Supplement A Improving Forced Air Heating Systems
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WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-1 Supplement A The Challenge Recent research and testing of new homes in the Pacific Northwest and across the United States shows the impor- tance of a properly installed HVAC system. Interactions between system components, the house envelope, and other equipment can seriously affect: • Occupant health and safety; • Occupant comfort; • Equipment and structural durability; and • Energy efficiency. The Washington State Energy Code (WSEC) provides spec- ifications for duct sealing, duct insulation, equipment sizing, equipment efficiency, and controls that provide a basis for a safe, efficient system. In forced air heating and cooling systems, attention to detail in duct installation is very important. Ductwork that is undersized, unbalanced or leaky can cause serious problems. Understanding these problems helps you build a better, safer home for your client that meets the intent, as well as the letter, of the Code. Health and Safety Leaky ducts can compromise health and safety. Because the air handler fan drives air into the ductwork, even small leaks can have a large impact. When duct leaks are connected to areas outside of the conditioned space, the leakage induces pressure changes across the envelope of the house. Supply leaks depressurize the house. Return leaks pressurize the house (see Figure A-1). Depressuriza- tion can cause combustion appliances to backdraft. Back- drafting allows flue gases to enter the living space and can Improving Forced Air Heating Systems
Transcript of Supplement A Improving Forced Air Heating Systems
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-1
Supplement A
The Challenge RecentresearchandtestingofnewhomesinthePacificNorthwestandacrosstheUnitedStatesshowstheimpor-tanceofaproperlyinstalledHVACsystem.Interactionsbetweensystemcomponents,thehouseenvelope,andotherequipmentcanseriouslyaffect:
• Occupanthealthandsafety;• Occupantcomfort;• Equipmentandstructuraldurability;and• Energyefficiency.
TheWashington State Energy Code(WSEC)providesspec-ificationsforductsealing,ductinsulation,equipmentsizing,equipmentefficiency,andcontrolsthatprovideabasisforasafe,efficientsystem.Inforcedairheatingandcoolingsystems,attentiontodetailinductinstallationisveryimportant.Ductworkthatisundersized,unbalancedorleakycancauseseriousproblems.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
Supplement A-2 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program
Figure A-1
Supply and Return Leakage
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-3
Figure A-2
Unbalanced Duct System
Supplement A-4 WSEC Builder’s Field Guide, 8th Edition, 2009 • 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,transfergrillesorsomecombinationoftheabove(seeFigureA-3).
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-5
Figure A-3
Unobstructed Return Air Options
Supplement A-6 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program
Figure A-4
Moisture Damage Resulting from Return Duct Leakage
Improper duct design and sizing can cause heating and cooling equipment to operate out of the 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 the manufacturer’s specifications, but generally
look for 400-425 cfm of airflow per ton of installed capacity.
WSEC Builder’s Field Guide, 8th Edition, 2009 • 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:ThephotographinFigureA-4showsmoisturedamagetotheexteriorsidingonanewhomecausedbya
Supplement A-8 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program
Figure A-5
System with Fully Ducted Returns
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-9
returnductleak,whichpressurizedthehouseanddrovemoistureintotheexteriorwall.Theareaofmostseveredamageistheexteriorwallforanupstairsbathroom.Theuseofaircleaningsystemswithcontinuousruntimesexaggeratesthisproblem,makingpropersealingandbal-ancingevenmoreimportant.
Energy Efficiency Theproblemsthatimpacthealthandsafety,comfortand
durabilitycanalsoadverselyaffectenergyefficiency.Leakyductscandeliverconditionedairtounconditionedspaces.Pressuredifferentialscreatedbyleakyductsorunbalancedsystemsincreasewholehouseairleakagerates.Toputthisinperspective,studieshaveshownthat,eveninnewhomes,manysystemsperformbelowtheirexpectedeffi-ciency(lossesof20-30percentarenotuncommon).
Ductworkanditsinteractionwiththehousehasasignifi-cantimpactonoverallsystemeffectiveness.Whensystemcomponentsoperateoutsideofthemanufacturer’sspeci-ficationsfortemperaturerise,pressuredrop,orairflow,efficiencyisoftenlost.
The Solutions
Duct Sealing Toachieveoptimumductsealingperformance,thefollow-
ingpracticesarerecommended:
• Donotusebuildingcavitiesasductwork.TestinginNorthwesthomeshasshownthatreturnductsleakfarmoreoftenthansupplyducts.Thisisassociatedwiththeuseofpannedfloorjoistsandbuildingcavitiesusedaspartofthereturnsystem.Makingatightwell-sealedductoutofabuildingcavityisoftenverydifficult.Foraqualitysystem,itmaybelessexpensive(inthelongrun)toinstallafullyductedreturn(seeFigureA-5).
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Figure A-6
Poor Duct Fittings Can Not be Well Sealed
Figure A-7
Tight fittings and Mastic Make the Best Seal
Tight fittings and assembly are required to control duct leaks over the long term.
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-11
• Selectfittingsthatdonotleavelargegapsinthesystem.Manyductsareassembledwithlargegapsbetweenfittings.Eveniftheyarewellsealedwhenfirstinstalled,thispracticewilleventuallyleadtosealingfailure.Ductsealingmaterialssimplyarenotdesignedtoseallargeholesforlongperiodsoftime.Tightfittingsandassemblyarerequiredtocontrolductleaksoverthelongterm(seeFigureA-6).
• Donotuse“ducttape”–instead,usedurablesealingmaterials.AdvancedagingtestsconductedoncommonlyusedductsealingmaterialsbyLawrenceBerkeleyNationalLaboratoryconcludedthatducttapeisapoorperformer.Clothorvinylbackedducttapeswithrubberadhesivesfailedveryquicklyandarenotrecommendedforductsealing.Themetal-backedtapeswithacrylicadhesiveworkedbetter.Masticswerebyfarthemostdurable(seeFigureA-7).
• Selectsealingmaterialsthatarecompatiblewithductsystemcomponents.Ductsealingmaterialsneedtobeinstalledaccordingtothemanufacturer'sinstructions.Manytapesrequirethattheductsbecleanandoilfreebeforeinstallation.UsingsealantsratedundertheUL181standardassurescompatibilitywithductboardandflexibleductsystems.
• Useperformancetestingmethodstoassureatightseal.Contractorsshouldconsiderducttightnesstesting.Whilethismaynotbepracticaloneveryjob,itisavaluablelearningexperience.Ducttight-nesstestingwillprovidefeedbackonproblemareasandsharpenacrew’sabilitytoprovidequalityinstallations.Testingcanalsoalertcontractorstopotentialliabilitiesassociatedwithcombustion
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applianceback-drafting,aswellasdurabilityissues.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.
StudiesconductedintheNorthwestindicatethatforcedairheatingsystemslocatedcompletelyindoorswillreduceenergyusebyabout30percent.Thiscanbeaccomplishedatnoadditionalcost.Whenthebuilderprovidesaspaceindoorsfortheequipmentandducts,itreducestheHVACcontractor'scost.TheHVACcontractorwilluselessduct-work,willnotneedtoinsulatetheductsandwillbeabletoworkinbetterconditions.TheHVACcontractorwillalsobeabletospecifysmallerequipment,reducingcostfurther.Thebuilderwillneedtoprovideanindoorspacefortheducts.Thismayaddtothecostofframinganddrywall.Perproject,theHVACcontractorandbuildercostwillusuallyoffseteachother.
Movingtheheatingsystemindoorsismosteasilyaccom-plishedintwo-storyhomes.Theductworkcanbeplacedinthespacebetweenthefloors.Muchofthesystemcanbeplacedinexistingfloorcavities(seeFigureA-8).
Wheretransitionsmustbemadebetweenfloorcavities,
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-13
Figure A-8
Ducts Placed Within the Floor Joists
Figure A-9
Drop Soffit Conceals Ducts
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dropsoffitsaretraditionallyused(seeFigureA-9).
Theincreasinguseofengineeredfloortrussesprovideadditionalopportunitiesformovingductseasilybetweenfloors.Mostcompositewoodtrussescanbecuttoaccom-modatelargetransitionsacrossthefloorjoist.Webtrussesprovidethegreatestopportunity.Steelwebswithwoodcordsprovideareaslargeenoughtoincorporatesubstantial
Figure A-10
Engineers Trusses Provide Space for Ducts Between Floors
WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program Supplement A-15
ductsystems(seeFigureA-10).
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.
Supplement A-16 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program
Recommended Practice FollowthesesuggestionstobuildqualityHVACsystems:
• MeetorexceedCoderequirements.CodesaffectingHVACinstallationsaresetuptoestablishminimumsafetyandefficiencystandards.Itisagoodplacetostart.
• Sizeanddesignductsystemsusingrecognizedengineeringprinciplesandcalculatedloads.Examples:ACCA Manual D and ACCA Manual J.
• Installequipmentaccordingtothemanufacturer’sinstructions.
• Testandadjusttheequipmentandthedistributionsystem.
• Whencombustionappliancesarepresent,provideforcombustionsafety.Ventproperly,providecombustionair,minimizenegativepressures,testforCO(carbonmonoxide)productionandbackdrafting,installCOalarmsandeducatethehomeowneraboutcombustionsafety.
• Wherepossible,installductsintheheatedspace.• Sealducts.• Insulateductsnotinstalledintheheatedspace.• Useasystemsapproachtoassurehealth,safety,
comfort,durabilityandenergyefficiency.
