure.

Duct leakage

If duct leakage occurs in the attic space during thecooling season, warm, moist air can be pulled intothe return, resulting in an increase in both the latentload and the sensible load on the HVAC system. Thisinfiltration into the return system will be aggravated ifthe return system is undersized. For example, con-sider a 3-ton system that requires 450 cfm per ton forcooling (1,350 cfm). Assume that the return isimproperly sized and can handle only 1,000 cfm. Dueto the undersizing of the return and the available sta-tic pressure of the blower, the blower will try to pull airfrom the attic space if leaks are present in the duct

system. If the leaks are large enough in the returnsystem, the 350-cfm shortfall will be drawn from thehigh-temperature air in the attic. In some areas, attictemperatures can reach 140 to 150°F db (dry bulb)even if the attic is vented properly. Under these con-ditions, the mixed air temperature entering the evap-orator coil will be 91.9 to 96.9°F db, instead of thenormal 75 to 80°F db. For most systems, the ∆Tacross the evaporator is 18 to 20°F db, which meansthat in this case the air leaving the evaporator will be73.9 to 76.9°F db instead of the normal 55 to 60°F db.If the relative humidity of the air in the attic is high, italso will increase the latent load on the evaporatorand degrade the system’s capacity to maintain thedesired comfort conditions in the structure.

As a general rule, when framing members are usedas part of the return air duct system, the followingapproximate cfm values are recommended:

2 × 4s on 16-in. centers = 150 cfm2 × 4s on 24-in. centers = 200 cfm2 × 6s on 16-in. centers = 210 cfm2 × 6s on 24-in. centers = 340 cfm2 × 8s on 16-in. centers = 340 cfm2 × 8s on 24-in. centers = 560 cfm2 × 10s on 16-in. centers = 500 cfm2 × 10s on 24-in. centers = 800 cfm2 × 12s on 16-in. centers = 700 cfm2 × 12s on 24-in. centers = 1,020 cfm

The approximate cfm values listed here are based onthe true net free areas. They do not take into accountthe complications encountered with sealing cavitiesagainst air infiltration and other problems discussedabove.

If the return air duct system is located in the crawlspace of the structure and panned joists are used forthe return system, duct leakage can add a largelatent load on the HVAC system, resulting in reducedsensible heat removal capacity and increased oper-ating costs for the homeowner. Complaints about thecomfort conditions within the structure are also likely.

As you can see, a properly sized, sealed return ductsystem is just as important as a properly sized,sealed supply duct system in ensuring that the com-fort conditions within the structure are maintained inan economical manner.

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CENTRAL RETURN SYSTEMS

Central return systems can be applied in smallerstructures with success. These systems consist ofone or more return inlets, strategically located withinthe structure and sized to return the total amount ofthe supplied air volume to the indoor blower. Both theinlet box and the return connecting ductwork shouldbe sized where the room air will flow back to theblower at a relatively low velocity. As emphasizedabove, the return duct system must be free of leaks.It is sometimes desirable to have some turns or otherfittings in the duct run in order to prevent blower noisefrom being transmitted down the duct and exiting thereturn air grille.The return air box should be lined withsound attenuation insulation and insulated on theoutside to prevent or retard heat gain and heat loss.

Air delivered to all rooms of the structure should havean unrestricted path back to the return grille. Thismeans that closed doors must not isolate a room.The most common method seen in constructiontoday is to undercut the doors, thus allowing air topass under a door if it is closed. The amount of theundercut is determined by using the same methodsas those used to size the return grille or the connect-ing return duct—that is, when the door is closed,there must be adequate free area to ensure that theroom does not become pressurized. If a room isallowed to become pressurized, the delivery of thesupply air will be impeded, comfort conditions in theroom will suffer, and the exfiltration of room air willoccur. Assume, for example, that the room in ques-tion has a cfm requirement of 200 cfm and a 32-in.entry door. Based on a design friction loss factor forthe return system of 0.05 in. w.g., the opening at thebottom of the door would need to be approximately 3 in. of “free” space (excluding any floor coverings) in

order to ensure a proper return air flow from theroom. A better solution would be to install a bootsleeve above the door and nonvision (non-see-through) transfer grilles on either side, or install non-vision transfer grilles in the door itself (a 12-in. × 8-in.transfer grille would be required in this case).

Larger structures require multiple returns in order toachieve satisfactory results. Each of the returns mustbe sized to return the amount of air delivered to therooms or zones where the return grille is located. It isworth repeating that the return system must be capable of handling the same amount of air as thatdelivered by the supply side of the system.

INDIVIDUAL ROOM RETURN SYSTEMS

In some structures it may be desirable to install indi-vidual room returns. Of course, this means that thereturn system is just as elaborate as the supply sys-tem. Each individual room return is sized to return thesame amount of air that is delivered to the room bythe supply system.The results are excellent if the lay-out and sizing are done properly—in fact, there is noother return system that performs as well as the indi-vidual room return system. The major drawback tothis approach is the initial cost.

SUMMARY

It is the goal of this document to offer an overview ofthe general principles of duct system design and tocall attention to some of the special considerationsthat need to be given to the process to ensure a sat-isfactory result. The ultimate objective is to provide adependable system that not only serves the needs ofthe consumer, but also increases profits for theinstalling contractor by reducing callbacks.

Refrigeration Service Engineers Society1666 Rand Road Des Plaines, IL 60016 847-297-6464