HVAC Design Guide
Transcript of HVAC Design Guide
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CALIFORNIARESIDENTIALNEWCONSTRUCTION
HVACDESIGNGUIDE
CALIFORNIAENERGYCOMMISSION
DESIGNGUIDELINE
JULY2005CEC-500-2005-118-A2
ArnoldSchwarzenegger,Governor
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PreparedBy:
Dr.RobertHammonBuildingIndustryInstituteSacramento,CAContractNo.400-00-037
PreparedFor:
CaliforniaEnergyCommission
PublicInterestEnergyResearch(PIER)Program
MarthaBrook,
ContractManager
AnnPeterson,
PIERBuildingsProgramManager
NancyJenkins
OfficeManager
ENERGYEFFICIENCYRESEARCHOFFICE
MarthaKrebs,Ph.D.
DeputyDirector
ENERGYRESEARCHANDDEVELOPMENTDIVISION
B.B.Blevins,ExecutiveDirector
DISCLAIMER
ThisreportwaspreparedastheresultofworksponsoredbytheCaliforniaEnergyCommission.ItdoesnotnecessarilyrepresenttheviewsoftheEnergyCommission,itsemployeesortheStateofCalifornia.TheEnergyCommission,theStateofCalifornia,itsemployees,contractorsandsubcontractorsmakenowarrant,expressorimplied,andassumenolegalliabilityfortheinformationinthisreport;nordoesanypartyrepresentthattheusesofthisinformationwillnotinfringeuponprivatelyownedrights.ThisreporthasnotbeenapprovedordisapprovedbytheCaliforniaEnergyCommissionnorhastheCaliforniaEnergy
Commissionpassedupontheaccuracyoradequacyoftheinformationinthisreport.
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Preface
ThePublicInterestEnergyResearch(PIER)ProgramsupportspublicinterestenergyresearchanddevelopmentthatwillhelpimprovethequalityoflifeinCaliforniabybringingenvironmentallysafe,affordable,andreliableenergyservicesandproductstothemarketplace.
ThePIERProgram,managedbytheCaliforniaEnergyCommission(Commission),annuallyawardsupto$62milliontoconductthemostpromisingpublicinterestenergyresearchbypartneringwithResearch,Development,andDemonstration(RD&D)organizations,includingindividuals,businesses,utilities,andpublicorprivateresearchinstitutions.
PIERfundingeffortsarefocusedonthefollowingsixRD&Dprogramareas:
xBuildingsEnd-UseEnergyEfficiencyxIndustrial/Agricultural/WaterEnd-UseEnergyEfficiency..RenewableEnergyxEnvironmentally-PreferredAdvancedGenerationxEnergy-RelatedEnvironmentalResearchxEnergySystemsIntegration
WhatfollowsisanattachmenttothefinalreportfortheProfitability,Quality
,andRiskReductionthroughEnergyEfficiencyprogram,contractnumber400-00-037,conductedbytheBuildingsIndustryInstitute.ThisprojectcontributestothePIERBuildingEnd-UseEnergyEfficiencyprogram.Thisattachment,CaliforniaResidentialNewConstructionHVACDesignGuide"(Attachment2),providessupplementalinformationtotheprogramfinalreport.
FormoreinformationonthePIERProgram,pleasevisittheCommission'sWebsiteat:
http://www.energy.ca.gov/research/index.htmlorcontacttheCommission'sPublicationsUnitat916-654-5200.
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TableofContents
Abstract.....................................................................................................1
1.0Introduction...................................................................................................2
1.1Purpose.......................................................................................................................2
1.2TargetAudience..........................................................................................................3
1.3Limitations...................................................................................................................4
2.0TheDesignProcess......................................................................................5
2.1DesigninghousesaroundtheHVACsystem..............................................................5
2.2Coordinationwithothertrades....................................................................................7
3.0DesignMethodology.....................................................................................8
3.1CodeissuesrelatedtoHVACdesign..........................................................................8
3.1.1ACCAManualDrequiredby2000UMC............................................................................8
3.1.2Title24loadcalculations.....................................................................................................9
3.2ACCAManualsJ/S/D................................................................................................11
3.2.1TheOverallDesignMethod..............................................................................................11
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4.0SpecialDesignTopics................................................................................34
4.1FurnaceLocation.......................................................................................................34
4.2RegisterLocation......................................................................................................35
4.3MultipleOrientationDesigns.....................................................................................37
4.4ZonalControl.............................................................................................................43
4.5WindowLoads...........................................................................................................44
4.5.1Heatingloadsfromwindows.............................................................................................44
4.5.2Coolingloadsfromwindows.............................................................................................45
4.6DuctLoads................................................................................................................48
4.7Two-storyConsiderations..........................................................................................49
5.0OtherMechanicalDesignRelatedIssues.................................................51
5.1CondenserLocationsandRefrigerantLines.............................................................52
5.2FurnaceLocations(alsoseepreviousdiscussion)....................................................53
5.3AtticAccessLocations..............................................................................................54
5.4Flue(b-vent)locationsandrouting............................................................................55
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5.5Ductsizesandlocations(soffits,joistbays,chasesanddrops)..............................56
5.6DuctInstallation,Insulation,andLocation.................................................................57
5.6.1DuctSealing......................................................................................................................57
5.6.2DuctLocationandInsulation.............................................................................................57
5.7Combustionairsupply...............................................................................................58
5.8Thermostatlocation...................................................................................................59
5.9VentilationandIndoorAirQuality..............................................................................60
5.9.1IndoorAirQuality..............................................................................................................60
5.9.2VentilationSystems...........................................................................................................61
5.9.3VentilationandIndoorAirQualityStandard......................................................................61AppendixA:References&Resources.........................................................63AppendixB:Glossary....................................................................................64
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TableofFigures
Figure1:CeilingRegisterLocations..........................................................................................16Figure2:ExampleHousePlan...................................................................................................18Figure3:ExampleHVACDesign...............................................................................................19Figure4:ExampleVoidinInteriorStairChasewhichoftenoccursadjacenttoroundroomorstairways....................................................................................................................................20Figure5:ExampleVoidinDeadSpace.....................................................................................20Figure6:ExampleExteriorChase.............................................................................................21Figure7:Walk-InClosetwithInteriorChase.............................................................................21Figure8:ClosetChaseExample...............................................................................................22
Figure9:MediaChaseAgoodlocationforcreatingchasesisinamedianiche.......................22Figure10:WaterClosetChaseAnothergoodlocationforcreatingchasesisinawatercloset23Figure11:ChimneyChaseChasescanalsobeinchimneys,evenasfalsechimneys............23Figure12:RiserCanInstallation................................................................................................24Figure13:RiserCanDetail........................................................................................................26Figure14:FloorJoistDetail.......................................................................................................27Figure15:FloorTruss..........................................................
......................................................28Figure16:Duct-to-RegisterConnections.........................................
..........................................29Figure17:SoffitChase..............................................................................................................30Figure18:ON/OFFruntimesforthreecoolingconfigurationswithceilingreturns:supplyregisterinteriorceiling;ceilingoverwindows;andin-wall...........................................................36Figure19:SampleSitePlanwithVaryingOrientation...............................................................38Figure20:ComparisonofHVACCycleTimeforCase1,2and3.............................................50
Figure21:FAUClearance.........................................................................................................53
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TableofTables
Table1:MatrixofTrades.............................................................................................................7Table2:OrientationEffectonHeatTransferMultiplier..............................................................37Table3:SubdivisionSitePlanOrientation.................................................................................39Table4:Plan1LoadsandEquipmentSizing............................................................................39Table5:Plan2LoadsandEquipmentSizing............................................................................40Table6:Plan3LoadsandEquipmentSizing............................................................................40Table7:Branchductdiametersundermultipleorientations......................................................41
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Abstract
Adequatetoolsandmethodsnowexisttodesignenergy-efficientHVACsystems.FailuretocorrectlyapplytheminproductionhomescostsCaliforniahomeowners.Thismajormissedopportunityisafunctionofbothafaultydesignprocessandinaccessibilityofthedesignmethods.Thecost-centricdesign-buildprocesscommonlyemployedbyproductionbuildersrarelyincludesaskilledHVACdesignerearlyinthedevelopmentphasewheretheycanmosteffectivelyintegrateHVACrequirementswiththehousedesign.CurrentlyavailableHVACdesigntoolsandmethodsrequiretimeandhighlevelsofskill,whichnegativelyaffectsthecost/profitagenda.Amoreintegrateddesignprocessandsimplifieddesignmethodsareessentialtoimproveusage,increaseHVACdesignquality,andreduceHVACenergy
consumption.
Thisdesignguideisnotintendedtobeastep-by-stepinstructionbookonhowtodesignanHVACsystembecauseadequatemethodologiesalreadyexistforthat.Rather,itisintendedtobeastep-by-stepguideforclarifyingthosemethodologiesandintegratingthemintotheoveralldesignprocessforanentirehouse.ItalsoaddressesimportanttopicsparticularlyimportanttoCalifornia,andspecifictonew-constructionproductionhomes.
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1.0Introduction1.1PurposeThepurposeofthisDesignGuideis:
1.TobeausefultoolfortheplanningandimplementationofagoodresidentialHVACdesignprocessandtoassistduringthatprocess.2.Toencouragecoordinationbetweenkeyplayerssuchasthearchitect,builder,structuralengineer,framer,HVACdesigner,HVACinstaller,energyconsultant,electricaldesigner,andplumbertominimizeconflictsduringtheinstallationofaproperlydesignedsystem.3.Tohelpidentifyhowallofthedesigners,consultants,andtradespeopleareimpactedbytheprocessandhowtheyneedtocommunicateinordertofurtherminimizeconflicts.
4.ToexplainandsimplifycurrentHVACdesignmethodologiessothattheyaremoreapplicabletoCaliforniaproductionhomes,moreuseful,andmorewidelyused.5.ToaddresstopicsnotwellcoveredbyexistingHVACdesignmethodologiesandprovideguidanceonissuesthathavebeenofparticularconcerninproductionhomes.Introduction1.1Purpose
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1.2TargetAudienceThetargetaudienceforthisdesignguideis:
1.HVACdesigners,whethertheyworkforthedesign-buildcontractorwhowilleventuallybeinstallinganHVACsystemoraconsultingengineeringfirmhiredtoprovideadetaileddesignforotherstofollow.2.ArchitectsdesiringtobetterincorporatetheHVACsystemintotheirhousedesigns.3.BuildersdesiringtobettercoordinatetheinstallationoftheHVACsystemintotheirhouses.4.RelatedtradesorconsultantsinterestedinbettercoordinatingtheirworkwiththatoftheHVACdesignerandinstaller.Introduction1.2TargetAudience
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1.3LimitationsThisdesignguideisnotintendedtowalkyouthroughallofthestepsnecessarytodesignanHVACsystem.Therearesomeverysophisticateddesignmethodologiescurrentlyavailablewhicharewell-supportedbytradeandprofessionalorganizations(e.g.,ACCAsManualsJ,S,andD).Unfortunately,theytendtobecomplexandoverlyprecise.Also,thetimenecessarytoproperlyusethem(nottomentionthetimeneededtolearnthem)doesnotfitwellwithinthecurrentdesignprocess.Theytendtobeslantedtowardissuesrelatedtocustomhousesandretrofittingolderhouses.TheyalsodevotemuchtimeandtexttobuildingpracticesatypicalofCaliforniaresidentialnewconstruction,suchasbasementsandsheetmetalducting.ThisDesignguideisintendedtosupplementthosemethodologiesandencouragewiderusebymakingthemmoreconsistentwithcurrentpracticesintheconstructionofCaliforniaproduction
homes.
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2.0TheDesignProcess2.1DesigninghousesaroundtheHVACsystemWouldntitbenicehousesweredesignedaroundtheHVACsystem?IfspecialconsiderationwasgiventothearchitecturaldesignformakingtheHVACsystemeasytodesignandinstall?Ifadequatespacewasprovidedforthefurnaceandalloftheductwork?Ifthehousewasdesignedwiththermodynamicsinmind,tominimizestratification,cross-zoneinterferenceandotherproblemsthataredifficultand/orexpensivetoremedywithstandardHVACpractices?
ThisisunlikelytohappenwithouttheinputofaqualifiedHVACdesigner,andthedesignersinvolvementneedstohappenearlyinthedesignprocess.Moretypically,ahouseisalmostcompletelydesignedbeforeanHVACdesignereverseesit,andtheHVACsystemdesignedwithanemphasisonfittingintothehouseratherthanefficientlyconditioningthehouse.
Unfortunately,HVACinstallershavebecomequiteproficientatgettingsystemstofitintohouses(whethertheywillworkornot!).Theresulthasbeenundersizedandinefficientductsthataredifficulttobalanceandcreateunnecessaryoperatingpressureonthefanmotor.Tocompensatefortheshortcomingsofsuchductsystems,manyinstallershaveincreasedthesizeofthefurnace,coilandcondenser.Thisisthesamelogicasputtingalargerengineinyourcarbecausethetiresaretoosmall.Thecarmightgofaster,butitsurewouldntperformwellorgetverygoodgasmileage.
Oftenthereasongivenforaparticularsizeductbeinginstalledis,thatsthelargestthatwouldfit.Ifadequatespaceisacriticalimpedimenttotheinstallationofaproperlydesignedsystem,thenadequatespaceandclearancemustbedesignedintothehomebythearchitectandbuiltintothehomebytheframer.NomatterhowwellanHVACsystemisdesignedonpaper,thedesigneffortsarewastedifthesystemcannotbeinstalledinthefield.
Typicallyahousegoesthroughthefollowingdesignprocess:
xConceptualDevelopment:Determinespricerange,squarefootage,numberofstories,lotsizes,generalfeaturesandstyles.xPreliminaryDesign:Developsfloorplansketches,numberofbedrooms,majoroptions,basiccirculationandfunctionlocations,aswellassomeelevationconcepts.Someearly
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ValueEngineering(VE)meetings.xDesignDevelopment:Preliminarystructural,mechanical,electrical,plumbingandTitle24energycompliance.SomeVEmeetings.
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ConstructionDocuments:finalworkingdrawingsreadyforbidding,submittal.Backcheckingandcoordinationbyconsultants.SomelateVEmeetings.
TheHVACdesignersneedtoprovideinputasearlyaspossible.TheyneedtotellthearchitectwhicharchitecturalfeaturescausecomfortissuesandaredifficultorimpossibletoovercomewithtypicalHVACpractices.Theyalsoneedtomakesurethearchitectallowsadequatespacetorunducts.Manyarchitectshavehadtore-designplansenoughtimesduetoHVACissuesthattheyknowfairlywellhowtoaccommodateHVACitems.Still,manyproblemscommonlyarisethatcouldbeavoidedthroughearlierinputandbettercoordination.
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2.2CoordinationwithothertradesThefollowingmatrixshowsthemaintradesandconsultantswhoareaffectedbytheHVACsystem.Thefirstcolumnliststheitemorissueandeachsubsequentcolumnhoweachtradeisaffectedbyit.
MatrixofTrades
ItemArchitectBuilder/Framer/StructuralEngineerHVACInstallerEnergyConsultantElectricalPlumberDrywallorinsulationFAUlocation
Roofpitch,furnaceclosets,clearanceingarageTrussdesign,platform,clearance,closets,bollards,atticaccessframingTypeofFAU(upflow,
horizontal),clearance,timingofinstallationModelingcorrectlocationofductsforcomputermodelPower,servicelight,control
wiring,etc.Condensatelines,gaspipingInsulationunderplatformmaybedifferentEquipment
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size,loadcalculationsClearances,#ofsystems,buildingfeaturesStructuralimpacts(weight)Materials,labor,costsEnergyfeaturesimpactsizingElectricalloadsSupplyregisterlocationsAesthetics,clearancesRegisterbootsupportMaterials,labor
SealingaroundregistersReturngrillelocationsAesthetics,noiseissuesFramedopeningsMaterials,laborSealingaroundgrillesCondenser
locationsandlinesetAesthetics,noiseissuesClearance,accessibilitytoyard(set-backissues),2x6walls,chasesMaterials,labor,serviceabilityPower,
servicedisconnectAtticaccessAestheticsFramedopening,trussissuesAccess,serviceabilityRoutingB-ventChases,clearances,
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aesthetics(onroof)Framedchases,roofcapMaterials,labor,installationNoconflictswithventChases,soffits,anddropsAesthetics,feasibilityFraming,clearancesforducts,conflictsMaterials,labor,installationNoconflictswithductsThermostatlocation
AestheticsMaterials,labor,installationWiringSealholeforwiresEquipmentefficiencyMaterialsEfficiencydeterminedbyenergyconsultantCombustionair
Atticventcalcs,routingforCAductsAdequateatticvents(roofer)Ducting,ifany
Table1:MatrixofTrades
TheDesignProcess2.2Coordinationwithothertrades
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3.0DesignMethodology3.1CodeissuesrelatedtoHVACdesign3.1.1ACCAManualDrequiredby2000UMCItisnotwidelyknownthatthe2000UniformMechanicalCode(2001CaliforniaMechanicalCode)requiresthatallresidentialductsystemsbesizedaccordingtoACCAsManualD,whichitselfrequiresManualJasaprerequisitedesignstep.Theexactlanguageis:
Chapter6,DuctSystems,Section601.1SizingRequirements.Ductsystemusedwithblower-typeequipmentwhichareportionsofaheating,cooling,absorption,evaporativecoolingoroutdoorairventilationsystemshallbesizedinaccordancewithChapter16,PartIIReferencedStandardsorbyotherapprovedmethods.
Chapter16,PartIIReferencedStandards.Residentialductsystems,ACCAManualD.
Veryfewjurisdictionsareenforcingthis,mostofthembecausetheyarenotawa
reofit.Thisofcoursedoesntmeanthatitisntrequired.ItisunclearwhatexactlyneedstobesubmittedinordertoverifythatahomehasbeendesignedtotheACCAmethod.Onewouldassumethataclearlydrawnmechanicalplanalongwithsupportingcalculationsand/orworksheetswouldberequired.
TheACCAmanualswerenotwrittenwiththeintentofbeingusedasspecificcodelanguage,thereforeitwillbeuptothelocaljurisdictiontodecideexactlyhowtoenforceadherenceto
them.TheUniformMechanicalCodestatesthatductsmustbesizedaccordingtoManualD.TherearemanysuggestionsandrequirementsinManualDthatdonotrelateductsizing,someofwhichareimpracticalorsimplyinappropriatetoCalifornianewconstruction.FlexibilityindesignisimportantandsincelittleofManualDisrelatedtohealthandsafety,muchofManualDoutsideofthesizingmethodologyshouldbeconsidereddiscretionary.
Note:ThenextrevisionoftheCMCmayaltertheManualDrequirementtobeonlyforhomesthatrequireoutdoorair.Ithasbeensuggestedthatthiswastheoriginalinten
tandwhyitisintheUMC.
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3.1.2Title24loadcalculationsChapter2.5.2ofthe2001ResidentialManualexpandsonSection150(h)oftheEnergyEfficiencyStandards,whichestablishesthecriteriaforsizingresidentialHVACsystemsinCalifornia.Itprovidesforthreedifferentmethodsforcalculatingthebuildingsdesignheatlossandheatgainrates(loads).Italsoestablishesthedesigntemperaturestobeusedforsizingequipment.
Forthepurposeofsizingthespaceconditioning(HVAC)system,theindoordesigntemperaturesshallbe70degreesFahrenheitforheatingand78degreesforcooling.[note:effective10/1/05,theindoordesigntemperaturewillchangeto75degreesFahrenheitforcooling]TheoutdoordesigntemperaturesforheatingshallbenolowerthantheWinterMedianofExtremescolumn.Theoutdoordesigntemperaturesforcoolingshallbefromthe0.5percentSummerDesignDryBulbandthe0.5percent
WetBulbcolumnsforcooling,basedonpercent-of-yearinASHRAEpublicationSPCDX:ClimateDataforRegionX,Arizona,California,Hawaii,andNevada,1982.[note:effective10/1/05,theoutdoordesigntemperaturesforcoolingchangesto1.0percentSummerDesignDryBulbandthe1.0percentWetBulbcolumnsforcooling]
ThethreeapprovedloadcalculationmethodsarewrittenandsupportedbythreedifferenttradeorganizationsASHRAE,SMACNA,andACCA.MicropasandEnergyPro,thetwomostcommonTitle24compliancesoftwareprograms,bothusetheASHRAEmethod.Theygeneratewholehouseheatlossandgaincalculationsinordertomeettherequirementof
submittingapprovedloadcalculationsaspartoftheenergycompliancepackage.Wholehouseloadsareusefulforsizingtheequipmentbutareoflittleusefordesigningaductsystem,whichrequiresroom-by-roomloads.However,itisveryusefultohaveawhole-houseloadcalculationtocomparetothetotaloftheroom-by-roomloads.Thisensuresconsistentandaccuratecalculationsandhelpscatcherrors.
TheResidentialManualalsoremindsusthattheUniformBuildingCodeaddressesthesizingoftheheatingsystem,thoughnotthecoolingsystem.Itstates:
ThesizingofresidentialheatingsystemsisregulatedbytheUniformBuildingCode(UBC)andtheStandards.TheUBCrequiresthattheheatingsystembecapableofmaintainingatemperatureof70Fatadistancethreefeetabovethefloorthroughouttheconditionedspaceofthebuilding.
NoneofthecalculationsapprovedbyTitle24addressthetemperatureatanydistanceabove
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thefloor.Theyallassumethatthetemperatureisthesameeverywhereinthehouse,thattemperaturebeingwhatevertheinsidedesigntemperatureis.Thespecificationof3feetabovethegroundsimplyprovidesareferencefortestinganactualsystem.Itisgenerallyassumedthatiftheheaterhasacapacityequaltoorgreaterthantheheatingloadcalculationsandareasonabledistributionsystem,itwillmeetthisrequirement.
Theresidentialmanualreiteratesthattheloadcalculationsareonlypartoftheinformationusedtosizeandselecttheequipmentandwhocanpreparethosecalculations(presumablybasedontheBusinessandProfessionsCode),butdoesnotgointomuchmoredetailaboutwhatelsegoesintothesizingandselectionprocess.
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Thecalculatedheatgainandheatlossrates(loadcalculations)arejusttwoofthecriteriaforsizingandselectingequipment.Theloadcalculationsmaybepreparedby:
(1)the[Title24]documentationauthorandsubmittedtothemechanicalcontractorforsignature,(2)amechanicalengineer,or(3)themechanicalcontractorwhoisinstallingtheequipment.Title24doesnotspecificallystatehowcoolingloadsshouldbeconsideredwhensizinganairconditioner.Itdoesntevenstatethatanairconditionerhastobeinstalledatall.MostjurisdictionstreattheTitle24coolingloadsasaminimumsizingcriteria.Inotherwords,asystemmustbeinstalledthathasacoolingcapacitythatatleastmeetstheTitle24coolingload.Insomeclimatezones,itiscommonpracticetoofferairconditioningasanoption.So,apparentlythesizingcriteriaonlyapplyifairconditioningistobeinstalled
.[note:2005amendmentstoTitle-24willofferanalternatesizingmethod.]
Thefollowinglinkwilldirectyoutoanon-linecopyoftheTitle24ResidentialEnergyManual,AppendixCCaliforniaDesignLocationData.AmapoftheCaliforniaclimatezonescanbefoundinthisappendixalongwithinformationonCaliforniaclimatezonerequirements.http://www.energy.ca.gov/title24/residential_manual/res_manual_appendix_c.PDF.Or,ifyouareconnectedtotheinternet,youcanclickonthelinkbelow:
Title24ResidentialManual,AppendixC--CaliforniaDesignLocationData
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3.2ACCAManualsJ/S/D3.2.1TheOverallDesignMethodTheoveralldesignstepsfortheACCAJ/S/Dmethodology,asitshouldbeusedintypicalCalifornianewconstructionproductionhomes,isdescribedinthefollowinglist.Throughouttheexecutionofthislist,certaindecisionsaremadethatmayaffectothertrades.Itisimportantthatthiscoordinationbemadeinacontinuousandconsistentmanner.TheMatrixofTrades(page10)isprovidedtohelpguideyouinthiscoordination.
Step1.DetermineZones
Step2.CalculateRoombyRoomLoads
Step3.Select/sizeEquipment
Step4.Layoutductsystem
-LocateFAU(s)
-Locategrillesandregisters-Routeducts-Subzones(trunks)Step5.Determineoperatingconditions
-Staticpressure-TotalCFM-Equivalentlengths-FrictionratesStep6.Sizeducts
-Roomairflowisproportionaltoroomload
-Frictionrateandroomairflowdetermineductsize
Step7.Finaltouches
-Locatethermostat-LocatecondenserDesignMethodology3.2ACCAManualsJ/S/D
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Step1.DetermineZones
Zones,asdiscussedhere,aredefinedasareasofthehousethataretobeindependentlycontrolled,typicallybytheirownthermostat.Smallerhousestypicallyonlyhaveonezone.Ifthemaincriterionforzoningahouseiswhetheritcanbeservedbyasinglesystemornot,thedesignermaywanttowaituntilafterdoingtheloadcalculations.Thenewloadcalculationsoftwareproductsallowyoutoeasilyassignandreassignroomstodifferentzonesandthisstepcanbeintegratedintothenextstepofperformingtheactualroom-by-roomloadcalculations.However,evaluatingahouseforpossiblezoneconsiderationsisstillausefulfirststep.
Thereareavarietyofwaystozoneahouseandthereareseveralfactorstotakeintoaccount.Theseincludeusepatternssuchaslivingareasandsleepingareas.Thermodynamiczonesplayanimportantroleaswell.Theseareareasofahouset
hatwillbehavesubstantiallydifferentbecauseoftheirrelativepositionorisolationfromeachothersuchasupstairsanddownstairs,eastwingandwestwing,etc.Sometimesusepatternsandthermodynamiczonesdonotcoincideandyoumayhavetoprioritizeoneovertheother.Usuallythermodynamicconsiderationstakeprecedence.
Zoningahouseforliving/sleepingcangenerateanenergyefficiencycredittowardTitle241compliance.Thisenergyefficiencycreditisbasedontheabilitytoprogramthe
thermostatscheduledifferentlyforthesetwozonestherebysavingenergy.Therealenergysavingsofthisstrategyishighlydependentontheoccupantsproperprogrammingandoperationofthethermostats.Itcaneitherbeaccomplishedbyasinglesystemwithzonalcontrol(singlesystemwithdualzonecomponents)orbyseparatesystems.SeeSection4.4.ZonalControlformorediscussiononzonalcontrol.IfthedualzonestrategyisusedforTitle24compliance,theHVACdesignmustensurethatitdoesnothaveanadverseaffectoncomfort.
Ifallofthespacesdefinedaseitherlivingareasorsleepingareasarenotlocatedinthermodynamicallysimilarzones,specialstepsmayberequiredtoensureconsistentcomfortthroughouteachzone.Forexample,ifatwo-storyhouselargeenoughtorequiretwosystemshasallofthebedroomsupstairsexceptthemasterbedroom,itmaybedifficulttozonethehouseforliving/sleeping.Becauseitisatwo-storyhouse,itwantstobezonedup/downforthermodynamicreasons.Thesleepingzoneissplit
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betweentwofloorsandmayrequirefurtherzonalcontroltoachievesatisfactorycomfort,resultinginatotalof3thermostats.
UsuallythefirstquestionaskedfromacostperspectiveisCantheentirehousebeservedbyasingleHVACsystem?Inotherwords,canthetotalcoolingloads,regardlessofotherconsiderations,bemetbyasingle5-tonairconditioner(thelargestsystemtypicallyusedinresidentialconstruction)?Thisisnotknownuntiltheloadsarecalculated.Apreliminaryestimatecanbemadebasedonsquarefootageandwindowareaandthenlaterrevisediftheresultsoftheloadcalculationschangetheassumptions.
1EnergyEfficiencyStandardsforResidentialandNonresidentialBuildingsPublicationNumber:400-01-024,August2001
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Ashomesgetmoreandmoreefficient,especiallyinregardtowindowtechnologies,largerandlargerhomescanbeservedbyasingle5-tonsystem.Atsomepoint,otherconsiderationsneedtobetakenintoconsideration.Thingssuchasadequateairflow(airchanges)needtobeconsidered.Doesasingle5-tonsystematapproximately2000cfmhaveenoughairmovingcapabilitytoadequatelydistributeairthroughoutaverylargehouse,evenifitcanmeetthesteadystatecoolingload?Also,howsusceptibleisthehousetonon-steadystateconditions?Inotherwords,whathappensifincoolingmodethetemperatureisinadvertentlyallowedtosubstantiallyexceedthecomforttemperature?Willthesystembeabletocatchupinareasonableamountoftime?Thiscanbeacriticalcustomerserviceissueinproductionhomesandisatopicthatneedsfurtherresearch.
Ifthehousecanbeservedbyalargesinglesystem(i.e.,5-tons)buthasdistinctzones(e.g.,upstairsdownstairs)itisrecommendedthatthosezonesbecontrolledindependently(separatethermostats).Thiscanbeaccomplishedbymultiplesystemsorbyasinglesystemwithzonalcontrols.SeeSection4.4formoreonzonalcontrol
Step2.Calculateroombyroomloads
Forroom-by-roomloads,ACCAsManualJisthemostwidelyusedandmostwidelysupportedstandardizedmethodology.Thereareatleasttwosoftwareversionsofit
(SeeAppendixAforresourceinformation).Eventhoughitwasoriginallyintendedtousehandwrittenformsandworksheets,itisnowvirtuallymandatorytouseacomputermethod(unlessyourareextremelyaccurateandpatientthetypeofpersonwhocanfilloutcomplicatedtaxformsbyhand.).BecauseACCAManualJisallbasedonpublishedtablesandworksheets,somepeoplehavewrittentheirownloadcalculationspreadsheetsbasedonManualJ.
Thetwoavailablesoftwarepackages(Right-Suite2andElite3)haveverysophisticated
featuresallowingComputerAidedDesign(CAD)-basedtake-offsforwindowandwallareas.ThismakesveryeasyandquickworkofenteringphysicalbuildingdataifyouhaveaccesstoanarchitectsCADfiles.ThesoftwarepackagesallowyoutoimportaCADfloorplanofthehomeandessentiallytraceoverittocreatetheroomsandzones.Windowsanddoorsaredrag-and-dropcomponents.Ifyoudonothaveaccesstothe
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architectsCADfiles,youcanusethesoftwaretodoaprettyreasonablejobofrecreatingthefloorplanofahouse.Thesesoftwarepackagesalsohaveusefulductlayoutdrawingfeatures.
Theunderlyingconceptofroom-by-roomloadsisthateachroom,orareaservedbyasupplyregister,istreatedasanindividualload.Thisprovidesforaveryaccuratedeterminationofhowtodistributetheair.Ifairisdistributedproportionallytoeachroomsload,theneachroomwillbeconditionedappropriately;resultingiseventemperaturedistributionacrossahome.ThisisthebasisforACCAManualD.Itsnotperfectinreality.However,itisthebestmethodwehaverightnowandworksquitewellformostproductionhomes.Themorecomplexandbrokenupthehouselayoutisarchitecturally,thelessthisassumptionisapplicable.
2WrightsoftSoftware,3EliteSoftware
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Step3.SelectandSizeEquipment
Usetotalofroom-by-roomloadsforeachzone
1.Oncethehousehasbeenzonedandtheloadsforeachofthezonesarefinalized,thesystemcanbesizedandselected.ACCAsManualSprovidesdetailedinformationfordeterminingheatingandcoolingcapacitiesofvarioustypesofequipment.InCaliforniaresidentialnewconstruction,themostcommonHVACsystemtypeissplit-systemDirect-Expansion(DX)coolingwithagasfurnace.Theheatingcapacityiseasytodeterminebasedontheratedheatingoutputofthefurnace,whichchangesverylittlebasedonactualconditions.Someadjustmentmayneedtobemadeforhighaltitudes.Determiningthecoolingcapacityatactualconditionsismorecomplex.Itdependsonseveralconditions:a)theoutdoortemperature,b)theindoorenteringwetbulb4anddrybulb5temperatures,andc)theairflow(cfm)acrossthecoil.Inordertoproperlyaccountfortheseconditionsitisnecessarytousedetailedcapacitytablesprovidedbythemanufacturer.Again,ACCAsManualSgoesintogooddetailonthisprocess.
InCaliforniaresidentialnewconstructionthefollowingconditionsaretypical:
1.Outdoortemperature:Thisisthetemperatureoftheairthatisblowingthroughthecondensertocooltherefrigerantandisusuallythesameoutdoortemperaturethatisusedforthecoolingloadcalculationsunlessitisknownthatthecondenserwillbelocatedinahotterlocationsuchasonaroof.2.Indoorenteringwetbulbanddrybulb:Thesedescribetheconditionoftheairblowingacrossthecoilandareusuallyassumedtobethesameastheindoorconditionsusedintheloadcalculations.Title24coolingloadsarecalculatedusinganindoortemperature(drybulb)of78degF.Somedesignersusea
lowertemperature,suchas75degreestobesafe.(Note:lowerindoortemperaturesdriveupthecoolingloadanddecreasethecalculatedcapacity,potentiallyrequiringalargersystem.)Exceptforsomecoastalareas,Californiaisconsideredadryclimate.Asafeindoorwetbulbtemperatureis65degreesF.Thiscorrespondsto78degreesFand50%relativehumidityonthepsychometrictable.(Note:Thehigherthehumidity,thehigherthewetbulbtemperature,andthelowerthecoolingcapacitywillbe.)4Thewetbulbtemperature(WBT)relatesrelativehumiditytotheambientairordrybulbtemperature.Whenmoistureevaporates,itabsorbsheatenergyfromitsenvironmentinordertochangephase(vialatentheatofvaporization),thusreducingthetemperatureslightly.TheWBTwillvarywithrelativehumidity.Iftherelative
humidityislowandthetemperatureishigh,moisturewillevaporateveryquicklysoitscoolingeffectwillbemoresignificantthaniftherelativehumiditywerealreadyhigh,inwhichcasetheevaporationratewouldbemuchlower.Thedifferencebetweenthewetbulbanddrybulbtemperaturethereforegivesameasureofatmospherichumidity.
5Drybulbtemperaturerefersbasicallytotheambientairtemperature.Itiscalleddrybulbbecauseitismeasuredwithastandardthermometerwhosebulbisnotwet-ifitwerewet,theevaporat
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ionofmoisturefromitssurfacewouldaffectthereadingandgivesomethingclosertothewetbulbtemperature.Inweatherdataterms,drybulbtemperaturereferstotheoutdoorairtemperature.
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3.Airflowacrossthecoil:Thisistypicallythesameasthedesignairflowforthesystem.Itcomesfromthefurnaceairflowtablesatthedesignstaticpressure(usuallybetween0.5and0.7incheswatercolumn,0.6isareasonablenumbertousebutitdependsonthespecificdesigncriteria)andrangesfrom350-425cfmpertonofthefurnace.Thefollowingbasicconceptsaregoodthingstokeepinmindwhendesigning(orevaluatingtheperformanceof)asystem:
1.Astheoutdoordesigntemperaturegoesup,thecoolingcapacityoftheACunitgoesdown(andtheloadonthehousegoesup).Thisisbecausetheoutdoorairistheheatsinkusedbytheairconditionertodumptheheatintothatisextractedfromtheindoorair.Astheoutsideairgetswarmer,itisharderfortheairconditionertodumpheatintoit.2.Astheindoordrybulbtemperaturegoesdown,thecoolingcapacitygoesdown.Thisisbecauseitishardertoextractheatfromcolderair.
3.Astheindoorwetbulbtemperaturegoesdown,thecoolingcapacitygoesdown.Thisisbecausetheairhasmoremoistureinitandcoolingcapacityisusedupwhenthismoistureiscondensedoutoftheair.4.Astheairflowacrossthecoilgoesdown,thecoolingcapacitygoesdown.Thisisbecausewithlessairpassingacrossthecoil,thereislessopportunityforthecoiltoextractheatfromtheairstream.Step4.LayOutDuctSystem
oLocateForcedAirUnit(s)(FAU)ThelocationoftheFAU(furnace)dependsona
varietyoffactors.Theseincludesuchthingsasclearance,accessibility,ductrouting,andventing.Personalpreferenceevencomesintoplay.Ananalysiswasdoneontheimpactsofenergyconsumptionandfurnacelocation(SeeSection4.1fordetailsofthisstudy)aspartoftheresearchprojectthatincludedthewritingofthisdesignguide.Itfoundthatfurnacelocationhadlittleimpactonenergyconsumptionandeffectivenessofthesystem.Theonlynotabledifferencebetweenafurnaceintheatticandafurnaceinagarage,forexample,wasthatthefurnaceinthegarage
tendedtohavesomewhatlongerducts,whichresultedinmoreconductivelosses/gainsandmoreresistancetoairflows.Italsoshowedabitmorefanpowerconsumptionduetothelongerductruns,butthiscanbecompensatedforbyusinglargerducts,iftheycanbeaccommodated.Firstcost(duetolabor)tendstobethebiggestconsiderationindecidingwheretoputthefurnace.Thegeneraltrendtodayistoputfurnacesinatticseventhoughtheyarelessaccessible.Floorarea,eveninagarage,isatapremium.
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Also,sinceanatticlocationismorecentrallylocated,ittendstohaveductrunsofmoreequallength.Inotherwords,therearelesslikelytobeverylongductruns.Also,ventingafurnaceismorestraightforwardfromanatticthanfromagarage,especiallyinatwo-storybuilding.Furnacelocation(seeSection5.2)isagooddiscussiontopicforvalueengineeringmeetings.
oSelectingandlocatinggrillesandregisters-ACCAalsopublishesaManualTTerminalSelection,whichcontainssomegoodinformationontheselectioncriteriaforsupplyregistersandreturngrilles.Itcoverssuchtopicsasregistertype(2-way,15
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3-way,etc.),pressuredrop,facevelocity,noisecriteria,andthrowdistance.Inresidentialnewconstructiongrillesareoftensizedbasedonthesizeoftheductservingthem,whichisaltogetherinadequate.Similarly,grilletypesareoftenselectedbasedonpersonalpreferenceandsometimesfaultyreasoning.Muchmorethoughtshouldgointothisprocess.
Inatypical,square-ishroomsuchasasecondarybedroom,therearefourbasiclocationsforasupplyregisters,fiveifyoucountfloorregisters,whicharealmostalwayslocatedunderawindow.ThefourmainlocationsareshownFigure1.
Figure1:CeilingRegisterLocations
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Astudyontheimpactsofenergyconsumption,comfortandsupplyregisterlocationwasperformedaspartoftheresearchprojectthatincludedthewritingofthisdesignguide.Thisstudyevaluatedandcomparedthemostcommonoftheselocations:2-wayoverawindow,3-waynearaninteriorwall,andhighsidewalloppositeawindow.SeeSection4.2fordetailsonthisstudy.
Givenachoice,theresultsofthisstudyprovideimportantconsiderations.Sometimes,however,thegeometryoftheroomdictateswhereyoumustplaceregisters.Forexample,inalongnarrowroomwheretheexteriorwallisonthenarrowdimension,youmaybeforcedtoputaregisteroverthewindowbecausetheinteriorwallistoofaraway.Also,structuralandarchitecturalconstraintssuchaslocationsofchases,floorjoistdirectionsandbeamsmaydictateregisterlocations.Anyofthelocationsmentionedabovecanbemadetoworkadequatelywellifcertainconsiderationsaremade.Whatevertheregisterlocation,thefollowingconsiderationsshouldbeemphasized:
1.
Registeroverwindoworonexteriorwall.Usea2-wayregisterorientedparalleltothewindow/exteriorwall.Thiswillcreateacurtainorsheetofsupplyairparalleltotheexteriorwallandtheairwillnaturallymoveawayfromthewallandmixwiththeairintheroom.Usinga3-wayregisterpointedawayfromthewindow/exteriorwallwillthrowthebackintotheroomtooquicklyandmaynotadequatelyconditiontheareadirectlyinfromofthewindow.Itmayalsoshortcircuittheairflowbythrowingitbackintothenaturalreturnpathbeforeithasachancetomixwiththereturnair.A3-wayregisterlocatednearawindowbutpointeddirectlyatitwillblowairdirectlyonthewindow.Thiswillheatandcoolthewindow,whichserveslittlebenefitwhenthepurposeistoheatandcooltheairinsidetheroom.Infact,thismostlikelywastessubstantialenergy.
2.Registernearaninteriorwall.Usea1-wayor3-wayregisterwiththeprimarydirectiontowardthewindow/exteriorwall.Itisimportanttoensurethattheregistersthrowdistanceisadequatetoreachnearthewindow/exteriorwall.3.Registercenteredinaroom.Usea4-wayregister.4-wayregistersdelivertheairequallyinallfourdirections.Considerationmustbegivenforinterferencewithlightfixturesorceilingfans.Ifthisisthecase,thenlocatetheregisteranaestheticallyappropriatedistanceawayfromthefixture,buttowardtheexteriorwall.4.Highsidewallregisters.Useabar-typeregisterthatthrowsair
perpendiculartothefaceoftheregister.Pointtheregistertowardthewindow/exteriorwall.Aswitharegisternearaninteriorwall,itisimportanttoensurethattheregistersthrowdistanceisadequatetoreachnearthewindow/exteriorwall.Bar-typeregisterslocatedinaverticalwalltypicallyhavemuch,muchgreaterhorizontalthrowdistancesthan3-wayor1-wayceilingregisters,andbetteroverallairflowcharacteristicsingeneral(morecfmpersquareinch,quieter,etc.).DesignMethodology3.2ACCAManualsJ/S/D(Step4)
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Thebasicthingstokeepinmindwhenselectingandlocatingaregisterare:
1.Goodairmixing:youwantthesupplyairtomixinwiththeroomairasmuchaspossible.Thisisaidedbydirectingtheairintheoppositedirectionofthenaturalpathbacktothereturn(e.g.,outthedoor).2.Goodairdistributionandnostagnantareas:youwantthesupplyairtoreachalloftheoccupiedareasofaroom,especiallyareasclosetoaload(e.g.,window).Throwdistanceisanimportantconsiderationforthis.oDeterminingsub-zones(trunks)andtheuseofbalancingdampersInproductionbuilding,adesigneristypicallydesigningthesystemforahomethatmaybebuiltinseveraldifferentorientations.(SeeSection4.3fordiscussionondesigningformultipleorientations.)Thesystemistypicallydesignedfortheworst-caseorientationwithconsiderationforairflowsneededinotherorientations.Thesystemmustat
leastbeabletobeeasilybalancedtoworkinallorientations.Astrategythathelpsaccomplishthisistodividethemainzonesofthehouseintosub-zones.Thesesub-zonesareareasinthemainzonethatwillbeaffectedsimilarlywhenthehouseisinanorientationotherthanworstcase.Forexample,Figure2showsabasicsingle-story,single-zonehouseinitsworst-caseorientation.Figure2:ExampleHousePlan
Ifthehouseisrotated180degrees,bedrooms2and3willgofromthesouthside
ofthehousetothenorthsideofthehouseandprobablyneedmuchlessair.Ifthesetworoomsareonthesametrunk,thiscanbeaccomplishedeasilybyusingamanualbalancingdamperlocatedrightatthesupplyplenum.Thefamily/kitchenarea,living/diningareamasterbedroommaybetreatedsimilarly.
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Figure3showsareasonablelayoutandapproachtoaccomplishorientation-dependentbalancingusingmanualbalancingdampersthatareeasilyaccessible.
Figure3:ExampleHVACDesign
oRoutingductsTheactualroutingofductsisafunctionofthenumberandlocationofsupplyregisters(andtoalesserextentreturngrilles),architecturalandstructuralconstraints,ductsize,ductlength,andotherpracticalissuessuchaspreferredtypesoffittings(t-wyesvs.duct-boardtransitionboxes).Inasingle-storyhousewithampleatticspacethisisprettystraightforward.Youcanlocatetheregistersfirstandthensimplysketchtheductsin.Inamultiple-storyhouse,thisisamuchgreaterchallenge,atleastforallbutthetopfloor.Assumingthesystemservingthef
irstfloorislocatedintheattic(atypicalscenario),theductsservingthefirstfloormustpassverticallythroughtheupperfloor(s),andthenhorizontally(unlessyouarelucky)totheceilingregistersonthefirstfloor.Thereisusuallyagreatdealofframing(suchastrusses,blocks,joists,beams,headers,andtop/bottomplates)betweenthefurnaceandtheregister.Infact,veryoftentheframingisthedecidingfactorindeterminingwhereregistersareultimatelyplaced.Thefollowingaresomeideasforgettingductsfromonepointtoanother.
VerticalDuctRuns
ChasesandvoidsTheseareshaftsbetweenwalls,eithercreatedintentionally(chases)orincidentally(voids)thatcanbeusedtorunductsfromtheattic,throughtheupperfloor(s),tothelowerfloor(s).
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SamplesofIncidentalVoids
Figure4:ExampleVoidinInteriorStairChasewhichoftenoccursadjacenttoroundroomorstairways
Figure5:ExampleVoidinDeadSpace(wherespacesofunequalsizeorshapeareadjacenttoeachother)
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SamplesofChases
Figure6:ExampleExteriorChaseVoidscanbefoundinthebumpoutsofexteriorarchitecturaldetails,butcaremustbetakentoensurethatthatparticulararchitecturaldetailoccursinallelevationstyles
Figure7:Walk-InClosetwithInteriorChaseChasescanbecreatedincornersofclosets.Thedeadcornerofawalk-inclosetisanidealplacebecauseithasminimalimpactorhangingspaceanditprovidesaconvenientwayfortheshelfandpoletobesupported.
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Figure8:ClosetChaseExampleChasesmayalsobeaddedtoeitherendofaflatcloset.Ifgiventhechoice,itispreferablenottohaveachaseadjacenttoanexteriorwallwhentheroofslopesdowntothatwall(i.e.,hiproof),becausetheroofcaninterferewiththeductgettingdownthroughthetopofthechase.Ifthiscannotbeavoidedtherearevariouswaystodroptheceilingintheclosettobetteraccommodatetheduct.
Figure9:MediaChaseAgoodlocationforcreatingchasesisinamedianiche
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Figure10:WaterClosetChaseAnothergoodlocationforcreatingchasesisinawatercloset
Figure11:ChimneyChaseChasescanalsobeinchimneys,evenasfalsechimneys
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RisercansThesearerectangularducts,usuallysheetmetal,whichfitinawallcavitybetweenthestuds.Theyarerelativelycommon,butduetopotentialnoiseproblems,highresistancetoairflow(highequivalentlength),structuralconstraints,andinstallationcosts,theyaretypicallyusedonlyasalastresort.Ifcareistakenintheirdesignandconstruction,theycanhoweverbeaviablesolutiontomanyroutingproblems.Youshouldkeepthefollowingthingsinmindifconsideringrisercans:
1.NoiseThermalexpansionandcontractioncancausesheetmetalrisercanstomakesubstantialamountsofnoise.Thisiscalledoilcanningandcanmanifestitselfinclicking,popping,clanking,squeakingandotherannoyingnoises.Manycontractorshavehadtotearoutrisercansduetocustomerservicecomplaints.Thisisaveryexpensiveandmessyretrofit.Somecontractorswillflat-outrefusetoinstallthem.Avoidputtingrisercansinbedroomwallsifatallpossible.Someprecautionstopreventingnoiseareusingheavier
gaugemetal,caulkingbetweenallmetal-to-metalseams,andusingleadtapeasasounddampener.Youmightalsoconsiderusingductboardratherthansheetmetal.Itrequiresalargercrosssectionalareathansheetmetalbutisvirtuallysilentandhasmuchbetterinsulationproperties.2.HighResistancetoairflowTheavailablespaceinatypical(16oncenter)2x4and2x6studwallis3x14and5x14.Thetypicalsizerisercansusedinthesewallsare3x14and5x14,whichcorrelatetoroundflexductequivalentsizesof8and9,respectivelyThehighresistancetoairflowcomesnotsomuchfromtherisercanitself,butfromtheround-to-rectangularandrectangular-to-roundtransitions.Itishighlyrecommendedthatsmooth,roundedtransitions
beusedwherepossible.Itishighlydiscouragedtosimplycutaroundholeinthesidefaceoftherisercan.Figure12:RiserCanInstallation
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Figure13:RiserCanDetail
Caremustbetakentoensurethatnotrusssitsontopofthestudbaythatyouintendtouseandthestudbaymustlineupwiththefloorjoistsbelow.TheuseofrisercansrequirescarefulcoordinationbetweentheHVACsubcontractor,thearchitect,thestructuralengineer,andtheframer.
HorizontalDuctRuns
FloorJoistBaysThesearethespacesbetweentheparallelfloorjoists.CaliforniabuildersoftenusewoodenI-beamtypefloorjoists.
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Figure14:FloorJoistDetail
Commonsizes(heights)are12,14,andsometimes16.Whileitispossibletocutholesinfloorjoistsasbigastheheightoftheweb,therearestrictlimitationsonthisandjoistpenetrationsmustalwaysbeapprovedbythestructuralengineer.EvenifyoudocuttheI-joistsitcanbedifficulttopullflexductthroughtheseholes.Theothercoordinationthatmusttakeplaceiswiththetradesthatwillbesharingthisspace,especiallyplumbers.Gaspiping,sanitarydrainsandwaterpipingcanallberuneitherperpendiculartoorparallelwiththeI-joists,andcaninterferewithducts.
SomebuildersusefloortrussesratherthanI-joists.Theseconsistofdiagonalframingmemberssimilartoarooftrussratherthansolidwebbing.
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Figure15:FloorTruss
Thesearemuchmoreaccommodatingofductswithoutcuttingholesbutsimilarcoordinationmustbemadewiththeplumbers.
Oneimportantthingtokeepinmindwhenrunningductsinfloorjoistbaysisthatthebestpracticeforconnectingtoaceilingregistermayrequireaspecialtransitionfittingratherthansimplymakinga90-degreebendintheduct.
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Figure16:Duct-to-RegisterConnections
DroppedceilingsandSoffitsSometimestheonlywaytogetpastabeam,wallorfloorjoistsistocreateadroppedorfalseceilingbelowtheobstructionthatprovidesroomtorunaduct.Whenconsideringtheseasanoptiononemustrealizethattheycanberelativelyexpensivetobuildandoftenhaveaestheticdisadvantagesbecausetheylowertheceilingheight.Usuallyloweringtheceilinginasmallroomsuchasabathroom,laundryroom,orhallwayisnotabigproblem.Thetotaldroprequiredtorunductsistheouterdiameteroftheductplus3fortheframing.Insmallerroomsthedroppedceilingcanbeflatstudded(withthe2x4sturnedsideways)andthenyouonlyneedtoadd1totheouterdiameteroftheduct.Mostbuildersandarchitectsdonotliketogowithlessthanan8ceilingheight,butmaysometimesallowa76ceilingheightifabsolutelynecessary.
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Figure17:SoffitChase
Soffitsaresimilartodroppedceilingsexceptthattheyarelocalizedandresembleahorizontalchase.Soffitsprovideaboxed-inareawhereawallmeetsaceilingasanalternativetodroppingtheentireceiling.Theyarecommoningarages.Whenbuildingasoffitinagaragecaremustbetakentomaintaintheintegrityofthe1-hourfireseparationbetweenthegarage(GroupUoccupancy)andthehouse(GroupRoccupancy).
Step5.DetermineOperatingConditions
oStaticpressureStaticpressureisthepressureatwhichthefan(inthefurnace,FAU,orfancoil)mustoperate.Itistheabsolutesumofthesupplypressure(positive)andthereturnpressure(negative).Thehigherthispressure,thelowertheairflowwillbe.
TheACCAmethodallowsyoutosizeyourductsaroundaspecifiedstaticpressure,ensuringthatthefanwilloperateatconditionssuitabletoproperairflowandfanperformance.
Mostfurnacesareratedatanominal400cfmperton.Thisusuallycorrespondstoastaticpressureof0.5inchesofwatercolumns(iwc).Becauseofthis,manysubcontractorsassumethattheyareoperatingat0.5iwcand400cfm/tonjustbecausetheyinstallacertainsizepieceofequipment.Manydontrealizejusthowdependentstaticpressureandairflowareonhowtheysizetheducts.Iftheductsizingmethodologydoesnotproperlyaccountforpressurelossesinthe
distributionsystem(e.g.,coils,fittings,filters,bends,andregisters),thestaticpressurewillbetoohighandpossiblyoutsidethefurnacemanufacturers
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recommendedrange,resultinginpoorperformanceandprematureequipmentfailure.Inaddition,theairflowwillbetoolow,decreasingtheperformanceofthesystemandpossiblyreducingcoolingcapacitytobelowthecoolingload(ineffectmakingtheairconditionertoosmall).
Adesignstaticpressurethatgivesgoodairflowandresultsinreasonablysizedductsis0.6iwc.ACCAutilizesavaluecalledAvailableStaticPressureinitsimportantequations.Itistheoperatingstaticpressureacrossthefurnacelessthestaticpressuredropsofvariousitemssuchas,thecoil,filters,heatexchangers(externaltofurnace),registers,grilles,etc.ThevaluesforallofthesepressurelossesaregiveninManualD.
oTotalCFMTotalCubicFeetperMinute(CFM)canbedeterminedbypickingthedesignstaticpressureandreferringtothefurnacemanufacturersairflowtablefortheairflowatthatstaticpressure.Usehighspeedforcooling.ThetotalCFMis
usedtodetermineactualdesigncoolingcapacity.Thisnumberisdistributedtoeachroomproportionalthateachroomsload.Aslongastheductsaresizedproperly,thistotalairflowwillbemetorexceededinthefield.
oEquivalentlengthsThepressuredropofductandductfittingsareaccountedforusingequivalentlengths.Theyareexpressedinunitsoffeet,whichmakesenseforalengthofductbutisabitunusualforafittingsuchasat-wyeorelbow.Itissimplyawayofaccountingforpressuredropofafittingbyequatingittoanequivalentlengthofduct.Equivalentlengthsareusedinthecalculationforfrictionrate.
oFrictionratesThefrictionrateisthecriticalfactorfordeterminingwhatsizeductisneededtoprovideacertainamountofCFM.Theunitsareinchesofwaterper100feet.Itdescribesthepressurelossforevery100feetofduct.Theequationforfrictionrateisfairlysimple:
FrictionRate(AvailableStaticPresssure*100)/(TotalEquivalentLength)
ItisusedinthefrictionchartsinAppendixAofManualD.Itisalsousedinductsliderules,whichareessentiallythefrictionchartsputintoaslideruleorwheelformat.Notethatthereisadifferentfrictionchartfordifferentducttypes.Chart7isforFlexible,SpiralWireHelixCoreDucts,a.k.a.flexductorvinylflex.Foracommonfrictionrateof0.1and200cfm,thechartshowsthatyouwouldneedbetweenand8anda9duct,soa9ductmustbeinstalledtoensurethatatleast200cfmisdelivered.
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IntypicalCaliforniaresidentialnewconstruction,frictionratesbetween0.9and
1.2aremostcommon.Lookingonchart7,thisisaverysmallareaonthechart.Also,whenyouconsiderthatthetypical5-tonsystemonlygoesuptoabout200031
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cfm,theareaofchart7thatiscommonlyusedisverysmallandtheaccuracyisquestionable.Itisrecommendedthatadesignernotusingthesoftwareuseagoodqualityductsliderulesuchasthewheel-typeduct-sizingcalculatorpublishedbyACCA.
Severalductsliderulemanufacturersrecommendthatyouuseafrictionrateof
0.1.Thisonlyworksifyoucandesignthesystemtoensurethecorrectavailablestaticpressureandtotalequivalentlength.However,simplyusingafrictionrateof0.1andtheroom-by-roomairflowsgeneratedbyManualJforaresidentialnewconstructionhomewouldbebetterthanmostrulesofthumbscurrentlybeingused.Herearesomeexamplesusingthefrictionrateequationandfrictionchart:
.Theavailablestaticpressure(ASP)iscalculatedtobeabout0.25Example1
iwc.Thetotalequivalentlengths(TEL)areestimatedtobeabout250feet.Theequationforfrictionrate(FR)yieldsavalueof0.1.If130cfmarerequired,
theductcalculatorshowsthata7flexductisnotadequatesoan8mustbeused.Inthefield,itisdeterminedthattheductcannotberunasexpectedandanewrouteisdetermined,whichadds30ofextralengthtotheduct.Willthisaffecttheductsizing?Inthiscase,no,itwouldnot.Adding30feetchangesthefrictionrateto0.09.Usingtheductcalculator,an8ductisstilladequate.Infact,an8ductwouldworkaslongasthefrictionratewas0.065orhigher.Thismeansthatupto130feetofextralength(actualorequivalent)couldbeaddedandtheduc
twouldstillsupplyatleast130cfm.
Thisisnotalwaysthecase,however.Eachductdiametercanhandlearangeofairflows.Itdependsonhowcloseyouaretotheupperlimitofthatrange.Theoretically,addingjustonefootofextralengthcouldrequireincreasingthe
ductsize.Example2:UsingthesamestartingpointasExample1(ASP=0.25,TEL=250andFR=0.1),thebuilderwantstoofferelectronicfiltersandneedstoknowiftheywouldaffecttheductsizing.Thefiltermanufacturerlistsastaticpressu
redropof0.10iwc.
Thischangesthefrictionratefrom0.1to(0.25-0.10)*100/2500.06,whichwouldrequirethata9ductbeusedtodeliver130cfmandbecausethefilteraffectstheentiresystem,manyotherductsmaybeaffectedaswell.
Thisscenarioassumesthatthedesignerintendstomaintaintheoperatingstaticpressureof0.6iwcinordertomaintainacertaintotalairflow.Adifferent
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approachwouldbetokeeptheductsthesamesizeandletthestaticpressurechange.Fortheductstostaythesamesize,thefrictionratemustnotchange.Forthistobetruetheavailablestaticpressureneedstostaythesame(assumingthattheequivalentlengthsarenotgoingtochange,inotherwordsthebasicductlayoutdoesnotchange),whichmeansthatthestartingstaticpressure
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acrossthefanhastogoupbythesameamountthattheelectronicfilterwilluseup.Ifweassumeanoperatingstaticpressureacrossthefanof0.7iwc(0.6originally+0.10forthefilter),themostobviousimpactwillbethattheairflowwillgodown.Thiscanbequantifiedusingthefurnacefanflowtable.Whatneedstobeconfirmedisthattheairflowisstilladequatetomeetthesensiblecoolingcapacity(rememberthatasairflowgoesdown,sodoescoolingcapacity).Also,maximumairvelocitiesmustbeconfirmedasdoesthefurnacemanufacturers
recommendedoperatingrangeforstaticpressure.Step6.SizeDucts
Roomairflowshouldbeproportionaltoroomload.Oncetheroom-by-roomloadshavebeencompletedandtheequipmenthasbeenselected,itisasimplemattertodeterminehowmuchaireachroomorspaceneeds.Theairflowrequiredineachroomisproportionaltoeachroomsload.Inotherwords,iftheroomaccountsfor10%oftheloaditmustget10%oftheairflow.
Frictionrateandroomairflowdetermineductsize.Onceairflowisdetermined,aductcalculator(ductsliderule)canbeusedtodetermineductsizeusingthefrictionrate.
Step7.FinalTouchesLocatethermostat(refertoSection5.8ThermostatLocation.)Locatecondenser(refertoSection5.1CondenserLocationsandRefrigerantLines.)
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4.2RegisterLocationAspartofthetaskofdevelopingthisdesignguide,astudywasconductedtoevaluatetheimpactoffurnaceandregisterplacementonenergy,comfort,andquality.
Threesupplyregisterconfigurationswereevaluatedusingacomputationalfluiddynamicsmodel(CFD)forbothheatingandcooling.ThesethreeconfigurationsrepresentthemostcommonpracticeinCaliforniaproductionhomebuilding:registercenteredintheceiling,registeroverwindow,andhighsidewall.Tworeturnlocations,ceilingandlow-wall,werealsoevaluated.
Thisstudyusedacomputersimulationandisnotaperfectmodelofreality.Forexample,interiorfurnishingswerenotincludedinthemodel.However,theresultsdoprovideareasonablepicturethatmatcheswellwithreal-worldexperience.DetailedinformationonthisstudyisavailablefromtheCaliforniaEnergyCommissionasAttachment2tothe
FinalReportfortheProfitability,Quality,andRiskReductionthroughEnergyEfficiencyprogram.ThereportisalsoavailablethroughtheBuildingIndustryInstitute(BII)orConSol.
Thestudiesindicatethatthemostenergyefficientlocation,withnonegativeimpactoncomfort,istoplacethesupplyregisteronahighsidewall.Thestudyresultsshowthatthislocationprovidesthebestmixingandisthepreferredlocation.Ingeneral,highwallregistersareagoodideasincetheyallowtheairstreamtomixwithroomairabovetheheadsoftheoccupantsand
minimizeairvelocityandtemperaturenon-uniformitiesintheoccupiedpartoftheroom.ThereareotherconsiderationsinselectingthesupplyregisterlocationandthesearecoveredinStep4oftheOverallDesignMethod.
Thefigurebelowisanexampleoftheinformationgeneratedbythisstudy.Thisexampleshowsthedutycycleforthethreesupplyconfigurationswithaceilingreturnundercoolingconditions.ThedurationoftheHVACONtimeisnotablyshorterforthein-wallsupply.Alsonotethatthetotaldutycycletimeforthein-wallconfigurationisnearly25%longerthanth
eothercases.
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75.0076.0077.0078.0079.0080.0081.000.005.0010.0015.0020.0025.0030.0035.0040.00Time(mins)TemperatureatThermostat(F)CeilingInteriorACON#1CeilingInteriorACOFF#1CeilingInteriorACON#2CeilingInteriorACOFF#2OverWindowsACON#1OverWindowsACOFF#OverWindowsACON#2OverWindowsACOFF#2InWallsACON#1InWallsACOFF#1InWallsACON#2InWallsACOFF#2Figure18:ON/OFFruntimesforthreecoolingconfigurationswithceilingreturns:supplyregisterinteriorceiling;ceilingoverwindows;andin-wall
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4.3MultipleOrientationDesignsInacoolingdominatedclimate,whichincludesmostofCalifornia,orientationhasadramaticimpactonequipmentsizingbecausemosthomes,especiallynewproductionhomes,havethelargestconcentrationofglazingonthebackofthehome.Therequiredcoolingequipmentofatypical2300squarefoothomecanchangefrom3.5-tonto5-tons,a30%increaseincapacity,justbyrotatingthehousefromsouth-facingtoeast-facing.Theorientationofahome,ormorepreciselyitswindows,iswhatdeterminesthemajorityofitsheatgain.East-andwest-facingwindowshavethegreatestheatgainbecausethesunislowerintheskyandshinesthroughthewindowatananglemoreperpendiculartothewindows,increasingtheamountofradiationenteringthehome.
Sunangleandwindoworientationareaccountedforintheheattransfermultipliersusedinthe
loadcalculationmethods.Heattransfermultipliers(HTM)arevaluesthatwhenmultipliedbytheareaofthewindowproducestheheatgainofthatwindowincludingconductiveaswellasradiativeheatgains.TheunitsareBtuh/sf.ThefollowingHTMsforadual-pane,low-e,
aluminum-framedwindowillustratetheimpactoforientationonheatgain.NorthEast/WestSouthSE/SWNE/NW21.461.032.853.144.3Table2:OrientationEffectonHeatTransferMultiplierAsthisshows,eachsquarefootofeast-orwest-facingglasshasnearlytwicetheheatgainof
southfacingglassandnearlytriplesthatofnorthfacingglass.Mosttypicalhomestendtohavethemajorityoftheglassonthebackofthehouse.Thisiswheremostoftheslidingglassdoorsandlargefamilyroom/greatroomwindowsaretypicallylocated.Whensomuchoftheglassisloadedononesideofthehouse,thevariationintotalcoolingloadismuchgreaterbetweenorientations.Conversely,iftheglazingareaofahousewereexactlyevenlydistributedonallfoursidesofthehome,thetotalcoolingloadwouldbeequalinallorientations.Thisisrarely,ifever,thecaseintypicalproductionhomedesign.
BecausethemajorityofhomesbuiltinCaliforniaareproductionhomesusingthemasterplanconcept(severalplantypesusedoverandover,andbuiltmultipletimesinvariousorientations),thevariationbetweenbestandworstcaseorientationmustbeconsidered.Standardpracticeistodesignforworst-caseorientation.Thisisanacceptablepracticeforthevastmajorityofplans.Theriskofthisapproachisthattheequipmentinthebest-caseorientat
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ionisoversizedtoadegreethatcannegativelyimpacteffectivenessandefficiency.
Notonlydoesorientationimpactthetotalcoolingloadofahome,ithasanevengreaterimpactonanindividualroomsload.Thekeytoagoodductdesignisevendistributionofairinamountsproportionaltotheloadfromeachroom.Ifahouseisbuiltinmultipleorientations,theneachofitsroomscanandwillfaceanyorientation.Thismeansthatanindividualroomscalculatedcoolingloadcanchangebyafactorofnearlythreetimes(recallthedifferencebetweentheNorthHTMandEast/WestHTM.)This,inturnmeansthataroomsairflowrequirementcannearlytriple.Thenetresultisthatductsizingrequirementsforagivenroomcanchangeastheorientationchanges,butitisextremelyimpracticaltorequiredifferentductlayoutsforasinglemasterplandependingonwhatorientationitistobebuiltin.Thus,theworst-caseorientationisusedeventhoughitmaynotprovidethebestlayoutforallorientations.
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Example:
Thefollowingexampleisfora30-lotsubdivisionwiththreeplantypes.Plan1isa2000squarefootsingle-storyhome.Plan2isa2400squarefoottwo-storyhome.Plan3isa2850squarefoottwo-storyhome.Eachplanistobebuilt10timesasshownbelow.
Table3:SubdivisionSitePlanOrientation
LotPlanFrontOrientation11N22N33NE41NE52NE63E71E82NE93NE
101N112NW123NW131NW142W153W
LotPlanFrontOrientation161SW172SW183SW191S
202S213S221SE232SE243SE251E262NE273NE281N292N303E
Theloadsandequipmentsizingcanbetabulatedasshownbelow.
Table4:Plan1LoadsandEquipmentSizing
Plan1OrientationLotsSensibleLoad(Btuh)Cond/coil/furnace(tons)N1,10,28290673.5/4/4NE4332014/4/4
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E7,25330714/4/4SE22268713.5/4/4S19250673/4/4SW16267213.54/4W-339724/4/4NW13328714/4/4
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Table5:Plan2LoadsandEquipmentSizing
Plan3OrientationLotsSensibleLoad(Btuh)Cond/coil/furnace(tons)N2,29349995/5/5NE5,8,26380715/5/5E-370885/5/5SE23332814/5/5S20330184/5/5SW17336974/5/5W14400215/5/5NW11358815/5/5
Table6:Plan3LoadsandEquipmentSizing
Plan3DownstairsSystemUpstairsSystemOrientationLotsSensible
Load(Btuh)Cond/coil/furnace(tons)SensibleLoad(Btuh)Cond/coil/furnace(tons)N-225553/3/3289003.5/4/4NE3,9,27240823/3/3307212.5/4/4E6,30236213/3/3300203.5/4/4SE24219213/3/3272223.5/4/4S21210022.5/3/3261993.5/4/4
SW18208222.5/3/3267893.5/4/4W15250173/3/3311103.5/4/4NW12232213/3/3291813.5/4/4
Plan1:Sinceonlylot19hadaloadlowenoughtomakeita3/4/4,itwouldberecommendedthata3.5/4/4beusedhereandontheotherlotswhereappropriate.Theotherlotswouldget4/4/4systems.
Plan2:Thesizingshownisareasonablebreakdown.Notethatthereisnosuchthingas4.5tonsystem.Iftherewere,therewouldbethreesizesofsystems.
Plan3:Thesizingshownisareasonablebreakdown.Notethatallofthelotshadthesameequipmentsizingupstairs.Thisisbecausethesecondfloortypicallyhasamoreevenwindowdistribution.
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Notethatthisapproachwouldresultintheopportunitytodownsize10outof40condensersbyatleastone-halftonatasubstantialcostsavings.
Anexampleofhowthefrontorientationofthehouseaffectstheductlayoutforanexamplehouseistabulatedbelow.Thenumbersarethediameterofthebranchductservingtheroomsshown.Thenumbersvarybecauseasthehouseturnstheorientationofeachroomchanges,whichchangeseachroomsloadandsubsequently,itsairflow.
Trunkductsarenotshownbutareaffectedsimilarly.
Table7:Branchductdiametersundermultipleorientations
RoomNNEESESSWWNWMaxLiving766777677Dining766777677Living766777677Family777777777
Family777777777Kitchen777777777Nook777777777Den666656666Bath3444444444Laundry555555555Mbed888878888Mbath666666666Mwic444444444Bed2666666566Bath2444444444Bed3666666666Bed4666666666
Asonecansee,therequiredductsizesnevervarymorethanonesizeforanyparticularroom.Also,manyroomsareunaffectedbyorientation.Thisparticularhousehadafairlygoodfenestrationdistribution.Asglazinggetsmoreloadedonanysingleside,thevariationinductsizesgetsgreater.
Designingtothemaximumsizeforeachroomdoesnotresultinalargeamountofchangeformosthomesbutitdoesinsurethatallroomswillhaveductinglargeenoughtoprovideitsfair
shareinallorientations.
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4.4ZonalControlZonalcontroltypicallyreferstoasingleHVACsystemwith2ormoreindependentzones.Thisindependenceisaccomplishedthroughacontrolpanelandmotorizeddampersthatsendairtothezonesthatrequireitandlimitorstopaltogethertheairgoingtozonesthatdonotrequireit.Eachzonehasitsownthermostat.
Ashomesgetmoreandmoreefficient,thesizeofahomeservedbyasinglesystemgetslargerandlarger.Thelargerahouseis,themoredifficultitcanbetoadequatelycontroltheindoortemperaturewithasinglethermostat.Zonalcontrolisaneffectivewaytoaddzoneswithouttheexpenseofmultiplesystems.Zonalcontrolshouldbeusedforcomfortonly.Itwillnotreducetheloadoftheenvelopenorwillitincreasethetotalcapacityofthesystematpeakconditions.
Indecidingwhetherzonalcontrolisneededornot,thedesignermustconsidert
hediversityofthehome.Forexamplea3000squarefoot1storyhousethatissprawlingandspreadoutwithmanywingsandappendageswouldbemorelikelytoneedzonalcontrolthanahousewiththeexactsamecoolingloadbutthatislargerbutmorecompact.
Thedesignermustalsoconsidertherelativeairflowrequirementsbetweenthetwozonesastheychangebetweenheatingandcoolingmodes.Forexampleatwo-storyhousemayrequiremoreairdownstairsthanupstairsinheatingmodebutthatmayreverseincoolingmode.
Becausetheductsaresizedforcoolingairflow(duetothehigherfanspeed)thehomemayneedtobebalancedseasonallybyclosingdampersand/orregistersinordertogetadequatecomfortdistributionbetweentheupstairsanddownstairsinheatingmode.Thisisnotanunreasonableexpectationbutazonalcontrolsystemwouldhelpalleviatethiseffort.Ifazonalcontrolisnotinstalledinthissituation,theoccupantsshouldbeinformedoftheseasonalbalancingrequirementandeducatedonhowtoperformit.
Formorediscussiononzonalcontrol,seeSection3.2.1TheOverallDesignMetho
d,Step1.
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4.5WindowLoadsWindowsaccountforaverylargefractionofcoolingandheatingloadsinabuilding.Theglazingtype,theamountofglazing,insulationandshadingdevicesusedallcontributetoasignificantportionoftheoverallcoolingloads(mainlysolargains)andheatingloads(conductiveheatlosses)inabuilding.
Asanexample,a1940squarefoothomewithan18.6%window-to-wallratiowasanalyzedin4climatezones(zones7,10,12,and14)andfourorientationsusingMicropas6.Heatingloadsattributedtoglazedsurfacesremainedapproximatelyequal(16.5%-18.0%,dependingonclimatezone).Coolingloadsvariedbetween32.0%and41.3%dependingonbothorientationandclimatezone.Becausewindowsrepresentsuchahighpercentageofheatingandcoolingloads,itisimportantthattheirimpactbeaccuratelyquantified.
4.5.1HeatingloadsfromwindowsIncalculatingheatingload,onlyconductiveheatlossiscalculatedbecausesolargainsreducethenetheatlossandactuallyassisttheheater.Heatlosscalculationsarethereforebasedonnighttimeconditionswhentherearenosolargains.AsimpleUA..Tcalculationisused:
qUA..TA..T
R
Inthisequation,Uistheoverallwindowu-valueincludingglassandframe;Aist
eroughopeningofthewindow;and..Tissimplythedifferencebetweentheindoorandoutdoorwinterdesigntemperatures.
TheabilityoftheUA..Tformulatopredictactualheatlossesislimitedbytheaccuracyoftheinputparameters.Areaisnotaproblemsinceitisafixedvalue.U-valueislimitedbytheaccuracyofgenericwindowdescriptionstoaccuratelyreflecttheactualU-valuesofallthedifferentbrandsofwindowsthatmaymeetthegenericdefinition.Ifthemakeandmodelofthe
windowtobeinstalledisknownanditisawindowthathasbeentestedtoNationalFenestrationRatingCouncil(NFRC)standardstherewillbeareasonablyaccurateU-valuethatcanbeusedforthatwindow.Eventestedvalueshavetheirlimitations.U-valuewithinaparticularmakeandmodelofwindowwillvarybywindowsizebecausetheframe-to-glassratiochanges.Asareasonablesimplificationandtokeepthecostoftestingwindowsdown,onlyasinglecommon
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sizewindowistestedandthattestedU-valueisusedforallwindowsinthatproductline.
Theactual..T(differencebetweentheindoorandoutdoorwinterdesigntemperatures)valuecanvarysomewhatfromthenumberusedinthecalculations.Ofcourse,outdoortemperaturevarieswithseasonandtimeofday,butthe..Tusedinthecalculationcanbewrongevenatthetimewhentheyaresupposedtobecorrect.Tounderstandthis,itisimportanttounderstandhowthesetemperaturesareselected.
Theindoordesigntemperatureisthedesiredindoortemperature.Itcanbethoughtofasthethermostatsetpoint.However,evenwhenathermostatreadsacertaintemperature,70degreesforexample,itwillnotbe70degreeseverywhereinahouse.Therecanbeplacesinthehousewherethetemperatureissubstantiallyhigherorlowerthan70degrees.Forexample,supplyairregistersarecommonlyplaceddirectlyaboveorbelowwindows.Whentheheaterisoperating,hotairofupto150degreesisblowingonornearthewind
ow.Withan
6Enercomp,Inc
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outdoortemperatureof30degrees,thisyieldsareal..Tof120degrees.Ifthedesigntemperatureswereassumedtobe70degreesindoorsand30degreesoutdoors,thereal..Tisthreetimesthedesign..Tof40degrees,triplingtheheatloss.
Theoutdoordesigntemperatureisastatisticallyderivedtemperaturebasedonhistoricaltemperaturedatacollectedatanearbydatacollectionpoint.Therearehundredsofthesethroughoutthestate.Becauseitisastatisticallyderivedvalue,ratherthanthecoldesttemperatureonrecord,forexample,itisunderstoodthatthistemperaturewill,bydefinition,beexceededacertainnumberofhoursperyear.Thestatisticalnumberthatisusedisdeterminedtobeonethatmakestheseexcessivetemperatures(i.e.,temperaturescolderthantheassumedoutdoordesigntemperature)anacceptableoccurrence.Variationsfromthisdatacanbecausedbymicroclimatesornormal(orabnormal)macroclimaticchangesandwillthrowo
ffthestatisticalaccuracyloadcalculations,butproblemswiththeindoortemperatureasdescribedabovewillhaveanevengreaterimpactinthestatisticalaccuracyoftheloads.Inotherwords,theactualnumberofhoursthattherealheatloadexceedsthecalculatedheatloadmaybedangerouslyhigh;theheatermaybeunablemaintainacomfortableindoortemperatureduringlongperiodsofextremecoldwhenrealityexceedsthedesignmargin.
4.5.2CoolingloadsfromwindowsCoolingloadslargelyconsistoftheincomingsolarradiationthroughthewindow
sandconductiveheatgain.Heatgaincalculationsaremadeupofaconductivecomponent,verysimilartoheatlosscalculations,buttheheatistravelingintothehouseratherthanoutofthehouse.Heatgaincalculationsaresusceptibletothesamefactorsthatmakeheatlosscalculationsinaccurate.Theyarealsomadeupofamuchlargerradiantcomponent.Thisistheheatgainassociatedwithsunlightpassingthroughthewindowsandiseffectedbyaverylargenumberoffactors,onlyafewofwhichareaccountedforintheloadcalculations,for
simplicityreasons.Also,forsimplicityreasons,theloadassociatedwithsunlightisaveragedthroughouttheday.Thisiscalleddiversityandhastodowiththefactthatthesuntravelsacrosstheskyandtheactualloadonroomsinahousewillnotmatchthisaveragedvalue.Somecalculationmethodsallowapeakloadtobecalculatedwhenappropriate.Thisisthehighestcoolingloadthatwilloccuratanytimeduringagivenday.
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Factorsthateffectwindowheatgainandloss,calculatedandactual,aresummarizedbelow:
xWindowareatotalandforeachorientation.Becausewindowsarealessefficientpartofthebuildingshellthanwalls,floorsorceilings,themorewindowsyouhave,thehighertheheatingandcoolingloadswillbe.Somewindowshaveahigherheatgainpersquarefootbecauseoftheirorientation.Seetheorientationdiscussioninthenextsection.xLocationThegeographiclocationofthehousecanimpactthecoolingloadsassociatedwithwindowsotherthansimplyaffectingtheoutdoordesigntemperatures.Thelatitudeofhousedeterminestheangleofsunandsunspathacrossthehorizon.Localfactorscanaffecttheintensityofsun.Theseincludecloudcover,pollution,andhumidity.xWindowsolarheatgaincoefficient(SHGC).Thisisapropertyoftheparticularwindow
andisdefinedastheratioofthesolarheatgainenteringthespacethroughthefenestrationareatotheincidentsolarradiation.Solarheatgainincludesdirectlytransmittedsolarheatandabsorbedsolarradiation,whichisthenradiated,conducted,orconvectedintothespace.TheSHGCofawindowisaffectedbythenumberofpanes,thicknessandclarityoftheglasspanes,anytintingorotherspecialcoatings,
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thicknessoftheframe,mullionsandotherdetails.SHGCcanbedramaticallyimprovedthroughtheuseofspecialcoatingsthatblockcertainwavelengthsoflight,particularlythoseresponsibleforheatgain.
xU-value.TheU-valuedescribesawindowassemblysabilitytotransmitheatconductivelyandisafunctionofthepropertiesofboththeframeandglasspanes.LiketheSHGC,itcaneitherbeagenericnumberbasedonthegeneraldescriptionofthewindoworitcanbeaNationalFenestrationRatingCouncil(NFRC)testedvalue.
xEmissivityofwindow.Thisnumberdescribestheamountofheatthatisemittedfromawindowduetoitsbeingwarmerthanthesurroundings.Thelowerthelevelofemissivity,themoreefficientthewindow.Emissivitylevelsgenerallyrangefrom0to1and
canbedramaticallyimprovedthroughtheuseofspecialcoatings.EmissivityisusuallyaccountedforinloadcalculationsbyadjustingthewindowU-value.
Shading.Shadingdevicesareeitherinteriororexterior.Theycanbefurthersubdividedintoremovable(orotherwisecontrollable)andfixed.Thiscontrollabilityisimportantbecausetheycanassistinreducingheatgainincoolingmodebuttheycanalsoreduceheatgaininheatingmodewhenheatgainmaybedesired(i.e.,ona
coldbutsunnyday).Anadditionaltypeofexteriorshadingincludesthosethatarenotnecessarilyintegraltothebuildingandarecategorizedasadjacentstructures.
Interiorshadingdevices.Curtains,blinds,rollershadesandothersuchinteriorwindowtreatments,thoughoftenaestheticinpurpose,canhaveasubstantialimpactonheatgainswhenusedcorrectly.Themoreopaqueandreflectivethematerial,themoreitwillreducesolarheatgain.Forexample,awhite,opaquerollershadew
illreducesolargainsbetterthanadarkdrape.Onedisadvantageofinteriorshadingdevicesisthatsolargainshavealreadyenteredthespacebythetimetheyareinterceptedbytheinteriorshadedevice.Thisheatistrappedbetweentheshadingdeviceandthewindow.Someoftheheatisreflectedorradiatedbackoutofthewindow,butmuchofitremainsinside.
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Exteriorshadingdevices.Thesearedevicesthatarepartofthebuildingorwindowassemblyandincludeoverhangs,bugscreens,solarscreens,andawnings.Overhangsareoftenoverlookedasveryefficientdevicesforreducingloadsandenergyconsumption.Architecturalfashiontypicallyoutweighstheirpracticality.Thoughapermanentcomponentofthebuildingtheycanbedesignedtomaximizethebenefitinthesummerandminimizetheirimpactinthewinter.BugscreensarenotconsideredanenergydevicebutcanhaveanoticeableimpactontheSHGCofawindowassembly.Sun-screens(a.k.a.solarscreens)canbeaverycosteffectivemeansofreducingheatgain.Also,becausetheyareremovable,theirimpactintheheatingseasoncanbeminimized.Awningsbehaveasanoverhangandarealsoseasonallyremovable.
Adjacentstructures.Thesecanincludebuildings,trees,fences,andterrainsuchashills.Theymayhaveasubstantialimpactonactualloadsbutarerarelyaccounted
forinthecalculations.Theymostcommonlyshadeawindowbutcanhavetheoppositeimpactofreflectinglightintoawindow.Inthisregard,thegroundadjacenttoabuildingisconsideredanadjacentstructurebecauseitcanreflectadditionallightintoawindow.Imaginethedifferenceinsolargainsbetweenahousesurroundedbylushlawnandahousesurroundedbyabrightwhiteconcretesurface.
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BestPractices
Bestpracticefornewconstructionloadswouldbetomodelnointernalorexternalshadesintheloadcalculations,buttomodeloverhangsbecausetheyarefixedarchitecturalfeaturesofthebuildingthatareunlikelytoberemoved.Internalandexternalshadesarefrequentlyleftopen,leftofforotherwiseremoved.Toassumethattheyareinplacewhencalculatingcoolingloadsisrisky.Somedesignersbelievethatinteriorshadesshouldbeassumedclosed.Thisresultsindramaticallylowersolargainsandcoolingloads.However,ifthecoolingequipmentissizedundertheseassumptions,thehomewillnotcoolproperlyonhotdaysifthehomeownerdoesnotclosethedrapes.Whileclosingdrapesonahotdayisapraiseworthybehavior,thisdesignphilosophyisnotconsistentwiththeexpectationsofmosthomebuyers.
TheapproachusedformodelingfeaturesinTitle24complianceisusuallyappropriateforloadcalculationsinnewconstruction.InManualJ,Version8,thedesignershouldalwaysassumeNFRCratedwindowswillbeusedinnewconstruction.Ifnon-ratedwindowsareuseddefaultperformancevaluescanbeusedthatareconsistentwithTitle24calculationsbutenteredintheloadcalculationsasthoughtheyareratedwindows.Assumethesameminimumfeaturesnecessaryforcompliance,ifslightlybetterfeaturesgetinstalled,fine.If,however,better
featuresgetinstalledthanwereassumedintheloadcalculations,thereisasmallriskofoversizingtheequipmenttoapointofreducedenergyefficiencyandconditioningperformance.However,thepotentialexpensetoabuilderofundersizingequipmentisfargreaterthanthatofoversizing.
Performancevaluesusedintheloadcalculations(U-value,SHGC,andshadingcoefficientofscreensandothershadingdevices)shouldbeconsistentwiththoseusedintheTitle24calculations.ThecurrentcomputerizedversionsofManualJ,Version8,forroom
-by-roomloadsandthecurrentmethodologyusedbyMicropasforwhole-houseloadsdoaveryadequatejobaccountingforloadsassociatedwithwindows.ItisausefulexercisetocomparetheMicropasloadtothetotaloftheroom-by-roommanual.Thisprovidesatrustworthychecktohelpensurethatnocalculationerrorshavebeenmade.Thisisanotherreasonwhyitisimportanttousethesamewindowperformancevaluesinbothcalculations.
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4.6DuctLoadsDuctleakageratesofupto45%werenotuncommoninnewhomesbuiltandtestedpriortothelate90s.Thisisadirectlossofconcentratedenergy;theheatedorcooledairisdumpeddirectlyintounconditionedspaces(e.g.,supplyleaksintoattics),orconditionedairisdisplacedbyunconditionedair(returnleaksinatticsorgarages).
ManualJdoesareasonablejobofaccountingforductleakageloads,givenaknownleakage.Theproblemliesnotinquantifyingaknownleakageratebutinestimatingtheactualleakageamount.Priortoconstructionand/orwithoutactuallytestingthesystemleakage,itisverydifficulttopredict.Field-testinghasshownthatusingverysimilarinstallationprotocolsontwosimilarhousescanstillresultinleakageratesthatarevastlydifferent.Eventhebrandoffurnacecanaffecttheleakageratebyone-thirdormore.
Title24softwareassumesthatthesystemistightifitisknownthatthehomewillbetested,andrepairediftheleakageisgreaterthan6%.Ifthehomeissubsequentlytestedandtheleakageisindeedlessthan6%thenthedesignercanrestassuredthattheloadcalculationsarevalid.Howeverifthesystemisnottestedandtheleakageissignificantlymorethan6%,theequipmentmaybeundersized.Commonly,ifthesystemisnotgoingtobetested,currentpracticeistoassumethatthesystemisguiltyuntilproveninnocenti.e.itleaksmorethan6%.Thesystemisassumedtobetypical,withaleakageof22%.Ifthedesigneras
sumesthishigherleakageandtheinstallerdoesanexcellentjobofinstallingthesystem,thesystemmaypotentiallybeoversized.
Eventestingasystemusingcommonproceduressuchasaductblastertestdoesnotguaranteethattheactualloadoftheductleakagewillbeaccuratelyestimated.Limitationsofcurrentductleakagetestsresultinsubstantialvariancesbetweentestedleakageandactualleakage.Theselimitationsincludetheinabilityofthetest,usingcommonpractices,to
distinguishbetweensupplyandreturnleaksandtheinabilitytoidentifythelocationofaleak,whichmaybelocatedinaveryhighpressurepartofthesystem(nearthefan)orinaverylowpressurepartofthesystem(neararegisterorgrille).Note:Theductblastertestpressurizestheentiresystemtothesamepressurelevelandtherebytreatsallleaksequally.
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Thebestwaytominimizevariancesbetweenestimatedandactualleakageistoassumethattheleakageisattainablylowandthenmaketheappropriateefforttoensurethatitisinstalledthatway.Moresophisticatedtestmethodsmayimprovetheaccuracyofmeasuringleakage,butthetighterthesystemsbecome,thelawofdiminishingreturns
makesmoretestingexpensiveandunnecessary.48
SpecialDesignTopics4.6DuctLoads
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4.7Two-storyConsiderationsAshomesbecomemoreandmoreefficient,theirheatingandcoolingloadsdecrease.TheresultofthisisthatlargerandlargerhomesarebeingservedbysingleHVACsystems.InatypicalCaliforniasubdivisionthatoffersfourfloorplans,threewillbetwo-storyhomes.Manyofthoseareservedbyasinglesystem,averycommondesigninCalifornianewconstructionandonethattendstohavemanycustomerservicecomplaintsrelatedtotemperaturevariations(stratification)inthehome.
ManyHVACsubcontractorsbelievethatatwo-storyhomewithasinglesystemmusthaveasubstantialamountofthereturnairtakenfromthefirstfloor.Whilethereisnoevidencetosupportthis,HVACsubcontractorswillinsistthatarchitectsandbuildersgotogreateffortandexpensetoaccommodatearelativelylargereturnductandgrilltothefirstfloor.Some
designersbelievethatareturnintheceilingofthesecondfloorisadequateaslongasthedownstairssupplyductsareproperlysized.
Thereisalsomuchdebateanddisagreementovertheproperlocationofathermostatinatwo-storyhomeservedbyasinglesystem.Somedesignerslocateitupstairsbecauseheatrisesandthatiswherethemostcoolingisneeded(coolingemphasized).Otherslocateitdownstairsbecauseinthewinterthefirstfloortendstobecolderandthatiswherethemostheatingisneeded(heatingemphasized).
Aspartofthetaskofdevelopingthisdesignguide,astudywasconductedtoevaluatetheimpactofthenumberandlocationsofreturnsandtheplacementofthethermostatinatwo-storyhomeservedbyasingleHVACsystem.
Threereturnconfigurationswereevaluatedforcoolingusingacomputationalfluiddynamicsmodel(CFD).ThesethreeconfigurationsweredesignedtoaddressthecommonpracticesinCaliforniaproductionhomebuilding:
xCase1:splitreturnsupstairsanddownstairs;thermostatupstairsxCase2:returnupstairs;thermostatupstairsxCase3:returndownstairs;thermostatdownstairs
Thefigurebelowisanexampleoftheinformationgeneratedbythisstudyshowingthetemperaturesanddutycyclesforthethreeconfigurations.Case2(returnupstai
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rs/thermostatupstairs)andCase3(returnupstairs/thermostatdownstairs)cycletwiceasoftenasCase1(returnsupstairsanddownstairs/thermostatupstairs).Case1,withsplitreturnupstairsanddownstairs,providesabettermixingofair,delayingthereturntoambienttemperature.
SpecialDesignTopics4.7TwoStoryConsiderations
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