Supplement A Improving Forced Air Heating Systems Forced Air Heating ... University Extension Energy...

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

Supplement A-2 WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program

Supply and Return Leakage

Figure A-1

WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�

Unbalanced Duct System

Figure A-2

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).

WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program Supplement A-�

Unobstructed Return Air Options

Figure A-3

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.

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

Supplement A-� WSEC Builder’s Field Guide, 7th Edition, 2006 • Washington State University Extension Energy Program

System with Fully Ducted Returns

Figure A-5

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).

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.

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

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).

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

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

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.

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.