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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 - www.extech.com [permission requested 9/12/12]
Discussed here: 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.
How is CFM measured? - Anon.
The Air flow rate for an HVAC system is defined as the volume of air being delivered at some speed or "rate", typically cubic feet per minute (CFM) or m/sec (meters per second), ft/sec (feet per second), or ft/min (feet per minute).
Air flow rate, or quantity of air being moved, is measured in m3/sec (cubic meters per second) or cubic feet per minute (CFM).
See TYPICAL AIR FLOW RATE CFM Specifications for HVAC equipment - for typical air flow rates in cooling and in heating mode.
We can also measure air velocity or air speed without necessarily knowing the total quantity of air being moved.
Air velocity or air speed is measured in m/s (meters per second) or feet/minute. Details are at AIR VELOCITY MEASUREMENT & STANDARDS
Some instruments can measure both air velocity (air speed) and air flow rate (air quantity).
Air flow rate is measured by calculating an average velocity for the conduit of interest, and then, multiplying this velocity by the cross sectional area of the duct at the measurement location.
The air velocity value may estimated using a single reading, or a survey across the duct at a station. - Flow Kinetics: - 
Our HVAC air duct register photos above and below illustrate two common air flow measurement points in a duct system: at the return air inlet (unsafe in the above photo left) and at a supply air register (below left).
We make the actual measurement of air flow in a cooling or heating system by using an instrument that is sensitive to the passage of air. This article describes the different types of air flow measurement instruments and how each works.
The accuracy of air flow rates measured in cooling or heating system air handler or duct systems is discussed at ACCURACY of CFM or other air flow measurements on HVAC systems.
If I held up a one-foot square sensor in front of an air source (say an air supply register) and the sensor measured air velocity at 12 inches per minute, I'd be measuring 1 CFM of airflow. (One cubic foot = 12 x 12 x 12 inches).
Or if we measured an air velocity at an air supply register of one foot per minute and we knew that the duct work was a 12-inch square duct, we'd figure we were seeing one cubic foot per minute of air supply at that location.
Actually here are more than one answer to your question about how airflow is measured in an HVAC system because there is a range of air flow measurement instruments on the market.
The measuring devices vary in price, accuracy, and in operating principle, and there are also of course multiple sources of CFM data: manufacturers specifications, theoretical numbers, and actual measurements. We are most interested in the last category.
Fans such as a blower assembly of an air conditioner or forced-air heating system are rated at a cubic feet per minute of air that the fan can move, presuming a particular rotating speed.
where CFM = cubic feet per minute or ft3/min.
Typically we need about 1 CFM of air flow per square foot of floor area of conditioned space provided that the ceiling height is about 8 feet above the floor, with a typical number of windows and doors and typical building insulation and heat gain or loss.
In those conditions, 1 CFM of air flow per square foot of floor area into a building space will give about 7.5 ACH or air changes per hour.
Watch out: the true CFM of a squirrel cage blower fan in a central warm air heating or cool air conditioning system can be 50% less than rated if the fan blades are dirty however.
Watch out: also, as we detail at PRESSURE TRANSDUCERS, air flow is not uniform in all cross-sectional areas of HVAC air ducts and air handlers and is different in various areas of rectangular vs. round ductwork.
(June 20, 2016) Anonymous said:
I just had my 20 yr old Hvac system replaced
I live in a 4 story urban town home Hvac unit on bottom floor no Ac gets to the 4 floor. What are or is the industry standard for Cfm in a residential setting supply side & return side?
Measured across the cooling coil, typical A/C air flow is about 400 to 450 CFM per ton of cooling capacity.
Typical air flow rates in CFM vary depending on the type of cooling system:
Also see details at DEFINITION of TONS of COOLING CAPACITY
My new home in Louisville Kentucky will be about 2000 square feet. How much cooling capacity in tons do I need?
Reply: rule of thumb calculation of required cooling capacity
For an average climate and building, you need
Now divide the total CFM required by 400 CFM (typical air flow per ton of cooling capacity of an air handler)
You need 5 tons of air conditioning capacity.
CFM measurements on HVAC systems should be considered an approximation not precision measurements. There are a number of sources of uncertainty even in the measurement itself.
For most HVAC air flow troubleshooting or air balancing applications, we are more interested in comparison measurements of air flow between different locations in the HVAC duct system than in high precision in statement of air flow itself.
OPINION: Therefore while pitot-tube type instruments and some electronic air flow measurement instruments can offer both precision and accuracy in HVAC or building air flow measurements, all of the instruments described in our article above can work suitably for heating and air conditioning design and maintenance.
Earlier at AIR FLOW RATE MEASUREMENT DEFINITION we explain that the total quantity of air entering (or leaving) an enclosed space is measured in cubic feet per minute or CFM, or in some countries cubic meters per minute or M3/Min.
Here we discuss air velocity or air speed measurements and standards.
Simple wind speed in an open space is measured by a wind speed anemometer.
We make similar measurements of air speed at air conditioning or heating system supply or return registers or at air handlers but we use smaller, simpler instruments like the vane anemometeer shown here.
For purposes such as rating air filters or air cleaning equipment we measure the simple speed with which air is moving, such as the speed of air movement across a cooling coil (evaporator coil) or the speed of air movement through an air filter.
Illustration: a vane anemometer, model RVA801, produced by Alnor Products, TSI Incorporated. See AIR FLOW & CFM DEVICE SUPPLIERS.
Air velocity is the rate of movement of air in a particular direction, expressed as feet per second or meters per second.
We are simply measuing the air flow rate or speed at a specific point or location.
The most common air flow measurement device used in HVAC systems is a hand-held vane anemometer or a hand-held hot-wire anemometer.
The vane anemometer is in essence a small fan driven by the movement of air across the fan blades.
The hot wire anemometer uses a heated wire that is cooled by the movement of air across the wire.
Some devices such as the Amprobe TMA 10A Anemometer with remote vane/sensor can measure air velocity (air speed in ft/min or meters/sec) and air flow rate (m3/sec or ft/min), and air temperature.
At AIR FLOW RATE CFM MEASUREMENT DEVICES & METHODS we describe various air flow measurement devices.
While the quantity of air being moved in an HVAC system (AIR FLOW RATE TYPICAL CFM Specifications for HVAC equipment) is an key overall figure in assessing the ability of an air conditioning or heating system to provide adequate cooling or heating in a building,
air velocity (AIR VELOCITY MEASUREMENT & STANDARDS) measured in feet per minute or liters or meters per minute is also critical for some HVAC equipment design and testing.
Examples of components for which air velocity (speed through the device) is particularly important are given in the HVAC Air Velocity table below.
Typical HVAC System Air Velocity Specifications
|HVAC Component||Recommended Air Velocity / Flow||Comments|
|Air Cleaner, electrostatic or electronic||Maximum: 700-750 fpm||Loses efficiency at higher air velocities
Also see MISSING / LEAKY AIR FILTERS
|Air Filter, replaceable, HEPA, other||Maximum: 700-750 fpm||
Loss of filter efficiency at higher velocities
Also see AIR FILTER EFFICIENCY
|Bypass Air 2||0.1 to 35% of total air volume||
More air bypasses at higher velocities.
|Condensing Coil 3 A/C or Heat Pump||Maximum 1000 fpm||Also see CONDENSING COIL|
|Cooling Coil / Evaporator Coil 4||
Minimum 400 fpm
Maximum 550-600 fpm
Sufficient air flow to avoid coil icing - see FROST BUILD-UP on AIR CONDITIONER COILS
Avoid condensate blow-off
Also see COOLING COIL or EVAPORATOR COIL
Also see DEHUMIDIFICATION PROBLEMS
|Heating Coil, water to air||Maximum 700 fpm||
Air coil heating systems
1. HVAC equipment operated at higher air speeds than those for which it was designed loses efficiency and for filters, will begin to bypass rather than remove particles from building air.
2. Bypass air is used in some heat pump systems to control system performance and economy by sending some air around rather than through the cooling or evaporator coil. Bypass air may be controlled by mechanically operated louvers, dampers, or other means.
Higher air speeds result in higher percentages of bypass air.
3. Condensing Coil in the condensing unit limits the air speed for efficient condensing of high temperature refrigerant gas back to a liquid form
4. Evaporator Coil (cooling coil) in the air handler limits the air speed in order to avoid loss of efficiency and to avoid blowing condensate downstream into the HVAC duct system (risking mold contamination)
5. A latent heat system is one that uses a cooling media that changes state - for example refrigerant that changes state beween a liquid and a gas form.
6. A sensible heat system is one that uses a cooling media that does not change state - for example water to air.
The cup type anemometer is familar if you've looked around a local weather station or airport. The movement of air across the rotating cups causes the shaft to turn at a rate that is measured by a wind speed indicator or in the case of this instrument from Spectris, a wind speed data recorder.
Here are excerpts from the manufacturer's product description:
The OM-CP-WIND101A is a complete system to accurately measure and record wind speed.
This low cost wind speed recording system comes complete with a data logger, weatherproof enclosure, a three-cup anemometer and all the necessary cabling to quickly get up-and-running.
Measurement Range: 0 to 100 mph (0 to 45 m/s)
Resolution: 0.085 mph at 10 second reading interval
Accuracy: ±2.0 mph from 0 to 10 mph; ±2.5% of reading from >10 to 100 mph
Starting Threshold: 1.75 mph & Reading Rate: 1 reading every second to 1 every 24 hours. - Omega, cited in detail at AIR FLOW & CFM DEVICE SUPPLIERS.
Above: the Dwyer Instruments FLST airflow rate monitoring device intended for monitoring air flow rates in an HVAC duct system.
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 http://www.dwyer-inst.com/PDF_files/FLST_iom.pdf
Above left is a wind speed anemometer - Wikipedia creative commons.
At page top we illustrate the Extech ExTech SDL300 Anemometer and data logger available from www.extech.com. 
Some anemometers are comparatively small inexpensive (and less precise) air flow measurement devices that use a hand-held fan like instrument such as the Kanomax vane anemometers 6800 series or the ExTech SDL300 shown at page top) to measure air flow in CFM or equivalent rates on other scales.
A hand-held portable fan blade anemometer device is held in the air path and moving air rotates a fan blade. The instrument measures fan blade rotation to calculate a flow rate or pressure equivalent that is combined with the known cross sectional area of the measurement device.
An advantage of measuring CFM with an anemometer is that you don't need to correct the measurement for temperature (variation in air density). Another advantage is that some of these instruments are very low in costs.
Choose a unit that measures in the range of air speed in CFM specified by the standards against which you are measuring for your application.
Tip: always choose an instrument whose scale focuses most-closely on the measurement value range in which you're interested. That will generally give more-accurate readings.
For example, choosing an anemometer that can measure between 0.3 m/second = 5.15 cubic feet per minute - and 30 m/s = about 515 cfm - covers the necessary air speeds (20 to 100 cfm) for tests of bathroom and kitchen ventilation fans in North America.
If you were interested in the bath and kitchen vent fan standard and measurements for its compliance, using New York City's standard as an example the air flow speeds of interest are
Choosing an instrument that is designed to measure wind (air) speeds of 100 mph is likely to get one that is inaccurate over its scale for very low air speeds such as at a building vent fan.
Choosing an instrument that is designed to measure air speeds extending only up to 5 meters/second would, on the other hand, be unable to register higher wind speeds of interest to meteorologists and sailors.
Watch out: a more-accurate measurement of air flow in a duct system probably requires the user to insert an instrument actually inside the duct - such as a hot wire anemometer.
Swing Vane Anemometers: using a vane or ball that moves along a curved scale are used to measure low velocity air (25 to 400 feet per minute) for checking wind speed or for measuring the air flow rate in duct work, at air filters (is the air filter dirty and needing replacement?), and to meet safety ventilation requirements for OSHA and the US EPA for safety exhaust hoods, spray booths and similar applications.
Pitot tube probes: a Pitot tube (invented by Henri Pitot (1732)) is a device that measures air (or other flowing gas or liquid) pressure when the tube is inserted or placed in the proper position (pointed into the direction from which air flow emanates) for sensing airflow.
The pressure is converted to a flow rate by considering the cross-sectional area of the duct or opening through which air is being delivered. (There are some assumptions behind this including that air flow rate is uniform across the cross section of the opening.)
By comparing the dynamic (moving air) pressure to static (non-moving air) pressure a pitot tube can give very accurate air flow velocity data.
Pitot tube image, Wikipedia creative commons. 
Quoting Flow Kinetics who offer instruments for air flow measurement as well as excellent technical publications on this topic illustrate a device used fro CFM measurement by measuring air pressure.
The (incompressible) velocity measured by a Pitot tube is calculated from the recorded differential pressure, Dp, and density, r, of the fluid. 
Of course in our case the "fluid" is air and we're interested in air movement through ductwork or out of a supply register into a building space.
Pitot tubes are familiar to air travelers who have noticed that little tube sticking down and pointing forward from the bottom of many aircraft where the pitot tube is used to measure the air speed of the craft. Indeed pitot tubes are used for high velocity airflow measurements where a vane anemometer could not possibly be up to the task.
Pitot tubes are the most accurate technology for measuring air flow rates and are generally used to provide the accuracy standard for comparison with other CFM measurement devices.
Pressure transducers: also measure pressure from a flowing gas or air and permit conversion to CFM measurements in the same manner as a pitot tube - knowing the cross sectional area of the duct or opening.
Pressure sensors measure the force exerted by a "fluid" including air or liquid by measuring the force that would be necessary to stop that movement. These devices are also called pressure transmitters, pressure senders, pressure indicators, piezometers, and in HVAC equipment and testing, manometers.
Actual measurements of airflow in an HVAC system or at air supply registers are expressed in cubic feet per minute and are most often made in the field using a hand held flow meter through which air moves. The flow meter is calibrated based on the its input area and the resistance offered by its own fan blades.
As air, say coming out of an air supply duct, blows through the handheld device it causes the device fan or sensor to move, giving a measurement of calculated air flow in cubic feet per minute at that location and time.
Watch out however: measuring CFM at a supply register is not at all the whole story since air flow varies throughout the system as it is affected by internal resistances such as bends, crimps, surface smoothness, duct length, etc.
And air flow through rectangular duct work is not identical to air flow CFM through a round duct of the same cross-sectional area.
An anemometer type device that uses a heated wire and measures the cooling effect of low velocity air flow can also be used to estimate air flow rates provided that air temperature is also considered to provide a correct estimate of air flow rate.
The GrayWolf Advanced Sense HVAC differential pressure manometer works on this principle using a hot wire probe inserted into the HVAC duct.
Also see the Kanomax A031 hot wire anemometer (photo at left, kanomax-usa.com, described below. ) 
Note: all of the air measurement instrument manufacturers listed in this article produce a range of air flow rate monitoring instruments (and other test equipment) providing a variety of functions, accuracy, and of course, price.
Capture Hoods can be used to make accurate measurements of air flow rates at HVAC system air supply registers. Capture hoods cover the entire supply air register and use a differential pressure device or a hot wire device to obtain an air flow CFM number.
Liquid Column gauges - liquid column manometers are a special form of liquid-column manometer used to measure low velocity air flow by comparing air pressure inside and outside of two spaces. At left the U-shaped plastic tube filled with a blue liquid is connected at its left end to the interior of a 6" plastic vertical exhaust duct forming part of a radon mitigation system.
The right end of the liquid column gauge is simply open to the atmosphere of the room, in this case a basement.
The differential in air pressure between the two ends of the tube is marked on a scale indicating the air flow rate inside of the column.
The difference in height between the two ends of the column of blue liquid is always in direct proportion to the difference between the two air pressures (inside & outside of the exhaust duct). If no air were flowing inside of the white exhaust duct, the two ends of the blue liquid would be at the same level.
In this application, air flowing past the end of the flexible plastic tube inserted into the column interior causes a reduction of air pressure in the tube that is a function of the speed of air flow past the tube opening. In this application the liquid column gauge reading of differential air pressure does not have to be precise as its function is simply to indicate that there is some difference in air pressure between the room interior and the exhaust duct interior.
As long as the room is at higher pressure than the column interior, the exhaust system is working and any radon gas below the floor slab (in this application) tends to exhaust through the duct rather than enter the room.
A simple test for air movement at the return air inlet is illustrated in our sketch.
Just hold a tissue or piece of toilet paper near the inlet grille face.
If air is moving into the grille the tissue will be pulled against the opening.
This toilet paper or tissue test can confirm air flow as well as the direction of air movement at an HVAC air supply or return air register, and is a useful, if trivial, demonstration that can help confirm air movement when air flow in the system is weak or uncertain.
Watch out: Obviously, this is a subjective, non-quantitative test for air movement at a building location. This test can tell you the direction of air flow but it is not accurate for much more than that.
Sketch above courtesy of Carson Dunlop Associates.
at AIR MOVEMENT in BUILDINGS we illustrate another toilet tissue test by taping a tissue to the bottom of an open window sash in a New York building.
Contact points for Dwyer Instruments who operate world-wide:
Could you please tell me is there any instrument for measuring of air flower rate in-line exhaust fans exist(at the end of a duct). We have not possibility and space for inside measuring at duct. We are thinking to find some way to measure from outside of duct. The diameter of duct( pipe) is 40-50 cm. Air temperature is 5-40 centigrade. Humidity =60-100 %
The duct is circular pipe with 40 Cm diameter.
Do you think the suggested instruments could measuring accurately on turbulence air flow?
Where we should put anemometer, before fan or after fan in Duct?
Please feel free to write me back for further questions.
I look forward to hearing from you. - D.K. by private email, 2016/03/21, The Netherlands
Above: the Dwyer MVA duct or vent airflow measurement probe system. Below the Dwyer DAFM airflow measuring probe.
Yes there are permanent, temporary, and hand-held probes that can measure air-flow inside of an air duct system.
I should have added you ought to be able to make an opening to insert a probe to measure airflow. That makes some older-style hand-held anemometers a bit large and inconvenient as you'd have to cut and perhaps seal around a larger opening, but other devices avoid that trouble as I'll cite below.
Dwyer instruments (and other HVAC duct airflow instrument manufacturers) make in-duct airflow measurement systems. You can install a permanently-mounted air flow measuring device in line in the duct system. Dwyer also has insertable probe devices.
There are at least 3 options. Here are an excerpta from Dwyer Instruments.
As for probe location ... it depends on what we're studying.
If you want to focus first on what is being delivered to the exhaust fan destination by your 60 cm supply duct, insert the 2-probes or the hand-held anemometer I cited into the ductwork at a convenient accessible location downstream from the blower fan itself. By definition, in the supply duct means you are measuring "downstream' of the blower fan itself. Measuring in the supply plenum is more difficult (rectangular) and requires more probes.
Keep in mind that measuring near the exhausdt fan itself or ahead of the exhaust fan (less convenient in your design) is not going to tell us the effects of downstream bends, obstructions, air registers, or other variations in the system. In fact comparing a measurement near the air handler with those made at points of supply into rooms can be diagnostic. - if I correctly understand the problems you're solving.
These instruments are not (I think) designed to analyse turbulence per-se, but rather to measure the liters (or cubic meters) of air being delivered by a duct system. Measuring in the round duct a meter or more from the blower assembly / air handler itself should tell us what the system is delivering into the supply system.
Be advised: I am not an HVAC engineer. These resources may be of assistance:
Continue reading at AIR LEAKS in RETURN DUCTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AIR FLOW MEASUREMENT FAQs - questions & answers posted originally at this article
Or see AIR MOVEMENT in BUILDINGS for a discussion of factors affecting the direction and amount of air movement in buildings.
Or see these
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Questions & answers about measuring air flow rates in mechanical systems, air conditioners, heating systems, other air ducts, posted originally at this article are now found at AIR FLOW MEASUREMENT FAQs
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