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4WaystoMeasureAirflowbyRobFalkePresident,NationalComfortInstitute

Its hard to find a practical standard describing how to measure ventilation airflows that can be applied inthe field. Heres how you can determine residential and commercial exhaust fan airflow on a job site.

Whileindustrystandardsspecifyingventilationratesarecomingoutofthewoodwork,apracticalstandarddescribinghowtomeasureventilationairflowsthatcanbeappliedinthefieldishardtofind.Letstakealookhowyoucandetermineresidentialandcommercialexhaustfanairflowinthefield.

FansatEitherEndoftheDuct

Thismaybeobvious,butitisagoodplacetobeginthisdiscussion.Althoughinlineexhaustfansexist,mostexhaustfansarefoundateitherthestartofaductorattheendofaduct.Thisisadefiningcharacteristicofsomeexhaustfansthataffectsthewaywemaydeterminefanairflowascomparedtoaforcedairsystemwherefansarelocatedbetweenthesupplyandreturnducts.

TheDangerousAssumption

Moststandardsarewrittentopersuadedesignerstospecifytheneededairfloworairchangerates.Thesestandardsrequireacalculation,andaspecificationrequiringthatthefanshoulddothejob.Withthesetaskscompleted,mostoftheengineeringworldbelievesthestandardismetandthatventilationneedsareassureddreamonfellas.

Theassumptionthatthefanandductthatwasspecifiedwilldothejobisfarfromrealized.Thedesignandspecificationmustrequirethattheventilationsystemalsoreceivetheverificationstepofmeasuring,adjustingandbalancingairflowbyaqualifiedairbalancer.Withoutthisstepthedesignisonlyawish.

TypicalMeasurementMethods

Therearemanydifferenttypesofventilationsystemsandnotalltherequiredmeasurementmethodscanbediscussedinashortarticle,sowewillreviewthemostcommontestmethods.

1.AirBalancingHoodMeasurement

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Exhaustfanperformancechart.

Ifyourelucky,andairexhaustfanairflowisbeingpulledfromtheroomthroughagrillethatisflatagainstaceilingorwall,andthevolumeofairflowisbetween30and2000CFM,useacalibratedairbalancinghood.Setyouthebalancinghoodtoreadinexhaustmode,securelyplacethehoodoverthegrilletocaptureallofthefanairflowcomingthroughthegrille.Thenreadandrecordairflow.

Airflowspecificationsareplusorminus10%ofdesignairflow.Withmostsmallerfansthisspecisadequate.Ifyoureallcaughtupinexactness,forgetaboutit.Ifyoureconcernedaboutthedeterminingthepreciseamountofductleakage,youreffortisbetterspentininstallingtheductsothatisdoesntleak.

Whenairflowcannotbemeasureddirectlywithabalancinghood,airflowcanbemeasuredintheexhaustductbyperforminganairflowtraverse.

2.TraversetheAirflowintheExhaustDuct

Anairflowtraverserequiresatleast5lengthsofstraightexhaustduct.Twoormore3/8in.testholesarethendrilledintotheduct.Ananemometer,atestinstrumentthatmeasuresairvelocityisusedtodeterminetheaverageairspeedintheduct.Thentheaveragefeetperminuteismultipliedbytheareaoftheductinsquarefeettodeterminetheairflowmovingthroughtheduct.

Traversetheairflowintheexhaustduct.

Example:Youhaveabathroomexhaustfandesignedfor200CFM.Thesystemhasan8in.exhaustduct.Theareaofan8in.ductis.35sq.ft.Youmeasurethevelocityatapointsintheductandfindtheaveragevelocityintheexhaustductis400FPM.Multiply400FeetperMinutetimestheareaoftheduct,whichis.35squarefeet,tofind140CFMexhaustfanairflow.

3.PlotFanAirflow

Eachfanmanufacturerpublishesfanperformancetablesforeachfan.Normally,thesefantablesareshippedwiththefan,ortheinformationcanbeeasilyfoundonlineatthemanufacturerswebsite.Inordertointerpretexhaustfanairflow,youmustmeasurethefanoperatingstaticpressureandthefanspeedorRevolutionsPerMinute.Thisfielddataisthenusedtoplotfanairflow.

Smallerfansareoftenconstantorsinglespeed.Withthesefans,youarenotrequiredtomeasurefanRPM.LargerfansrequirethatyoumeasurethefanRPM.ThisisnormallydoneusinganoncontacttachometerthatreadsRPMfromreflectivetapeaffixedtothefan.

Fanstaticpressureismeasuredusingamanometer(apressuregauge)andahoseortubeandastaticpressuretip.(Seestaticpressurekitspecificationsavailableattheendofthearticle.)

Sincemanyfansarelocatedateitherendoftheduct,theoperatingfanstaticpressureismeasuredasairentersorexitstheexhaustfan.Drillatestholeintheductandreadandrecordthefanstaticpressure.

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Exhaustfantest.

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Armedwiththefanoperatingstaticpressureandthefanspeed,gotothemanufacturersfantablematchingthefanbeingmeasuredandplotfanairflow.

DrawalineintersectingthefanspeedandthefanoperatingstaticpressuretorevealthefanoperatingCFM.

4.ACombinationofOneorMoreTestMethods

Becausesomefansarenotinstalledsuitabletomeasurefanairflowandbecauseyouarethejudgeandjuryresponsibleto

determinefanairflow,youwillfindtimeswhereyoumayemploytwoormoreofthetestmethodsabovetogathersufficientdatatrenderyourjudgmentofoperatingfanairflow.Themoredatayougather,themoreaccurateyourairflowdecision.

AirflowAwareness

Justbybecomingawarethatairflowshouldandcanbemeasuredwillgetyouintouchwiththeairsideofthesystemsyoudesign,installandservice.

Reviewingthespecificationsofthefansyouuseregularlywillmakeyouawareofthelimitationseachfanhasandtheinstallationconditionsthatcancausefanstoperformat50%oftheexpectedcapacity.

Theprimaryreasontomeasureairflowisforyoutodiscoverventilationissuesthatyourcustomershavebeensufferingwithforyears.Measuringexhaustairflowbringsopportunityforyourcustomerstoreceiveimprovedventilationandcomfort.Opportunitycomesyourwaythroughprovidingbetterserviceandincreasedearnings.

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