Efficient Humidity Control With Heat Pipes

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Efficient Humidity Control With Heat Pipes
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Fact sheet
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Johannesen, Roy
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University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
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Gainesville, Fla.
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"Publication date: December 1991."
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"EES-75"

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FactSheetEES-75 December1991 EfficientHumidityControlwithHeatPipes1 RoyJohannesenandMichaelWest2Buildingmoistureisusuallycontrolledbyair conditioning(AC),butsomeinstalledsystemscannot controltheextrememoistureloadencounteredin Florida.Oneefficientapproachtoremovingthis excessmoistureistheheatpipe.Aheatpipecan greatlyincreasethemoistureremovalabilityofanAC systemandsaveenergyatthesametime.Another advantageisthatheatpipeshavenomovingpartsand areessentiallymaintenancefree. AnACsystemthatdoesn'tcontrolhumiditycan induceavarietyofhealthandcomfortproblems. Thisfactsheetexplainshowaddingaheatpipe systemtosomeairconditionerswillcontrolthe humidityinahomeorbusiness.Informationisalso presentedonpossibleenergysavings,andon installationandoperatingcosts. Thekeytounderstandingwhataheatpipesystem willdoistoknowhowanACsystemremoves moisture.Thenextsectionwillexplainthisandwhy somesystemsareinadequate.HUMIDITYCONTROLBYAIRCONDITIONINGAbuilding'sairconditioningsystemisresponsible forremovingmoisturefromtheairinorderto provideforbothhumancomfortandmold-andmildewcontrol.Insidetheairconditioner,warm moistairisblownthroughacoolingcoil.Inthecoil, theairiscooledbelowitsdewpointtemperature. Thedewpointtemperatureisdefinedasthe temperatureoftheairwhentherelativehumidityis 100percent.Relativehumidityisdefinedasthe amountofmoistureintheairrelativetothemost moisturetheaircanholdatthesametemperature. Asairiscooleditlosesitsabilitytoholdmoisture. So,relativehumidityisincreasedbycoolingtheair, aswellasbyaddingmoisturetoit.Forexample,as theaircoolsonamuggynighttherelativehumidity increases.Whentherelativehumidityreaches100%, theairhasbeencooledtoitsdewpointanddew formsonsurfaces. Similarlyfortheairconditioner,oncetheairis cooledbelowthedewpoint,theairreleasesmoisture whichcollectsinadrainpan,anddrainsoutofthe system.Thecooledanddriedairisdeliveredtothe building.Theairnowhasalowerdewpointcalled theexitdewpoint. Manyairconditioningsystemsdonotremove adequateamountsofmoistureforFlorida'sclimate. MostACsystemsaredesignedtohandlepeakload conditions--thehottestafternoonofthesummer. Accordingly,theyworkbestduringthehottesttimes oftheyearbutnotsowellatothertimes. 1.ThisdocumentisFactSheetEES-75,aseriesoftheFloridaEnergyExtensionService,FloridaCooperativeExtensionService,InstituteofFood andAgriculturalSciences,UniversityofFlorida.Publicationdate:December1991. 2.RoyJohannesen,FormerEnergyExtensionSpecialist;MichaelWest,AssistantEnergyExtensionSpecialist,MechanicalEngineeringDept., CooperativeExtensionService,InstituteofFoodandAgriculturalSciences,UniversityofFlorida,GainesvilleFL32611. TheFloridaEnergyExtensionServicereceivesfundingfromtheFloridaEnergyOffice,DepartmentofCommunityAffairsandisoperated bytheUniversityofFlorida'sInstituteofFoodandAgriculturalSciencesthroughtheCooperativeExtensionService.Theinformation containedhereinistheproductoftheFloridaEnergyExtensionServiceanddoesnotnecessarilyreflecttheviewsoftheFloridaEnergyOffice. TheInstituteofFoodandAgriculturalSciencesisanequalopportunity/affirmativeactionemployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,age,handicap,ornational origin.Forinformationonobtainingotherextensionpublications,contactyourcountyCooperativeExtensionServiceoffice. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/ChristineTaylorStephens,Dean

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EfficientHumidityControlwithHeatPipes Page2ACsystemsaredesignedtoremoveacertain amountmoistureatpeakconditions.Thisiscalled thelatentheatratioofthesystem.Sensibleheatis heatintermsofdegreesonereadsonacommon thermometer.Latentheat,theotherkindofheat,is heatintermsofmoistureremoved.Sensibleplus latentisthetotalheatremoved.Thelatentheatratio ofanACsystemistheportionoflatentheatitcan removeoutofthetotalheatitcanremove.Itis typicallyaround30percentatpeakconditions(95F outdoors),andafewpercenthigheratnight conditions(75Foutdoors). Thebuildingloadalsohasalatentheatratio:it istheportionoflatentheatthatneedstoberemoved fromthebuildingoutofthetotalheatthatneedsto beremoved.Atpeakconditionsthereismuchmore sensibleheatthanlatentheat.Atnightandoncooler daystheamountofsensibleheatshrinksbutthe amountoflatentheatdoesnot.Andonwetdays,the amountoflatentheatgrows.Thelatentheatratio mayriseto50ormore.Duringhumidand/orcool weathertheACsystemindeedcoolsthebuilding,but itcan'tdehumidifyadequately. Tomakethingsevenworse,SOMEnewair conditioningunitshavesacrificedlatent(moisture removal)capacityinordertoincreasetheirnameplate SEERratings.(SEERstandsforSeasonalEnergy EfficiencyRatioandisameasureofenergy efficiency.)OnewaymanufacturersincreaseSEER istoraisethecoolingcoiltemperature. Unfortunately,thismeansthattheairblownthrough thecoildoesnotreachalowdewpointtemperature. Someofthesehighefficiencyunitshavealatentheat ratioof15percentorlessatdesignconditions.INCREASINGMOISTUREREMOVALIdeally,anACsystemisproperlysizedand designedbeforeitisinstalled.Acompleteand accuratebuildingheatloadanalysis,withspecial attentionpaidtointernalmoisturesourcesand infiltration,allowstheselectionoftheproper equipment.Unfortunately,equipmentsizingisoften doneusingtime-savingrules-of-thumb.Forexample, residentialmoistureloadisrarelycalculated:itis assumedtobe30percentofthesensibleheatload! Furthermore,buildingusageandoccupancyoften change,andACsystemrequirementschangewith them. Traditionalmethodsofincreasingthemoisture removalcapacityofACunitsincludeundersizing, reducingfanspeed,andaddingreheatwitheitherhot gasbypassorelectricalstripheating.Reheatmethods resultinsubstantialincreasesinelectrical consumption,especiallyelectricstripreheatwhichwill triple therequiredenergyinput. IfnewACequipmentisbeingselected,remember thatcoolingcoilswithmoretuberowsorgreater numberoffinshaveahigherlatentheatratio. ConventionalACequipmentthatcanhandlehigh latentloadsmayhavealowSEER.Make comparisonsbetweendifferentmanufacturers. AnundersizedACunitrunslongersinceitdoes noteasilysatisfythethermostat.Longerruncycles allowthesystemtoremovemoremoisturefromthe air,butindoortemperaturemayrise4to7degrees duringthelateafternoon. Reducingtheindoorfanspeedcausesthecoil temperaturetodrop,andalsoallowstheairto remaininthecoillonger.Thislowersthedewpoint oftheexitair,butsystemefficiencyisreduced. Reheatmethodsheattheairafteritpasses throughthecoolingcoil.Thisallowstheremovalof moisturewithoutover-coolingtheair.Hotgasbypass useshotrefrigeranttoreheattheair,andelectric stripreheatusesresistancecoils.Whenreheatis used,thecoolingabilityoftheACsystemgoesto waste.Electricreheatusesovertwicetheelectricity toreheattheairasittooktocoolit! Dehumidifiersuseaformofreheat.Theheat removedfromtheairtocoolittoitsdewpointisput backbeforetheairisblownoutoftheunit.Again, potentialcoolingabilityistotallywasted.HEATPIPEHUMIDITYCONTROLHeatpipetechnologyoffers enhancedmoisture removalforverylittleadditionalenergyinput .Since heatpipeshavenomovingpartsandaresealedunits, theyofferreliabilityequaltotheairconditioning systemtowhichtheyarefitted. Heatpipesareasimpleyetelegantwaytomove heatfromonepointtoanother(Figure1).Ifone endofasmallheatpipeisdippedintoacupofhot coffee,theotherendbecomesveryhotveryrapidly. Thesealedpipeisfilledwithcertainamountofa refrigerant.Therefrigerantisinasaturatedstate (thatis,thereisliquidandvaporatthesame

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EfficientHumidityControlwithHeatPipes Page3temperatureandpressure;forexample,waterboiling Figure1. Asimpleheatpipemovesheatfromcandletoairvery quickly. Figure2. Topdiagramshowsheatpipeinstalledaroundaconventional ACcoil.Bottomgraphcomparesdropinairtemperature.inapotandthesteamaboveitarebothat212Fand atatmosphericpressure).Whenheatisappliedto oneendoftheheatpipesomeoftheliquid refrigerantboilsintoavapor.Thisvaporquicklyrises tothehigherendofthepipeandcondenses,thereby releasingheatintotheairatthatendofthepipe. Condensedliquidrefrigerantreturnstotheboiling endoftheheatpipebygravity.Theoveralleffectis thatheatistransferredfromtheflametotheother endofthepipeveryquickly. Tohelpunderstandhowaheatpipeisappliedto airconditioning,theprocesshasbeendividedinto threeprinciplesteps(Figure2).Instep1,theairis pre-cooledbytheheatpipesystem.Instep2, moistureandheatisremovedfromairbytheairconditioningcoolingcoil.(Thecoolingcoilhas refrigerantorchilledwaterflowingthroughit.)In step3,heatisaddedtotheairbytheheatpipe system. Therearetwofluidstreamsthatarebeing affectedbytheprocess,namelyair(whichpassesover thecoolingcoilandheatpipe)andrefrigerant(which flowswithintheheatpipe).Onlytheeffectsonthe airstreamweredescribedinthethree-stepprocess mentionedabove.Amorein-depthheatexchange explanationfollowsinthenextparagraph.Anair temperaturegraphthatcorrespondstothesystem drawingshowndirectlyaboveitisalsodepictedin Figure2. Asshowninthesystemdrawing,theheatpipeis fittedaroundanairconditioningcoolingcoil.One endoftheheatpipeisplacedinfrontofthecoiland theotherendisplacedaftercoil.Oncomingair transfersheatintotheheatpipe(consequently droppingairtemperature),andcausestherefrigerant intheheatpipetoboil.Thepre-cooledairnext travelsacrossthecoolingcoilwhereheatand moistureareremoved.Therefrigerantvaporinthe heatpipetravelstothecondensingendoftheheat pipe.Finally,airexitingthecoilabsorbsheatfrom theheatpipecausingtherefrigerantintheheatpipe tocondense,completingtheheatpipecycle. Thetemperaturegraphshowshowtheheat-pipe fittedcoolingcoilcompareswithaconventional coolingcoil.Comparedtotheconventionalcoil,the airenteringtheheat-pipefittedcoilisnowatalower temperatureandthereforeclosertoitsdewpoint.In boththeheat-pipefittedandconventionalcoil,the temperaturedrop acrossthecoil isnearlythesame. Thatmeansthattheheat-pipefittedcoilwillchillthe airtoalowertemperaturethantheconventionalcoil. Sincetheairiscooledfurtherbelowitsinitialdew point, theheat-pipefittedcoilremovesmoremoisture fromtheair

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EfficientHumidityControlwithHeatPipes Page4Whentheairleavestheheat-pipefittedcooling coil,itistoocold.However,asitpassesoverthe other(condensing)endoftheheatpipethe refrigerantvaporintheheatpipetransfersheatinto thecoldairandwarmstheairtoatolerable temperature.Thecondensedrefrigerantisreturned totheboilingendoftheheatpipe.Theairisnow conditionedbythesystemtobeattheright temperature andhumidity tomeetabuilding's moistureload. Theoveralleffectofthisprocess,theoretically,is toalmost doublethemoistureremovalcapacity ofthe coolingcoil(atthesameindoortemperatureand humidity)whilereducingtotalcoolingcapacityby onlyafewpercent.Because total airconditioning systemcapacityisnotsignificantlyaffected,heatpipes areanattractiveretrofitsolutionforahumidity problem.HEATPIPEPERFORMANCEIt'svaluabletoknow how theheatpipesystem worksand,tohelppredicttheperformanceofaheat pipesystem,itisalsousefultoknow why itworks. Theheatpipesystemtakesadvantageofaninherent characteristicofallcoolingcoils: (1) Thecoolertheairenteringthecoil,thedryeritexits. Forexample,ifthethermostatisloweredfrom80(at 50percentrh)to75F,atypicalAC'slatentheatratio increasesfrom25percentto40percent.Theheat pipeprecoolstheair,sothecoil"thinks"the thermostatislowered,andit"gives"dryerair. Unfortunately,ACcoilbehaviorismore complicatedthan(1)suggests.Thereisanothercoil characteristicwhichlimitsheatpipeperformance: (2) Thedryertheairenteringthecoil,thelessmoisture isremoved. Dueto(1)alone,theincreasedlatentcapacitywould reducethehumiditytoabout50percent(at75F). However,dueto(2)thelatentcapacityofatypical ACisactually less at75F-50percentrhthanat80F50percentrh(itdecreasesfrom25percentto18 percent).Sothehumiditywillactuallybe somewherebetween50percentand60percent. Tomakemattersmoreconfusingathird characteristiccomesintoplay: (3) Thehotteritisoutside,thelessmoistureisremoved --abouta2to3 percentlowerlatentheatratio foreach10degreesFhigheroutdoor temperature. So,thetruelatentcapacityofaninstalledsystem isatabalancepointbetweenthecapacityincrease duetotheheatpipe,thecapacitydecreaseduetothe resultingdryerair,andcapacitychangesdueto thermostatsetting,outdoorconditions,andmoisture generationinthebuilding.Thebottomlineisthat humidity will decrease. Howmuch itdecreases dependsonmanyinterrelatedfactors.ENERGYSAVINGSThereisanadditionalbenefittoheat-pipemoisture-removalsystems.Itispossibletosaveenergy andmoneywithsuchasystem.Here'swhy: Forhumanstoremaincomfortable,both temperatureandhumiditymustbeattolerablelevels. Peoplecoolthemselvesbyevaporatingmoisturefrom theirskin.Iftheairhastoomuchmoistureinit, evaporationislimitedandnotenoughcoolingoccurs. Iftheairisdry,liketheArizonadesertat110Fand 3percentrh,apersoncanstillbecomfortablein extremeheat. Traditionally,anindoortemperatureof75Fand 50percentrelativehumidityisconsideredanideal state(whichisdifficulttoachieveinFlorida). Actually,thereisa range ofhumidityandtemperature whichisquitecomfortable.Withinthiscomfort range,thelowerthehumidity,thehigherthetolerable temperature.Ifonecanlowerbuildinghumidity, thenonecanraisethethermostatsettingandremain comfortable. Forexample,let'ssaythattheindoorrelative humidityis65percent.Accordingtoextensive studiesdoneonpeopleinroomsatdifferent temperaturesandhumidities,theaverageperson couldbecomfortableatatemperatureofabout75F. Ifwelowerthehumidityto40percent(possiblein Floridaconditionsbyretro-fittingwithheatpipes), thenwecanincreaseindoortemperatureto80Fand stillstaywithinthecomfortzone.Thisrepresentsa 5degreeFincreaseinACthermostatsetting. Aconservationruleofthumbsaysthatforevery degreeincreaseinsetpointtemperature,atwo percentreductioninairconditioningenergyconsumptionoccurs.Aftersubtracting4percent(becauseof

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EfficientHumidityControlwithHeatPipes Page5lossintotalsystemefficiencyduetoheatpipe Table1. Anenergyefficiencycomparisonofthreereheatmethods. ReheatOptionsWhereMoistureLoadis40%oftheTotalCoolingLoad ReheatType EnergyConsumptionRelative toaConventionalSystem* HeatPipes 1.06 Hotgasbypass 1.60 Electricalreheat 3.04 Thefollowingstudieswereusedindeterminingrelativeenergyconsumption: 1991airconditioningloadandenergyconsumptioncalculationsperformedattheUniversityofFlorida MechanicalEngineeringDepartmentbyMichaelWest.Acasestudyofawellinsulated,block construction,1300sq.ft.,3bedroom,2bathhomeintheGainesville,FLareawasconducted.The calculatedmoistureloadis4700Btuh(26%oftotalat95Foutdoortemperature).An18,000Btuh nominalhighefficiency(EER=9.75)heatpumpwithoutheatpipeshadalatentcapacityof2250Btuh (12%oftotalcapacity).Withheatpipes,latentcapacityincreasedto4300Btuh(23%oftotal capacity).Indoorrelativehumiditydecreasedfrom65%to50%.Annualelectricitycost($0.078/kWh) forcoolingincreasedfrom$304to$347.Anincreaseinthermostatsettingfrom75Fto80F decreasedannualcostto$322.CalculationsconductedinaccordancewithstandardASHRAE procedures. 1987testdataofa1.5tonheatpipeinstallation.TestconductedbyAppliedResearchLaboratoriesof Miami,FL.Resultsshowtheheatpipesincreasedlatentcapacitybyafactorof1.9,decreasedEER by4.2%,anddecreasedtotalcapacityby4.3%. Cromer,J.C."DesiccantMoistureExchangeforDehumidificationEnhancementofAirConditioners."Fifth annualSymposiumonImprovingBuildingEnergyEfficiencyinHotandHumidClimates.1988. *Forourpurposes,aconventionalsystemisconsideredtobeanairconditioningsystemwhichhasthe sametotal (sensible+latent)heatremovalcapacitybutnoreheatcapability.Notethat,underthe 40%latentcoolingloadconditionstatedforthiscomparison,thisconventionalsystemwouldnot be abletoremovetherequiredamountofmoisture.installation),thisresultsina6percentsavingsin energycostforthissimplecase. Inanactualapplicationwithaconventional thermostat,savingswillbeless.Thisisbecause increasedlatentcapacitycomesattheexpense decreasedsensiblecapacity.Thesystemwillhaveto runlongerbeforeitsatisfiesthethermostat,andit willruncorrespondinglymorehoursoverthecourse ofthecoolingseason.Onewaytomaximizesavings istoinstallahumidistatsothattheACcyclesonand offbasedoncomfort,notsimplytemperature. Airconditioningloadsareinactuallyaffectedby manyinterrelatedfactorssothisexampleshouldnot beconsidereddefinitive,butmerelyillustrative.In fact,mostresidentialheatpipeinstallationshave resultedinsmallenergysavings.Itseemsthatmost homeownerswhoinstallheatpipesdoexperience dryerandmorecomfortableconditionsbutare reluctanttochangethethermostattoahighersetting. Incommercialapplicationsthereismorepotential forsavingmoney,especiallyinanybuildingwhich mustemployareheatsystemtoachieveadequate moistureremovalcapacity.Reheatworkslikethis: Theairconditioningsystemissizedtoremovethe requiredamountofmoisture.Thisresultsinexcess capacitytoremovesensibleheat.Ifnothingwere done,thebuildingwouldbeovercooledand uncomfortable.Tocompensateforovercooling,heat isaddedtotheconditionedair.

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EfficientHumidityControlwithHeatPipes Page6Thisheatcanbesuppliedinanumberofways. Table2. Heatpipeinstallationcostsbasedonsizeandtype required. CostofVariousTypesofHeatPipeInstallations Type Capacity (tons) Installed Cost Range (dollars) Flat: (Installedinsupply andreturnducting) 2 400-760 3 490-920 5 590-1120 Split: (Heatinputremote fromheatoutput) 2 680-970 3 750-1070 5 940-1330 FittedCoil: (Integratedcoilheatpipeunit) 3 920-1300 4 1140-1620 5 1390-1970 AirHandler: (Fittedcoilwith blowerandhousing) 3 1410-2000 4 1530-2170 5 1730-2460 Note:Manufacture'sretailpricedatasuggest an18%averageincreaseinpriceperadditional tonofcapacity a40%averageincreaseinpricepertypeupgradeOneistheheatpipesystem.Anotheriscalledhot gasbypass.Inahotgasbypasssystem,someofthe high-temperaturerefrigerantwhichleavesthe compressorisroutedthroughabypasslinetoaheat exchangerlocatedattheairexitofthecoolingcoil. There,thishotgasaddsheattotheover-cooledair. Themostcommonmethodofaddingreheatto conditionedairisalsotheleastenergyefficient.This methodiscalledelectricalstripreheatanduses electricalresistanceheatingelementslocatedinthe airexitsideofthecoolingcoil.Anenergyefficiency comparisonofthesethreereheatmethodsshowsthe benefitsoftheheatpipe(Table1).COSTThetotalcostforbuyingandinstallingaheat pipesystemwasestimatedforarangeofsizes(Table 2).Theflatheatpipe(thefirstoptioninTable2)is rectangularinshapeandisthecheapestoftheheat pipeoptions.Itisinstalledinductinginaplace wherethesupplyandreturnductsrunupanddown (perpendiculartotheground)andareside-by-side. Iftheductsdonothaveasectionlikethis,consider asplitheatpipeinstallation. Sinceheatpipesystemshaveahighinitialcost,it paystotrytoreducethebuildingmoisture(andsensible cooling)loadfirst ,beforepurchasingadditional moistureremovalcapacity.Toreducemoistureloads, takethesesimpleandinexpensivesteps: 1) Callyourlocalutility andarrangetohave themconductafreeenergyauditofyourhomeora nocost/lowcostenergyauditofyourbusiness.Tell theauditorthatyouareespeciallyconcernedabout identifyingACductleaksandotherareaswhere moistoutsideairmightbeinfiltratingintoyour building. 2) Followup onthelow-costweatherstripping andsealingactionsrecommendedintheauditreport. 3) Increaseventilation toproblemareassuchas bathroomsandclosets.Conditionedairmustbe allowedtocirculateintheseareas.Installinglouvers indoorsorincreasingtheclearanceatthebottomsof doorsaretwolowcostwaysofincreasingairflow. Exhaustfans,boxfans,andpaddlefansalsohelpaid circulation. 4) Reduceinternalmoisturegeneration bycovering cookingpotsandusingcoolerwaterforwashing.Use anexhaustfantoremovemoistairfromthe bathroomandkitchenbeforeitdissipates ThefollowingExtensionpublicationoffersadditionalinformationonlow-costmoistureandmildew control: SayGoodbyetoMildewandSaveEnergy(EES65) Toobtainthisfreepublicationandotherenergyand humidityrelatedpublications,contactyourlocalcounty Extensionoffice.

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EfficientHumidityControlwithHeatPipes Page7GLOSSARYDewPoint -Thetemperatureofairwhenitis saturatedwithmoisture. RelativeHumidity -Theratiooftheamountof moistureintheairtothe amountinairsaturatedat thesametemperature. SensibleHeat -Energythatchangestheair's temperature. LatentHeat -Energythatisreleasedwhen moistureintheaircondenses. EER -EnergyEfficiencyRatio,thecoolingcapacity ofanACsysteminBtuh,dividedbyitspower consumptioninkWatastandardsetof operatingconditions. SEER -SeasonalEnergyEfficiencyRatio,the EERaveragedoveratypicalcooling season.Thistakeschangesinefficiency atnightandoncoolerdaysintoaccount, givingamorerealisticpredictionof actualoperatingcost. Btuh -BritishThermalUnitsperhour.TheBtuis aunitofheat,ittakes1Btutoraisethe temperatureof1poundofwaterby1degree Fahrenheit. Ton -AunitofACcoolingcapacity,equalto 12,000Btuh.Thisunitoriginatedasthe amountofrefrigeratingcapacityrequiredto freezeonetonoficein24hours.