hvac design guide

8/2/2019 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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HVACDesignGuideVersion1.0

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.

Introduction1.3Limitations

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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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TheDesignProcess2.1DesigningthehousearoundtheHVACSystem


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ConstructionDocuments:finalworkingdrawingsreadyforbidding,submittal.Backcheckingandcoordinationbyconsultants.SomelateVEmeetings.

TheHVACdesignersneedtoprovideinputasearlyaspossible.TheyneedtotellthearchitectwhicharchitecturalfeaturescausecomfortissuesandaredifficultorimpossibletoovercomewithtypicalHVACpractices.Theyalsoneedtomakesurethearchitectallowsadequatespacetorunducts.Manyarchitectshavehadtore-designplansenoughtimesduetoHVACissuesthattheyknowfairlywellhowtoaccommodateHVACitems.Still,manyproblemscommonlyarisethatcouldbeavoidedthroughearlierinputandbettercoordination.

TheDesignProcess2.1DesigningthehousearoundtheHVACSystem

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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.

DesignMethodology3.1CodeissuesrelatedtoHVACDesign

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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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DesignMethodology3.2ACCAManualsJ/S/D(Step1)


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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.

SpecialDesignTopics4.2RegisterLocation

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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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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


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.

SpecialDesignTopics4.4ZonalControl

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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

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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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HVACDesi