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).

Supplement A-10 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program

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

Supplement A-12 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program

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

Supplement A-14 WSEC Builder’s Field Guide, 8th Edition, 2009 • Washington State University Extension Energy Program

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.