1、10.1CHAPTER 10SMALL FORCED-AIR HEATING AND COOLING SYSTEMSComponents. 10.1Common System Problems 10.2System Design . 10.3Detailed Duct Design 10.4Small Commercial Systems . 10.8Testing for Duct Efficiency. 10.9HIS chapter describes the basics of design and componentTselection of small forced-air hea
2、ting and cooling systems,explains their importance, and describes the systems parametriceffects on energy consumption. It also gives an overview of testmethods for thermal distribution system efficiency, and considersthe interaction between the building thermal/pressure envelope andthe forced-air he
3、ating and cooling system, which is critical to theenergy efficiency and cost-effectiveness of the overall system. Thischapter pertains to residential and certain small commercial sys-tems; large commercial systems are beyond the scope of this chapter.Applicable standards includeAHRI Standard 260AHRI
4、 Standard 610ASHRAE Standard 90.2ASHRAE Standard 103ASHRAE Standard 152ASTM Standard E1554NFPA Standard 90BNFPA Standard 501COMPONENTSForced-air systems are heating and/or cooling systems that usemotor-driven blowers to distribute heated, cooled, and otherwisetreated air to multiple outlets for the
5、comfort of individuals inconfined spaces. A typical residential or small commercial systemincludes (1) a heating and/or cooling unit, (2) supply and returnductwork (including registers and grilles), (3) accessory equipment,and (4) controls. These components are described briefly in the fol-lowing se
6、ctions and are illustrated in Figure 1.Heating and Cooling UnitsThree types of forced-air heating and cooling devices are (1) fur-naces, (2) air conditioners, and (3) heat pumps.Furnaces are the basic component of most forced-air heating sys-tems (see Chapter 33). They are manufactured to use specif
7、ic fuelssuch as oil, natural gas, or liquefied petroleum gas, and are aug-mented with an air-conditioning coil when cooling is included. Thefuel used dictates installation requirements and safety considerations.Common air-conditioning systems use a split configurationwith an air-handling unit, such
8、as a furnace. The air-conditioningevaporator coil (indoor unit) is installed on the discharge air side ofthe air handler. The compressor and condensing coil (outdoor unit)are located outside the structure, and refrigerant lines connect theoutdoor and indoor units.The preparation of this chapter is a
9、ssigned to TC 6.3, Central Forced AirHeating and Cooling Systems.Fig. 1 Heating and Cooling Components10.2 2012 ASHRAE HandbookHVAC Systems and EquipmentSelf-contained air conditioners contain all necessary air-conditioning components, including circulating air blowers, and mayor may not include fue
10、l-fired heat exchangers or electric heatingelements.Heat pumps cool and heat using the refrigeration cycle. Theyare available in split and packaged (self-contained) configura-tions. Generally, air-source heat pumps require supplemental heat-ing; therefore, electric heating elements are usually inclu
11、ded withthe heat pump as part of the forced-air system. Heat pumps offerhigh efficiency at mild temperatures, but may be combined withfossil-fuel furnaces to minimize heating cost. Heat pump supple-mental heating also may be provided by thermostat-controlled gasheating appliances (e.g., fireplaces,
12、free-standing stoves).Ground-source heat pumps (GSHPs) are becoming more com-mon in residential housing, especially in colder climates. Becauseunderground temperatures are mild year-round, GSHPs typically donot use supplemental heating except in emergency mode (i.e., whenthe heat pump does not provi
13、de enough heat).DuctsDucts convey air to and from the fan in a heating or cooling unit.Registers and grilles are perforated covers over the openings whereductwork meets room walls, ceilings, or floors. In the extreme, a sin-gle return grille may connect directly to the fan cabinet. Supply reg-isters
14、 often allow control of flow volume and direction. Duct systemscan have very significant impacts on system efficiency and occupantcomfort, and should be carefully designed, not just constructed byrules of thumb.Accessory EquipmentForced-air systems may be equipped with accessories that fur-ther cond
15、ition the air. They may modify humidity, remove contam-inants, mix outdoor air with the recirculating air, or transfer energyin other ways.Humidifiers. Humidifiers add moisture to the airstream directlyas steam or an atomized spray, or by evaporation from heated pansor porous media. Chapter 22 of th
16、is volume contains more informa-tion on humidifiers.Dehumidifiers. Dehumidifiers remove moisture from the air-stream, typically by cooling it below the condensation point. Alsosee the section on Dehumidifiers in Chapter 1 of the 2011 ASHRAEHandbookHVAC Applications.Electronic Air Cleaners. Far more
17、effective than the passive fil-ters normally found in forced-air systems, these units use static elec-tricity to capture fine dust, smoke, and other particles. Electronic aircleaners usually have a washable prefilter to trap lint and largerparticles as they enter the unit. The remaining particles ta
18、ke on anelectric charge in a charging section, then travel to the collector sec-tion where they are drawn to and trapped on the oppositely chargedcollecting plates. These plates must be washed periodically. SeeChapter 33 for more information on air cleaners.Ultraviolet Lamp Sterilizers. Short-wavele
19、ngth ultraviolet lampsin the fan cabinet can kill organisms in the circulating air and pre-vent mold build-up on the wet cooling coil. See Chapter 17 fordetails.Energy/Heat Recovery Ventilators. These devices provideventilation air to the conditioned space and recover energy/heatfrom the air being e
20、xhausted outdoors. They can be operated asstand-alone devices or as part of the forced-air distribution system.Economizer Controls. These devices monitor outdoor tempera-ture and humidity and automatically shut down the air-conditioningunit when a preset outdoor condition is met. Damper motors openo
21、utdoor return air dampers, letting outdoor air enter the system toprovide comfort cooling. When outdoor air conditions are no longeracceptable, the outdoor air dampers close and the air-conditioningunit comes back on.Custom Accessories. Solar collectors, off-peak storage, andother custom systems are
22、 not covered in this chapter. However, theircomponents may be classified as system accessories.ControlsA simple thermostat controlling on/off cycling of central equip-ment may be all that is used or needed for temperature control. Suchthermostats typically have a switch for automatic or continuous f
23、anoperation, and another to choose heating, cooling, or neither.More complex systems may provide control features for timedvariations (from simple night setback to a weekly schedule of tem-peratures); multiple independent zones, power stages, or fan speeds;influence of outdoor sensors; humidity; aut
24、omatic switching betweenheating and cooling modes, etc. (see Chapter 47 of the 2011 ASHRAEHandbookHVAC Applications). Energy conservation has increasedthe importance of control, so methods that once were considered tooexpensive for small systems may now be cost-effective.COMMON SYSTEM PROBLEMSAn ide
25、alized forced-air heating or cooling system would be vir-tually unnoticed by occupants. It would maintain a comfortableindoor environment at all times, with negligible noise, no noticeableodors, no circulating allergens or other pollution, and no localizedtemperature differences. In addition, it wou
26、ld need no maintenance,would never fail in any way, and would be extremely efficient.Real systems, of course, can vary from this ideal in every way.One of the most common problems is an oversized cooling sys-tem that does not need to run very long to cool the air, so it does nothave time to remove m
27、uch moisture. High humidity is uncomfort-able, and also can encourage mold growth.Too narrow ducts create high resistance to airflow that cancause excess power consumption, high pressures, and low flowrates, especially if combined with an oversized system. High pres-sures can increase noise from air
28、flow and from the fan itself. Lowflow rates can result in ice formation on the cooling coil (possiblyshutting off flow completely) and even on refrigerant piping; whenthat ice eventually melts, water can enter places that should stay dry.Long ducts running to perimeter walls not only increase resist
29、ance,but often run through unconditioned space where they can beexposed to extreme temperatures. Leaky ducts are, unfortunately,very common. Leaks not only reduce efficiency, they can draw inhumidity, dust, pollen, etc. Leaks also can allow relatively humidindoor air to leak out to cold surfaces whe
30、re the water can condense,causing staining, mold, rot, and other problems. In cold climates, anair leak in the attic can result in ice formation and the potential forsignificant damage. Ducts should be in the conditioned space.Poor installation practices primarily affect duct systems, butcondensate
31、removal also can be a problem area. Air leakage problemsas described previously, come from failure to seal properly all open-ings in an air-handling cabinet, joints in the duct system, joints whereducts meet register flares, and openings where register flares pene-trate the room. Crude duct-taped jo
32、ints sometimes come apart after afew years. Flexible ductwork is easy to install, but is also easy toinstall sloppily, with kinks and unnecessary bends that restrict flow.Cooling systems installed above ground level commonly have agravity drain for condensate, and a catch pan under the unit as aback
33、up, with its own gravity drain. Sometimes an installed drainthat is nearly horizontal actually runs uphill. If the main condensatedrain runs uphill, the problem becomes evident almost immediately.If the problem is with the catch pan drain, it may be years before amajor freeze-up generates enough wat
34、er to overflow the pan andcause significant damage. Over time, mold and dirt can build upenough to block a drain line. Periodic cleaning may be necessary.Dirty filters are common because homeowners neglect to cleanor replace them. They are not merely an air cleanliness problem;they can cut down airf
35、low enough to raise fan power and even causecooling coils to freeze up.Small Forced-Air Heating and Cooling Systems 10.3Air-source heat pumps in colder climates typically are equippedwith auxiliary electric heat strips. Since resistance heating normallyis much less efficient than a heat pump, less-s
36、ophisticated controlsystems can result in unnecessary electric resistance heating andhigher electric bills. The resistance heating is necessary when con-tinuous operation of the heat pump is insufficient to maintain indoortemperature; however, it may also come on when the thermostat set-ting is rais
37、ed (manually, or automatically after night setback). Con-trol systems may turn on resistance heating each time the system isreversed to defrost the outdoor coil, in order to prevent indoor coldblow.This is by no means an exhaustive list of the types of problems towatch out for in forced air heating
38、and cooling systems, but it shouldalert designers to some of the most common ones.SYSTEM DESIGNThe size and performance characteristics of components areinterrelated, and the overall design should proceed in the organizedmanner described. For example, furnace selection depends on heatgain and loss a
39、nd is also affected by duct location (attic, basement,etc.), duct materials, night setback, and humidifier use. Here is arecommended procedure:1. Estimate heating and cooling loads, including target values forduct losses.2. Determine preliminary ductwork location and materials ofductwork and outlets
40、.3. Determine heating and cooling unit location.4. Select accessory equipment. Accessory equipment is not gen-erally provided with initial construction; however, the systemmay be designed for later addition of these components.5. Select control components.6. Select heating/cooling equipment.7. Deter
41、mine maximum airflow (cooling or heating) for each sup-ply and return location.8. Determine airflow at reduced heating and cooling loads (two-speed and variable-speed fans).9. Finalize heating/cooling equipment.10. Finalize control system.11. Finalize duct design and size.12. Select supply and retur
42、n grilles.13. When the duct system is in place, measure duct leakage andcompare results with target values used in step 1.This procedure requires certain preliminary information such aslocation, weather conditions, and architectural considerations. Thefollowing sections cover the preliminary conside
43、rations and discusshow to follow this recommended procedure.Estimating Heating and Cooling LoadsDesign heating and cooling loads can be calculated by followingthe procedures outlined in Chapters 17, 18, and 19 of the 2009ASHRAE HandbookFundamentals When calculating designloads, heat losses or gains
44、from the air distribution system must beincluded in the total load for each room. In residential applications,local codes often require outdoor air ventilation, which is added tothe building load. Target values for duct losses may be set by codes,voluntary programs, or other recommendations. If duct
45、s are locatedin the conditioned space, losses can be reduced essentially to zero.If this is not possible, losses should be limited to 10% of the heatingor cooling load.Locating Outlets, Returns, Ducts, and EquipmentThe characteristics of a residence determine the appropriate typeof forced-air system
46、 and where it can be installed. The presence orabsence of particular areas in a residence directly influences equip-ment and duct location. The structures size, room or area use, andair-distribution system determine how many central systems will beneeded to maintain comfort temperatures in all areas
47、.For maximum energy efficiency, ductwork and equipmentshould be installed in the conditioned space. ASHRAE Standard90.2 gives a credit for installation in this location. The next bestlocation is in a full basement. If a residence has an insulated,unvented, and sealed crawlspace, the ductwork and equ
48、ipment canbe located there (with appropriate provision for combustion air, ifapplicable), or the equipment can be placed in a closet or utilityroom. Vented attics and vented crawlspaces are the least preferredlocations for ductwork and HVAC equipment. The equipmentsenclosure must meet all fire and s
49、afety code requirements; adequateservice clearance must also be provided. In a home built on a con-crete slab, equipment could be located in the conditioned space (forsystems that do not require combustion air), in an unconditionedcloset, in an attached garage, in the attic space, or outdoors. Duct-work normally is located in a furred space, in the slab, or in the attic.Cummings et al. (2003) tested air leakage in 30 air handler cabinetsand at connections to supply and return ductwork and found leakagerates averaged 6.3% of overall system airflow.Duct construction must conform to local
