supplement a improving forced air heating systems forced air heating ... university extension energy...
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�
Supplement A
The Challenge RecentresearchandtestingofnewhomesinthePacificNorthwestandacrosstheUnitedStatesshowstheimpor-tanceofaproperlyinstalledHVACsystem.Interactionsbetweensystemcomponents,thehouseenvelope,andotherequipmentcanseriouslyaffect:
• Occupanthealthandsafety;• Occupantcomfort;• Equipmentandstructuraldurability;and• Energyefficiency.
TheWashingtonStateEnergyCode(WSEC)providesspecificationsforductsealing,ductinsulation,equipmentsizing,equipmentefficiency,andcontrolsthatprovideabasisforasafe,efficientsystem.Inforcedairheatingandcoolingsystems,attentiontodetailinductinstallationisveryimportant.Ductworkthatisundersized,unbalanced,orleakycancauseseriousproblems.Understandingtheseproblemshelpsyoubuildabetter,saferhomeforyourclientthatmeetstheintent,aswellastheletter,oftheCode.
Health and Safety Leakyductscancompromisehealthandsafety.Because
theairhandlerfandrivesairintotheductwork,evensmallleakscanhavealargeimpact.Whenductleaksareconnectedtoareasoutsideoftheconditionedspace,theleakageinducespressurechangesacrosstheenvelopeofthehouse.Supplyleaksdepressurizethehouse.Returnleakspressurizethehouse(seeFigureA-1.)Depressuriza-tioncancausecombustionappliancestobackdraft.Back-draftingallowsfluegasestoenterthelivingspaceandcan
Improving Forced Air Heating Systems
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Supplement A-2 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
Supply and Return Leakage
Figure A-1
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�
Unbalanced Duct System
Figure A-2
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Supplement A-� WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
beveryserious,potentiallyleadingtodeathfromcarbonmonoxideexposure.Returnleaksinaconfinedareasuchasautilityroomorbasementcanalsoinducedepressuriza-tion.Thiscanbackdraftanadjacentgas-firedwaterheater,orsucksoilgases(suchasradon)intoabasement.Returnleaksinagarage,crawlspace,oratticcanpotentiallyintroducepollutantsintoahouse,adverselyaffectingindoorairquality.
Unbalancedductsystemsalsocausepressureproblems.Insystemswithcentralreturns(verycommoninnewerhomes)supplyregistersandreturngrillesareoftenisolatedwhenbedroomdoorsareclosed.Withthedoorclosed,thezonewiththereturndepressurizes,againcreatingpotentialbackdrafts(seeFigureA-2).
Testinginnewhomesshowsthatapproximately1squareinchofunobstructedreturnairpathwayisrequiredforeachCFMofsupplyairdeliveredtoazone.Thismaybeaccomplishedwithductedreturnsineachzone,undercutdoors,transfergrilles,orsomecombinationoftheabove(seeFigureA-3).
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�
Unobstructed Return Air Options
Figure A-3
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Supplement A-6 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
Moisture Damage Resulting from Return Duct Leakage
Figure A-4
Improper duct design and sizing can cause heating and cooling equipment to operate out of manufacturer’s specifications for temperature rise, pressure drop, or air flow. This impacts efficiency and equipment lifetime. Airflow through the system is especially important for air conditioning and heat pump equipment. Inadequate airflow is a major cause of premature compressor failure. Follow manufacturer’s specifications, but generally look for 400-425 cfm of airflow per ton of installed capacity.
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-7
Comfort Leakyductscanalsocausecomfortproblems.Whensupply
ductsleak,theairdeliverytodifferentpartsofthehomemaynotmatchheatingandcoolingloads.Iftheductsarewellsealed,theregisterdamperscanbeadjustedtodistrib-uteconditionedairwhereitisneeded.Iftheductsleak,thiscontroloptionislost.
Returnleaksconnectedtotheoutsidecanchangethereturnairtemperature,adverselyimpactingsystemperfor-mance.
Example:Alargereturnleakinanatticinthesummertimemaydraw150°Fairintothesystemratherthan75-80°Fhouseair.Thehigherreturntemperaturecanoverwhelmthesystemcapacityandmakeitimpossibletocoolthehome.
Ductleakagemayalsochangetheairinfiltrationrateofthehomeleadingtoexcessivedrynessduringtheheatingseasonandhighhumiditylevelsduringthecoolingseason.Botharecommoncomfortcomplaints.
Improperlysizedductworkandpoorregisterplacementoftencontributetocomfortproblems.Inadequateorexcessiveairflowmayresultfrompoordesign.Heatpumpsystemswithlowerdeliverytemperaturesareespeciallyvulnerabletocomfortcomplaintsbecauseofthehighervolumesofairthatmustbemoved.
Durability Leakyreturnductsconnectedtotheoutsidepressurizea
home.Unbalancedsystemsalsoinducepressurizationinzoneswheresupplyairdoesnothaveanadequatereturnpathway(seeFigureA-1).Pressurizedzonesinhomesinheatingclimatesmayadverselyaffectstructuraldurability.Duringtheheatingseason,positivepressureinthehomecauseswarmmoistairtoflowintobuildingcavities.Astheairreachescolderexteriorsurfaces,themoisturecondensesandwetsthebuildingmaterials.
Example:ThepictureinFigureA-4showsmoisturedamagetotheexteriorsidingonanewhomecausedbyareturn
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Supplement A-� WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
System with Fully Ducted Returns
Figure A-5
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�
ductleak,whichpressurizedthehouseanddrovemois-tureintotheexteriorwall.Theareaofmostseveredamageistheexteriorwallforanupstairsbathroom.Theuseofaircleaningsystemswithcontinuousruntimesexaggeratesthisproblem,makingpropersealingandbalancingevenmoreimportant.
Energy Efficiency Theproblemsthatimpacthealthandsafety,comfort,and
durabilitycanalsoadverselyaffectenergyefficiency.Leakyductscandeliverconditionedairtounconditionedspaces.Pressuredifferentialscreatedbyleakyductsorunbalancedsystemsincreasewholehouseairleakagerates.Toputthisinperspective,studieshaveshownthat,eveninnewhomes,manysystemsperformbelowtheirexpectedeffi-ciency(lossesof20-30%arenotuncommon.)
Ductworkanditsinteractionwiththehousehasasignifi-cantimpactonoverallsystemeffectiveness.Whensystemcomponentsoperateoutsideofmanufacturer’sspecifica-tionsfortemperaturerise,pressuredrop,orairflow,effi-ciencyisoftenlost.
The Solutions
Duct Sealing Toachieveoptimumductsealingperformance,thefollow-
ingpracticesarerecommended:
• Don’tusebuildingcavitiesasductwork.TestinginNorthwesthomeshasshownthatreturnductsleakfarmoreoftenthansupplyducts.Thisisassociatedwiththeuseofpannedfloorjoistsandbuildingcavitiesusedaspartofthereturnsystem.Makingatightwell-sealedductoutofabuildingcavityisoftenverydifficult.Foraqualitysystem,itmaybelessexpensive(inthelongrun)toinstallafullyductedreturn(seeFigureA-5).
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Supplement A-�0 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
Poor Duct Fittings Can Not be Well Sealed
Figure A-6
Tight fittings and Mastic Make the Best Seal
Figure A-7
Tight fittings and assembly are required to control duct leaks over the long term.
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-��
• Selectfittingsthatdon’tleavelargegapsinthesystem.Manyductsareassembledwithlargegapsbetweenfittings.Eveniftheyarewellsealedwhenfirstinstalled,thispracticewilleventuallyleadtosealingfailure.Ductsealingmaterialssimplyarenotdesignedtoseallargeholesforlongperiodsoftime.Tightfittingsandassemblyarerequiredtocontrolductleaksoverthelongterm(seeFigureA-6).
• Don’tuse“ducttape”–instead,usedurablesealingmaterials.AdvancedagingtestsconductedoncommonlyusedductsealingmaterialsbyLawrenceBerkeleyNationalLaboratoryconcludedthatducttapeisapoorperformer.Clothorvinylbackedducttapeswithrubberadhesivesfailedveryquicklyandarenotrecommendedforductsealing.Themetal-backedtapeswithacrylicadhesiveworkedbetter.Masticswerebyfarthemostdurable(seeFigureA-7).
• Selectsealingmaterialsthatarecompatiblewithductsystemcomponents.Ductsealingmaterialsneedtobeinstalledaccordingtomanufacturers’instructions.Manytapesrequirethattheductsbecleanandoilfreebeforeinstallation.UsingsealantsratedundertheUL181standardassurescompatibilitywithductboardandflexibleductsystems.
• Useperformancetestingmethodstoassureatightseal.Contractorsshouldconsiderducttightnesstesting.Whilethismaynotbepracticaloneveryjob,itisavaluablelearningexperience.Ducttight-nesstestingwillprovidefeedbackonproblemareasandsharpenacrew’sabilitytoprovidequalityinstallations.Testingcanalsoalertcontractorstopotentialliabilitiesassociatedwithcombustionapplianceback-drafting,aswellasdurability
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Supplement A-�2 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
issues.TestsarerequiredtoobtainEnergyEfficientMortgagesandtheEnergyStar®Homesfivestarrating.Makesuretheductsarefullyinsulated.ThecoderequiresR-8insulationforductsoutsidetheconditionedspace(R-5inslabsortheground.)TheSuperGoodCents®energyefficiencystandardsuggeststhatinsulatingductsuptoR-11iscost-effectiveinelectricallyheatedhomes.SeeChapter7oftheBuilder’s Field Guideformoredetail.
Move the Ductwork Inside Ratherthanplacingtheairhandlerandductworkinthe
garage,crawlspaceorattic,considermovingitinside.Bybringingthecomponentsinsidetheinsulationandairbarrierofthehome,significantenergysavingscanbeachieved.Bykeepingthesystemcompletelyindoors,thechanceofbringingpoorqualityairinfromthegarageoratticspaceiseliminated.Anadditionalbenefitisthatductsinstalledintheheatedspacedonotrequireinsulation.
StudiesconductedintheNorthwestindicatethatforcedairheatingsystemslocatedcompletelyindoorswillreduceenergyusebyabout30%.Thiscanbeaccomplishedatnoadditionalcost.Whenthebuilderprovidesaspaceindoorsfortheequipmentandducts,itreducestheHVACcontrac-tors’cost.TheHVACcontractorwilluselessductwork,willnotneedtoinsulatetheducts,andwillbeabletoworkinbetterconditions.TheHVACcontractorwillalsobeabletospecifysmallerequipment,reducingcostfurther.Thebuilderwillneedtoprovideaindoorspacefortheducts.Thismayaddtothecostofframinganddrywall.Perproject,theHVACcontractorandbuildercostwillusuallyoffseteachother.
Movingtheheatingsystemindoorsismosteasilyaccom-plishedintwo-storyhomes.Theductworkcanbeplacedinthespacebetweenthefloors.Muchofthesystemcanbeplacedinexistingfloorcavities(seeFigureA-8).
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-��
Wheretransitionsmustbemadebetweenfloorcavities,dropsoffitsaretraditionallyused(seeFigureA-9).
Ducts Placed Within the Floor Joists
Figure A-8
Drop Soffit Conceals Ducts
Figure A-9
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Supplement A-�� WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
Theincreasinguseofengineeredfloortrussesprovideadditionalopportunitiesformovingductseasilybetweenfloors.Mostcompositewoodtrussescanbecuttoaccom-modatelargetransitionsacrossthefloorjoist.Webtrussesprovidethegreatestopportunity.Steelwebswithwoodcordsprovideareaslargeenoughtoincorporatesubstantialductsystems(seeFigureA-10).
Engineers Trusses Provide Space for Ducts Between Floors
Figure A-10
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WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-��
Bringingcombustionfurnacesinsidetheenveloperequirescareinmeetingcombustionairandventingrequirements.Directventandsealedventequipmentarelikelythebestsolutions.
Heating System Design and Installation Heatingsystemsdonotperformtopromisedspecifications
whentheductsarenotthecorrectsize.Ductssupplyingairtoindividualroomsneedtobesizedtoprovideairflowthatmatchestheheatingrequirementoftheroom.Theentiresystemmustbedesignedsothatitallowsthecorrectairflowthroughtheheatingandcoolingequipment.Inmanycases,thesetwocriteriaarenotmet.Thisleadstocomfortproblems,andlossofequipmentefficiency.
Tocorrectlysizeductsystems,userecognizedengineer-ingprinciplesandcalculatedloads.ThemostwidelyusedmethodsareACCA* Manual JheatlossmethodandACCA Manual Dductsizingmethod.
Thehomedesignerneedstoprovidespacefortheduct-work.Forcedairheatingsystemsneedtobeincludedinthehomeplanningprocess.Toooftensystemsfailbecausethehomedesignerhasnotprovidedspaceforequipmentandductwork.HomedesignersshouldconsulttheHVACcontractorearlyinthedesignprocess.
* Air Conditioning Contractors of America Association, Inc.
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Supplement A-�6 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program
Recommended Practice FollowthesesuggestionstobuildqualityHVACsystems:
• MeetorexceedCoderequirements.CodesaffectingHVACinstallationsaresetuptoestablishminimumsafetyandefficiencystandards.It’sagoodplacetostart.
• Sizeanddesignductsystemsusingrecognizedengineeringprinciplesandcalculatedloads.Examples:ACCA Manual D and ACCA Manual J.
• Installequipmentaccordingtomanufacturer’sinstructions.
• Testandadjusttheequipmentandthedistributionsystem.
• Whencombustionappliancesarepresent,provideforcombustionsafety.Ventproperly,providecombustionair,minimizenegativepressures,testforCO(carbonmonoxide)productionandbackdrafting,installCOalarms,andeducatethehomeowneraboutcombustionsafety.
• Wherepossible,installductsintheheatedspace.• Sealducts.• Insulateductsnotinstalledintheheatedspace.• Useasystemsapproachtoassurehealthand
safety,comfort,durabilityandenergyefficiency.