ASHRAE FUNDAMENTALS IP CH 17-2013 Residential Cooling and Heating Load Calculations.pdf

1、17.1CHAPTER 17 RESIDENTIAL COOLING AND HEATING LOAD CALCULATIONSResidential Features. 17.1Calculation Approach 17.1Other Methods . 17.2Residential Heat Balance (RHB) Method 17.2Residential Load Factor (RLF) Method. 17.2Common Data and Procedures 17.3Cooling Load 17.8Heating Load 17.11Load Calculatio

2、n Example. 17.12Symbols 17.14HIS chapter covers cooling and heating load calculationTprocedures for residential buildings, including detailed heat-balance methods that serve as the basis for cooling load calculation.Simple cooling load procedures, suitable for hand calculations, areprovided for typi

3、cal cases. Straightforward heating load calculationprocedures are also included.Procedures in this chapter are based on the same fundamentals asthe nonresidential methods in Chapter 18. However, many charac-teristics distinguish residential loads, and Chapter 18s proceduresshould be applied with car

4、e to residential applications.Additional information about residential heating and cooling isfound in Chapter 1 of the 2011 ASHRAE HandbookHVAC Appli-cations and Chapter 10 of the 2012 ASHRAE HandbookHVACSystems and Equipment.RESIDENTIAL FEATURESWith respect to heating and cooling load calculation a

5、nd equip-ment sizing, the following unique features distinguish residencesfrom other types of buildings:Smaller Internal Heat Gains. Residential system loads are pri-marily imposed by heat gain or loss through structural componentsand by air leakage or ventilation. Internal heat gains, particularlyt

6、hose from occupants and lights, are small compared to those incommercial or industrial structures.Varied Use of Spaces. Use of spaces in residences is more flexiblethan in commercial buildings. Localized or temporary tempera-ture excursions are often tolerable.Fewer Zones. Residences are generally c

7、onditioned as a singlezone or, at most, a few zones. Typically, a thermostat located inone room controls unit output for multiple rooms, and capacitycannot be redistributed from one area to another as loads changeover the day. This results in some hour-to-hour temperature vari-ation or swing that ha

8、s a significant moderating effect on peakloads, because of heat storage in building components.Greater Distribution Losses. Residential ducts are frequentlyinstalled in attics or other unconditioned buffer spaces. Duct leak-age and heat gain or loss can require significant increases in unitcapacity.

9、 Residential distribution gains and losses cannot beneglected or estimated with simple rules of thumb.Partial Loads. Most residential cooling systems use units of rel-atively small capacity (about 12,000 to 60,000 Btu/h cooling,40,000 to 120,000 Btu/h heating). Because loads are largely deter-mined

10、by outdoor conditions, and few days each season are designdays, the unit operates at partial load during most of the season;thus, an oversized unit is detrimental to good system performance,especially for cooling in areas of high wet-bulb temperature.Dehumidification Issues. Dehumidification occurs

11、during cool-ing unit operation only, and space condition control is usually lim-ited to use of room thermostats (sensible heat-actuated devices).Excessive sensible capacity results in short-cycling and severelydegraded dehumidification performance.In addition to these general features, residential b

12、uildings can becategorized according to their exposure:Single-Family Detached. A house in this category usually hasexposed walls in four directions, often more than one story, and aroof. The cooling system is a single-zone, unitary system with asingle thermostat. Two-story houses may have a separate

13、 coolingsystem for each floor. Rooms are reasonably open and generallyhave a centralized air return. In this configuration, both air andload from rooms are mixed, and a load-leveling effect, whichrequires a distribution of air to each room that is different from apure commercial system, results. Bec

14、ause the amount of air sup-plied to each room is based on the load for that room, proper loadcalculation procedures must be used.Multifamily. Unlike single-family detached units, multifamilyunits generally do not have exposed surfaces facing in all direc-tions. Rather, each unit typically has a maxi

15、mum of three exposedwalls and possibly a roof. Each living unit has a single unitarycooling system or a single fan-coil unit and the rooms are rela-tively open to one another. This configuration does not have thesame load-leveling effect as a single-family detached house.Other. Many buildings do not

16、 fall into either of the precedingcategories. Critical to the designation of a single-family detachedbuilding is well-distributed exposure so there is not a short-duration peak; however, if fenestration exposure is predominantlyeast or west, the cooling load profile resembles that of a multifam-ily

17、unit. On the other hand, multifamily units with both east andwest exposures or neither east nor west exposure exhibit load pro-files similar to single-family detached.CALCULATION APPROACHVariations in the characteristics of residences can lead to surpris-ingly complex load calculations. Time-varying

18、 heat flows combineto produce a time-varying load. The relative magnitude and patternof the heat flows depends on the building characteristics and expo-sure, resulting in a building-specific load profile. In general, an hour-by-hour analysis is required to determine that profile and find itspeak.In

19、theory, cooling and heating processes are identical; a commonanalysis procedure should apply to either. Acceptable simplificationsare possible for heating; however, for cooling, different approachesare used.Heating calculations use simple worst-case assumptions: no solaror internal gains, and no hea

20、t storage (with all heat losses evaluatedinstantaneously). With these simplifications, the heating problem isreduced to a basic UAt calculation. The heating procedures in thisThe preparation of this chapter is assigned to TC 4.1, Load Calculation Dataand Procedures.17.2 2013 ASHRAE HandbookFundament

21、alschapter use this long-accepted approach, and thus differ only indetails from prior methods put forth by ASHRAE and others.The cooling procedures in this chapter were extensively revised in2005, based on the results of ASHRAE research project RP-1199,also supported by the Air-Conditioning Contract

22、ors of America(ACCA) (Barnaby et al. 2004, 2005). Although the complexity of res-idential cooling load calculations has been understood for decades,prior methods used a cooling load temperature difference/coolingload factor (CLTD/CLF) form requiring only hand-tractable arith-metic. Without such simp

23、lification, the procedures would not havebeen used; an approximate calculation was preferable to none at all.The simplified approaches were developed using detailed computermodels and/or empirical data, but only the simplifications were pub-lished. Now that computing power is routinely available, it

24、 is appro-priate to promulgate 24 h, equation-based procedures.OTHER METHODSSeveral residential load calculation methods have been publishedin North America over the last 30 years. All use the UAt heatingformulation and some variation of the CLTD/CLF approach forcooling.ACCA. Manual J, 8th edition (

25、ACCA 2011) is widely used in theUnited States. Cooling loads are calculated using semiempiricalheat gain factors derived from experimental data taken at the Uni-versity of Illinois in the 1950s. These factors, associated over-view, and references are found in the 1985 and earlier editions ofthe ASHR

26、AE HandbookFundamentals. The 8th edition retainsthe underlying factors but provides increased flexibility in theirapplication, in addition to other extensions.ASHRAE. The 1989 to 2001 editions of the ASHRAE Hand-bookFundamentals contain an updated method based onASHRAE research project RP-342 (McQui

27、ston 1984). In thiswork, cooling factors were re-derived using a transfer-functionbuilding model that included temperature-swing effects.F280. This Canadian adaptation of the CLTD/CLF procedure(CAN/CSA Standard F280) also uses cooling methods based onASHRAE RP-342. Heating procedures include detaile

28、d groundheat loss estimates.A key common element of all cooling methods is attention totemperature swing, via empirical data or suitable models. Through-out the literature, it is repeatedly emphasized that direct applicationof nonresidential methods (based on a fixed set point) results inunrealistic

29、ally high cooling loads for residential applications.RESIDENTIAL HEAT BALANCE (RHB) METHODA 24 h procedure is required to accurately determine the coolingload profile of a residence. The heat balance (HB) method allowsdetailed simulation of space temperatures and heat flows. ASHRAEresearch project R

30、P-1199 adapted HB to residential applications,resulting in the residential heat balance (RHB) method. AlthoughRHB provides the technical basis for this chapter, it is a computer-only technique and is not documented here. HB is described inChapter 18 and Pedersen et al. 1998; Barnaby et al. (2004, 20

31、05)document RHB enhancements.RP-1199 produced an implementation of the RHB method,called ResHB (Barnaby et al. 2004). This application is derivedfrom the ASHRAE Toolkit for Building Load Calculations (Peder-sen et al. 2001) and has the following features:Multizone. Whereas the original Toolkit code

32、supported a singlezone, ResHB can analyze projects that include multiple systems,zones, and rooms.Temperature swing. ResHB calculates cooling load with tem-perature swing. That is, the code searches for sensible capacitysufficient to hold the space temperature within a specified excur-sion above the

33、 set point.Master/slave control. ResHB allows control of cooling output in“slave” rooms based on the cooling requirements of a “master”room, where the thermostat is located. Rooms with incompatibleload profiles will exhibit poor temperature control.Residential defaults. ResHB includes default values

34、 suitable forresidential problems.In its current form, ResHB is a research-oriented referenceimplementation of RHB. ResHB FORTRAN source code is avail-able under license from ASHRAE.RESIDENTIAL LOAD FACTOR (RLF) METHODThe procedure presented in this chapter is the residential loadfactor (RLF) method

35、. RLF is a simplified procedure derived fromdetailed ResHB analysis of prototypical buildings across a range ofclimates. The method is tractable by hand but is best applied usinga spreadsheet. Two main applications are anticipated:Education and training. The transparency and simplicity of RLFmake it

36、 suitable for use in introductory courses on building loadcalculations.Quick load estimates. In situations where detailed analysis isimpractical, the RLF method is a possible alternative. For exam-ple, the method might be implemented as a spreadsheet on a hand-held device and used for on-site sizing

37、 of replacement coolingequipment.Note that, although room-by-room calculations are possible withthe RLF method, computerized methods based on RHB are moresuitable for performing full room-level calculations required forequipment selection and distribution system design.RLF was derived from several t

38、housand ResHB cooling loadresults (Barnaby and Spitler 2005; Barnaby et al. 2004). A range ofclimates and building types were analyzed. Statistical regressiontechniques were used to find values for the load factors tabulated inlater sections. Factor values were validated by comparing ResHBversus RLF

39、 results for buildings not involved in the regressionanalysis. Within its range of applicability, RLF cooling loads aregenerally within 10% of those calculated with ResHB. The RLFderivation was repeated for 2009 using the updated temperature pro-file and clear-sky model (see Chapter 14), resulting i

40、n minor revi-sions to load factors and other coefficients. Additional revisions toChapter 14 occurred in 2013; those changes would alter RLF valuesvery little, so the 2009 factors are retained.The RLF method should not be applied to situations outside therange of underlying cases, as shown in Table

41、1.Note that the RLF calculation sequence involves two distinctsteps. First, the cooling and heating load factors (CFs and HFs) arederived for all project component types. These factors are then ap-plied to the individual components by a single multiplication. (Thetwo-step approach is demonstrated in

42、 the Load Calculation Exam-ple section.) For a specific location and representative construc-tions, CFs and HFs can be precalculated and used repeatedly. Inessence, the structure of RLF allows assembling location-specificversions of the rigid tables found in prior editions, and also docu-ments the e

43、quations used to generate tabulated values. Using theseequations, a complete implementation of the RLF method, includ-ing CF and HF calculation, is well within the capabilities of currentPC spreadsheet applications.Residential Cooling and Heating Load Calculations 17.3COMMON DATA AND PROCEDURESThe f

44、ollowing guidelines, data requirements, and proceduresapply to all load calculation approaches, whether heating or cooling,hand-tractable or computerized.General GuidelinesDesign for Typical Building Use. In general, residential sys-tems should be designed to meet representative maximum-loadconditio

45、ns, not extreme conditions. Normal occupancy should beassumed, not the maximum that might occur during an occasionalsocial function. Intermittently operated ventilation fans should beassumed to be off. These considerations are especially important forcooling-system sizing.Building Codes and Standard

46、s. This chapter presentation isnecessarily general. Codes and regulations take precedence; consultlocal authorities to determine applicable requirements.Designer Judgment. Designer experience with local conditions,building practices, and prior projects should be considered whenapplying the procedure

47、s in this chapter. For equipment-replacementprojects, occupant knowledge concerning performance of the exist-ing system can often provide useful guidance for achieving a suc-cessful design.Verification. Postconstruction commissioning and verificationare important steps in achieving design performanc

48、e. Designersshould encourage pressurization testing and other procedures thatallow identification and repair of construction shortcomings.Uncertainty and Safety Allowances. Residential load calcula-tions are inherently approximate. Many building characteristics areestimated during design and ultimat

49、ely determined by constructionquality and occupant behavior. These uncertainties apply to all cal-culation methods, including first-principles procedures such asRHB. It is therefore tempting to include safety allowances for eachaspect of a calculation. However, this practice has a compoundingeffect and often produces oversized results. Typical conditionsshould be assumed; safety allowances, if applied at all, should beadded to the final calculated loads rather than to intermediate com-ponents. In addition, temperature swing provides a built-in safetyfactor for sensible cooling: a 20