Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipmentAir flow rate (CFM) Measurement FAQs
Q&A on tools, & air flow measurement

  • AIR FLOW MEASUREMENT FAQs - CONTENTS: how to measure air movement or flow rates in buildings; how to measure HVAC duct supply or return air flow rates in CFM or by other standards. What tools to use to measure air flow rate, accuracy, procedures, & where to buy. Definitions, Procedures & Tools for Measurements of Air Flow Rates (CFM) in Buildings
  • POST a QUESTION or READ FAQs about air flow rate or CFM measurement methods & typical rates in buildings and in HVAC systems
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Air flow rate data & instruments: this article defines air flow rate or cubic feet per minute (CFM) as the term is used to describe building air conditioners, heating systems, or building air movement rates.

We describe the types of devices or instruments used to measure air flow, comparing the features, operation, and accuracy of each approach. We include examples of manufacturer's air flow rate or CFM data for HVAC equipment like air conditioners and furnaces.

We also include a list of air flow rate measurement instrument or tool suppliers - where to buy CFM measurement equipment. Page top photo illustrates an example of a vane anemometer produced by Extech, the Extech ExTech SDL300 Anemometer and data logger - [permission requested 9/12/12]

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Definitions, Procedures & Tools for Measurements of Air Flow Rates (CFM) in Buildings

Dwyer round FLST air flow monitoring equipment for air ducts (C)   see Dwyer Instruments cited in this article

Questions & answers about measuring air flow rates in mechanical systems, air conditioners, heating systems, other air ducts, posted originally at AIR FLOW MEASUREMENT CFM - topic home

On 2017-05-11 by (mod) re: suitability of Dwyer Instruments FLST airflow rate monitoring equipment in a wood dust handling system

Thanks for the question, David.

The short answer is maybe: there may be a problem with a requirement for frequent cleaning of the sensor system.

[Click to enlarge any image] Shown here, a round Dwyer Instruments FLST airflow monitoring device described in more detail below. Contact information for Dwyer is also below.

For other readers, the Dwyer Instruments FLST (illustrated here) is indeed an air duct flow rate sensing system (used in connection with additional instrumentation) described by the manufacturer as:

The SERIES FLST Airflow Measurement Station utilizes an airflow averaging element generating a velocity pressure signal similar to the orifice, venturi, and other primary elements. Single or multiple airflow elements are factory mounted and pre-piped in a casing designed for flanged connection to the ductwork.

Multiple elements are joined together for connection to a differential measurement device (gage, transmitter, etc.) for flow measurement and indication purposes.

... Standard airflow stations can be operated (in air) continuously in temperatures up to 350°F or intermittently in temperatures up to 400°F. ... monitoring airflow rates from 100 to 10,000 fpm (0.51 to 51 m/s)

The airflow averaging element, utilized in the FLST, is a head type device, which generates a differential (velocity) pressure signal similar to the orifice, venturi, and other head producing primary elements.

The FLST is constructed so that strategically located sensing ports (based on duct size) continually sample the total and static pressures, when inserted normal to flow. The total pressures sensed by the upstream ports are continually averaged within the element in an isolated chamber. The static sensing ports (located where the influence of the velocity head is zero) are averaged in a second isolation chamber

. Multiple elements are manifolded together for connection to a differential measurement device (gage, transmitter, etc.) for flow measurement and indication purposes.

In essence a grid of sensing tubes is mounted in a rectangular, round, or oval frame to mount in the HVAC duct system.

Watch out: However, depending on the particle size and dust levels in the air system you describe "carrying wood dust and flakes", there is likely to be a problem with fouling or clogging of the sensing holes in the FLST tube openings. The company's description of maintenance recommendations gives some insight into this problem, though you'll want to discuss your specific application with them further.

Since the sensing elements have no moving parts, only periodic cleaning may be required. The sensing elements should be inspected for fouling of the sensing holes as part of an annual preventative maintenance program.

Installations having viscous airborne particles may require more frequent inspection. If the sensing holes on the elements have become fouled or plugged, the following procedure is recommended. Caution, all instruments must be isolated (removed) from the sensing lines prior to performing the following cleaning procedure.


Cleaning: In applications where the sensing elements are subject to viscous contaminants it is recommended that the surface be washed with a cleaning agent. The cleaning agent used must be suitable for use on the type of material the sensing element is constructed of (i.e. aluminum, stainless steel, etc.)

The Series FLST Duct Mounted Airflow Measurement Station is not field serviceable and should be returned if repair is needed (field repair should not be attempted and may void warranty).

Be sure to include a brief description of the problem plus any relevant application notes. Contact customer service to receive a return goods authorization number before shipping. - retrieved 2017/05/11, original source

Since you didn't give your location I'll provide several contact points for Dwyer Instruments who operate world-wide:

On 2017-05-11 by David Howard

Dwyer Instruments FLST. Would this be something I could use in a negative air system carrying wood dust and flakes for constant monitoring?

On 2016-02-29 by (mod) re: air flow in CFM versus return air inlet size


The return air inlet size does not tell us the CFM or air speed delivered by the air handler. That's a function of the air handler's blower size and speed, impacted by duct design, constrictions, lengths, elbows, etc.

I'd ask your expert to look at the entire duct system for design defects, crimps, disconnections, leaks, before making further changes.

Search for RETURN AIR ADEQUACY for more information.

On 2016-02-27 by Lee M

We had a baseboard return vent that was (vent opening) 6" x 29" by 1 inch. Of course the grate itself was a little larger. We were led to believe this was 300 cfm.

We needed an additional 300 cfm so this was replaced with an baseboard angled vent that is (vent opening) 6" x 29 , with the top 1" out from the wall, and the base board, on the floor section 3.5 " out from the wall.

Of course some floor board was cut to accommodate the additional approx 2.5 " on the floor. Does this really give us the additional cfm we need? Thank you , Lee M

Extech ExTech_SDL300_Anemomete air speed or air flow rate measurement device and data logger - On 2016-02-07 by (mod) re: using an anemometer for heating or air conditioning system duct airflow rates


An anemometer is a wind velocity device used to measure outdoor windspeed. A conventional outdoor Anemometer ccould indeed be used to measure airspeeds in duct systems, as we discuss in response to a reader question at the end of AIR FLOW MEASUREMENT CFM , but because of difficulties of duct access a conventional outdoor wind-speed anemometer is not really convenient for measuring air flow iside of a heating or air conditioning system.

Several companies including Dwyer Instruments address this with specific hand-held instruments:

Handheld anemometers are an excellent, portable tool for performing tests on HVAC system performance; however, large rotating vanes can prevent easy access to ducts. Dwyer introduces the Model VT-200 Vane Thermo-Anemometer to eliminate this problem. Additionally, simple keypad programming enables the user to view volumetric flow rates in CFM or CMM. Data logging software is also available to easily record and view data on a PC or laptop. - Retrieved 2016/03/21, original s:

On 2016-02-07 by rico lingcuran

Magkanu po Yung anemonerter na baka post?? Tnx

On 2015-09-22 by (mod) re: measuring airflow of a blower assembly before it is installed

You cannot measure the air flow of a blower assembly if it is not running. You could calculate theoretical air movement out of the air handler by knowing the squirrel cage fan specifications in CFM it is rated to produce at a given RPM, then you'd need to reduce that by all of the frictional and constrictive losses in the duct system.

If the blower is running you can rent, buy, or borrow an air flow rate measurement instrument such as those cited in the article above.

On 2015-09-22 by Anonymous

we have a blower fix with 40kw, 400v, 50hz motor as spare now we wish to install.Before installation we would like to measure the blower total flow.
Tell me know how we can do it?

Question: ventilation for a nail & spa salon

(Feb 27, 2013) Sam Xu said:


I'm doing a tenant fit-up for a nail and spa salon. How do I show/calculate the HVAC systems to provide ventilation of 15CFM/person as described in IEBC 709.2?

Thanks for your help in advance.

(June 23, 2014) NK Gandhi said:

In an HVAC system air duct I am examining there is a diffuser fitted which has air flowing in four direction. So while measurintg four reading have to be taken and then average of four reading is ideal process or should we make an duct near diffuser and take a single reading?



Unless the duct configuration is quite asymmetric I'd expect the arriving air to hit uniformly on all four louvered sides of the diffuser, generating pretty much the same air flow at each outlet side. Why not make a few actual reading tests to confirm that the uniformity I predict is what you're finding?

On the other hand if an air duct takes a sharp 90deg. turn then connects to the diffuser very closely, the air flow may be non-uniform, in which case you've pointed out a potentially important source of variation.

If you find that's the case you might make a temporary adapter hood that is simply held in place over the diffuser to momentarily direct all air flow in a single direction to obtain an approximate reading of CFM.

Question: re: CFM needed for an 1800 sq.ft. room

(Sept 17, 2014) Matt said:

If I have a room that is 1800 cubic feet, and want to have an effective HEPA/UV HVAC system, how many CFM is necessary? Does the total amount of air need to be through the filter every minute? I guess what I'm asking, is will a 300 CFM system be good enough for a 1800 CF room to maintain dust/particle control? Or does it have to be 1800CFM to be a "Cleanroom"?


I can't answer this question as we know nothing about the room's air leak rate, access doors, air exchange rate, etc.


Continue reading at AIR FLOW MEASUREMENT CFM or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see AIR MOVEMENT in BUILDINGS for a discussion of factors affecting the direction and amount of air movement in buildings.

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