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 Oct 3, 2014 Tweet  COMMENTS 0  Where experts and professionals share insights and inspirations to grow business and build careers 4 Ways to Measure Airflow by  Rob Falke Pr esident, National Comfort Institute It’s hard to find a practical standard describing how to measure ventilation airflows that can be applied in the field. Here’s how you can det ermine residential and commercial exhaust fan ai rflow on a job site.  While industry standards specifying ventilation rates are coming out of the woodwork, a practical standard describing how to measure ventilation airflows that can be applied in the field is hard to find. Let’s take a look how you can determine residential and comme rcial exhaust fan airflow in the field. Fans at Either End of the Duct This may be obvious, but it is a good place to begin this discussion. Although in-line exhaust fans exist, most exhaust fans are found at either the start of a duct or at the end of a duct. This is a defining characteristic of some exhaust fans that affects the way we may determine fan airflow as compared to a forced air system where fans are located between the supply and return ducts. The Dangerous Assumption Most standards are written to persuade designers to specify the needed airflow or air change rates. These standards require a calculation, and a specification requiring that the fan should do the job. With these tasks completed, most of the engineering world believes the standard is met and that ventilation needs are assured; dream on fellas. The assumption that the fan and duct that was specified will do the job is far from realized. The design and specification must require that the ventilation system also receive the verification step of measuring, adjusting and balancing airflow by a qualified air balancer. Without this step the design is only a wish. Typical Measurement Methods There are many different types of ventilation systems and not all the required measurement methods can be discussed in a short article, so we will review the most common test methods. 1. Air Balancing Hood Measurement JUN 30, 2015 JUN 30, 2015 JUN 30, 2015 JUN 30, 2015 Connect Latest NEWS HARDI Announces Speakers for September Conference SPONSORED Titus: Redefine Your Comfort Zone PRODUCT Marathon Data Systems, Changes Name to WorkWave, and Acquires Foxtrax GPS, a Cloud-based Leader in Fleet Tracking Technology PRODUCT Tall, Supertall, Megatall: ASHRAE Provides Design Guidance for All HOME > SERVICE > 4 WAYS TO MEASURE AIRFLOW 232 Art icles SHARE  0 Recommend HPAC IdeaXchange Contractor Comfort ech HVAC-Tal k The Nate Magazin e Subscribe  Newsle tte rs Adve rtise  Cont act Us HVACR Jobs HVACR Distributio n Business Re si den tial HVAC Com me rcial HVAC Contr ols IAQ & Venti lation Refri gerati on Service Resources  REGISTER  LOG IN

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  • 7/1/2015 4WaystoMeasureAirflow|ServicecontentfromContractingBusiness

    http://contractingbusiness.com/service/4waysmeasureairflow 1/4

    Oct3,2014

    Tweet COMMENTS 0

    Whereexpertsandprofessionalsshareinsightsandinspirationstogrowbusinessandbuildcareers

    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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    ContractingBusiness.comCommercialHVAC ResidentialHVAC Controls IAQ&Ventilation Refrigeration Service Resources

    Exhaustfantest.

    Tweet

    Armedwiththefanoperatingstaticpressureandthefanspeed,gotothemanufacturersfantablematchingthefanbeingmeasuredandplotfanairflow.

    DrawalineintersectingthefanspeedandthefanoperatingstaticpressuretorevealthefanoperatingCFM.

    4.ACombinationofOneorMoreTestMethods

    Becausesomefansarenotinstalledsuitabletomeasurefanairflowandbecauseyouarethejudgeandjuryresponsibleto

    determinefanairflow,youwillfindtimeswhereyoumayemploytwoormoreofthetestmethodsabovetogathersufficientdatatrenderyourjudgmentofoperatingfanairflow.Themoredatayougather,themoreaccurateyourairflowdecision.

    AirflowAwareness

    Justbybecomingawarethatairflowshouldandcanbemeasuredwillgetyouintouchwiththeairsideofthesystemsyoudesign,installandservice.

    Reviewingthespecificationsofthefansyouuseregularlywillmakeyouawareofthelimitationseachfanhasandtheinstallationconditionsthatcancausefanstoperformat50%oftheexpectedcapacity.

    Theprimaryreasontomeasureairflowisforyoutodiscoverventilationissuesthatyourcustomershavebeensufferingwithforyears.Measuringexhaustairflowbringsopportunityforyourcustomerstoreceiveimprovedventilationandcomfort.Opportunitycomesyourwaythroughprovidingbetterserviceandincreasedearnings.

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