DUCT SYSTEM DESIGN CONSIDERATIONS - RSES.org

1 INTRODUCTIONOne of the most important components of any HVAC SYSTEM is the duct SYSTEM . The layout and sizing ofthe ductwork and the selection and location the ter-minal devices (grilles, registers, and diffusers) are crit-ical to the overall SYSTEM performance. The bestequipment manufactured will produce unsatisfactorycomfort control if it is installed with a faulty or poorlydesigned duct SYSTEM . In some cases, HVAC equip-ment failure can result from a poorly designed orinstalled duct SYSTEM . The duct SYSTEM is made up ofboth the supplysystem and the returnsystem. Eachof these duct SYSTEM components is equally importantin ensuring that the installed HVAC SYSTEM will pro-vide the proper control of temperature and humidity tofulfill the comfort needs of the building s must be the first priority of the duct systemdesigner to make sure that his or her DESIGN is capa-ble of delivering the proper amount of air (cfm), at theproper velocity (ft/min), through a properly selectedterminal device (grille, register, or diffuser) to condi-tion each room or zone of the structure properly forthe occupant s comfort.

2 The only valid method ofdetermining the room cfm requirements is to com-plete a room-by-room heat gain and heat loss calcu-lation, such as an ACCAM anual J load conditioned air also must be delivered to theroom or zone in such a manner that the primaryairstream does not come in contact with the primary airstream should be deliv-ered into the unoccupied zone of each room (seeFigure 1). On the surface this may sound like a tallorder, but it can be accomplished by paying closeattention to the details and following a few simplerules of good duct SYSTEM layout and SYSTEM LAYOUT CONSIDERATIONSY oumust take into account the physical layout andcharacteristics of the structure s floor plan whendeciding on the type and location of the duct systemto best condition the space.

3 The possibility of usingmultiple systems (two or more), or an individuallycontrolled, zoned SYSTEM within the structure mayneed to be single-floor structuresStructures with less than 1,500 to 1,800 ft2and rela-tively simple floor plans lend themselves well to theutilization of one SYSTEM , as long as the indoor airhandler or furnace is located as close as possible tothe geographic center of the structure. However, if thefloor plan is complicated or if the indoor air handler orfurnace location necessitates extremely long ductruns, either zoning or the use of two or more systemsmay be desirable.

4 Figure 2 on page 3 shows a smallsingle-floor residence. The indoor section of the sys-tem (H/A) is located near the geographic center of thestructure. Note also the location of the thermostat (T).1 Refrigeration ServiceEngineers Society1666 Rand RoadDes Plaines, Illinois 60016 duct SYSTEM DESIGN CONSIDERATIONSPart 2by Roger M Hensley, CMS 2005 by the Refrigeration Service Engineers Society, Des Plaines, ILSupplement to the Refrigeration Service Engineers 11 AFigure 1. Occupied zone of a roomLarge single-floor structuresEven a structure with more than 1,800 to 2,500 ft2maybe served by a single SYSTEM if the indoor unit can be located near the geographic center of the struc-ture and the floor plan is relatively simple.

5 Figure 3 onpage 4 shows a roughly 2,500-ft2structure with a single ceiling distribution SYSTEM . Note the locationsof the indoor unit, the thermostat, and the supply andreturn ducts. Structures like this one also lend them-selves very successfully to systems that employ indi-vidual zone controls and zone dampers. Figure 4 onpage 5 shows the same floor plan as Figure 3, butwith the addition of zone controls and zone , rambling floor plans present challenges for thesystem physical layout of such a struc-ture even if the indoor air handler or furnace can belocated near the geographic center of the structure often necessitates long duct 5 on page 6shows a large residence (approximately 2,800 ft2)with a single floor distribution SYSTEM .

6 The long ductruns mean that other factors, such as increased resis-tance to air flow and increased heat gain and heatloss within the duct SYSTEM , must be taken running cycles will be required to sat-isfy the desired comfort conditions in the parts of thestructure served by long supply and return duct conditions in the rooms that are the farthestaway from the indoor unit can suffer. In a typical sce-nario (depending on the location of the control ther-mostat), the rooms closer to the indoor unit and thethermostat will achieve the desired comfort tempera-ture before the rooms that are supplied by longerduct runs.

7 Look at Figure 6 on page 7, for example. Itshows the same floor plan as Figures 3 and 4. Againthere is a single SYSTEM , this time with floor distribu-tion and ceiling returns. But note that the indoor unithas been moved to the garage. The result is a varietyof long and short duct runs. Customer complaints inthese situations include rooms that are cooler thanthe thermostat setpoint during the heating seasonand warmer than the thermostat setpoint during thecooling season. Large temperature gradients fromroom to room within a structure should be avoided. Incomfort conditioning, it is desirable to maintain thetemperature across the various rooms and zoneswithin 3 the type of structure and floor plan shown in Figure 6, consideration should be given to zoning thestructure with individual zone controls (as in Figure 4)or utilizing multiple systems (see Figure 7 on page 8).

8 The use of a zoned SYSTEM with individual zone con-trols allows the proper amount of supply air to bedirected into each zone based on the zone s individ-ual heating and cooling requirements. The use ofmultiple systems results in shorter duct runs and pro-vides for better comfort control within the individualrooms of the structuresUsing two or more systems in multistory structures isoften a wise decision. Due to the natural effect ofwarmair rising and cool air falling, trying to utilizeone SYSTEM to condition a multistory structure canresult in unsatisfactory performance and lead to cus-tomer dissatisfaction and theheating season, the upper floor will tend to overheatbecause the warm air that is being delivered to thelower floor will migrate to the upper floor by naturalconvection currents within the structure.

9 The lowerfloor will tend to be cooler for the same reason thewarmair is rising to the upper level of the the cooling season, too, the upper floor will bewarmer and the lower floor will be tem-perature control of both upper and lower floors canbe extremely hard to achieve. The use of a designthat employs two or more systems can help over-come this problem. Systems that utilize individualzone controls and either multispeed or variable-speed fans, blowers, and compressors (or a combi-nation of these components) can be used verysuccessfully in multistory problem of designing an air distribution systemfor a particular structure can lead to a variety of dif-ferent solutions.

10 There is rarely a single right systemfor any given structure although there may be many wrong ones! The challenge for the designer is toselect the best SYSTEM type for each application. Itmust be the goal of the installer to install the systemas designed and installed airdistribution SYSTEM for the structure will significantlyreduce customer complaints and 3. Large single-floor residence with overhead distribution SYSTEM and indoor unit near center of structure5 Figure 4. Large single-floor residence with zoned overhead distribution system6 Figure 5. Large single-floor residence with rambling floor plan7 Figure 6.