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Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipmentAir Flow Rate in HVAC Systems
CFM & fpm air flow speed data for building air ducts, air handlers, air conditioners & heating furnaces

Air flow rate data: 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 include examples of manufacturer's air flow rate or CFM data for HVAC equipment like air conditioners and furnaces.

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]

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Air Flow Rates (CFM) in Buildings

Air conditioner air flow rate notes (C) Carson Dunlop AssociatesDiscussed here: what are the typical and target air flow rates in fpm or CFM in residential and commercial building ductwork, air handlers and through other HVAC components?

Carson Dunlop Associates' sketch points out that the (typical) desirable rate of cool air flow in an air conditioning system is around 400 to 450 cubic feet per minute.

But here we give other air flow fpm data for various components and air conditioning, heating, or ventilation system types.

Table of Air Velocity Rates fpm for HVAC Ducts & Equipment

While the quantity of air (CFM - cubic feet per minute) being moved in an HVAC system 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,

see those details in the table given just below on this page.

Air velocity measured in feet per minute (FPM) or liters or meters per minute (LPM) is also critical for some HVAC equipment design and testing.

See those details in our separate article found

at AIR VELOCITY MEASUREMENT & STANDARDS

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 Air Velocity - Feet per Minute - FPM

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 Ducts - main trunk, typical 1200 - 1800 fpm Industrial
Air Ducts - main trunk, typical 1000 - 1300 fpm Commercial, e.g. hotel, hospital
Air Ducts - main trunk, typical 700 - 900 fpm Residential
Air Ducts - branch, typical 800 - 1000 fpm Industrial
Air Ducts - branch, typical 600 - 900 fpm Commercial, e.g. hotel, hospital
Air Ducts - branch, typical 600 - 700 fpm Residential
Air Ducts - target 400 fpm Residential Ducts run through conditioned space
Air Ducts - target 400 - 600 fpm Residential Un-conditioned attic, ducts very well insulated
Air Ducts - typical 600 - 750 fpm Residential Un-conditioned attic, ducts exposed, little insulation
Air Filter, replaceable, HEPA, other Maximum: 700-750 fpm

Loss of filter efficiency at higher velocities

Also see AIR FILTER EFFICIENCY

Air filter, low-velocity disposable 7 300 fpm  
Air filter, high velocity, washable 7 650 fpm  
Bypass Air 2 0.1 to 35% of total air volume

More air bypasses at higher velocities.

Bypass air is defined below at the bottom of this table.
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

Also see FAN COIL & FAN CONVECTOR HEATERS & HYDRONIC COILS

Notes to the table above

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.

Bypass air is also used to describe leakage at HVAC air filters.

See MISSING / LEAKY AIR FILTERS.

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 between a liquid and a gas form.

See also DEFINITION of LATENT HEAT

6. A sensible heat system

is one that uses a cooling media that does not change state - for example water to air.

See also DEFINITION of SENSIBLE HEAT

See also DEFINITION of TONS of COOLING CAPACITY

And see DEFINITION of HEATING, COOLING & INSULATION TERMS

7. Air velocity through air filters in HVAC systems

Air velocity must not exceed 300 feet per minute through low velocity disposable filters.

Air velocity must not exceed 650 feet per minute through high velocity washable filters.

Undersized filters reduce airflow and can adversely affect furnace and cooling system operation. - source: Thermopride HIGH-EFFICIENCY UPFLOW FURNACE INSTALLER'S INFORMATION MANUAL [PDF]

8. Earth pipe air velocity

HVAC Duct Size Rules of Thumb - Cooling

Typical Manufacturer's Air Flow Rate CFM Specifications for HVAC equipment

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop AssociatesFans 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.

Sketch courtesy Carson Dunlop Associates, a Toronto home inspection, education, & report writing tool company.

Definition of standard building HVAC air flow required: 1 CFM / sq.ft.

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.

Question: What is the industry standard CFM for a residential HVAC system?

(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?

Reply: Definition of standard HVAC air flow rate per ton: 400 CFM per ton of cooling capacity

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

Question: how many cubic feet per minute of air flow is needed to cool a 2000 square-foot home?

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.

Air Flow Rate References

 

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2022-08-13 by InspectApedia-911 (mod) - Can UV Lights Disinfect the Air Moving Through Ductwork

@Scott Makrauer,

Thank you for a helpful and interesting question that I’d re-phrase as

Can UV Lights Disinfect the Air Moving Through Ductwork

It’s reasonable to take the cross-sectional area of the duct to complete the calculation that you describe.

You need to take the actual duct dimensions and from that determine the duct length that gives you one cubic foot.

Then that “foot” of air will move the number of feet through that duct determined by the blower’s CFM air movement rate.

Using your data, and MAKING UP some numbers as you don’t provide all of the measurements:

Suppose our theoretical HVAC duct is 12” x 12”.

One cubic foot will occupy a duct length also of 12” - we have 1 cubic foot of air in a 12” x 12” x 12” section of ductwork,
ignoring for simplicity any air compression effects and friction losses or effects of duct restrictions such as bends or elbows.

A typical air conditioner air handler and duct flow rate (measured at the cooling coil) is 400 to 450 CFM per ton of cooling capacity.

So a 1 ton A/C unit moves say 400 CFM.

Your calculation of 37.5 cubic feet per SECOND would be (37.5 CFM per second x 60 seconds per minute) 2250 CFM - possible if your cooling capacity is rated at about 5.5 to 6 tons.

Let’s use your numbers.

At 37.5 cubic feet per second, in a 12” x 12” cross-section duct, the first cubic foot of air has moved 37.5 feet in one second, or (37.5 x 5) 187.5 feet in five seconds.

So to effectively “disinfect” the air in the ductwork, IF the claim that a 5 second exposure of air is sufficient to kill pathogens in that air, your UV light would have to be almost 200 feet long, or would have to illuminate, at sufficient UV light strength, a 200 foot area of ductwork.

Bottom line: UV light can effectively disinfect and in some cases kill mold ON DUCT SURFACES in the UV-light-exposed areas (not elsewhere in the duct system) but I’ve not found good evidence to support the claim that the light can disinfect the air moving through the ductwork as you described.

See for example,

Levetin, Estelle, Richard Shaughnessy, Christine A. Rogers, and Robert Scheir. EFFECTIVENESS OF GERMICIDAL UV RADIATION FOR REDUCING FUNGAL CONTAMINATION WITHIN AIR-HANDLING UNITS [PDF], Applied and Environmental Microbiology 67, no. 8 (2001): 3712-3715.

and see

DUCTWORK CONTAMINATION

and also

UV DISINFECTION in HVAC SYSTEMS

found in AIR FILTER OPTIMUM INDOOR

From your email address I infer that you may be an employee or owner of PureAirEverywhere [dot] com - if so you should disclose that fact and explain your interest so as to maintain credibility.

InspectAPedia.com provides building and environmental diagnostic and repair information to the public, without cost or fee.

In order to absolutely assure our readers that we write and report without bias, we do not sell any products or services, nor do we have any business or financial relationships that could create such conflicts of interest.

On 2022-08-12 by Scott Makrauer

in a typical residental system the air leaving the Air Handler of a 2.5 ton to a 4 ton system about how far away from the air handler will the air be after 5 seconds of being blown towards the vents in the home.

I am asking this question because UV Germicidal lights require 5 + seconds of contact with the light and also needs to be within 18" of the light and still in the line of site of the light in order for the UV Germicidal light to have any effect on any pathogens in the air. I suspect the air moves way to fast by the light.

My calculations are that air moves about 33 to 37.5 cubic feet per second in the duct. I do not know how to turn that into a lineal feet measurement. I realize the size of the duct will also change the answer.

I am only looking for a general answer based on a typical duct size that feeds off of most residential air handlers. Any thoughts or comments will be very much appreciated. Scott@PureAirEverywhere.com Thank you!

On 2021-09-21 by inspectapedia.com.moderator

@Carl,

Yes indeed, heating or cooling can be inadequate for a building or a room in the building with ANY heat or cooling delivery method.

There are many options besides increasing the CFM delivery, such as insulation, low-E glass, and most-important, stopping air leaks that give un-wanted heat loss.

On 2021-09-20 by Carl

@danjoefriedman, Thanks, that makes sense. I was thinking of ACH as a room by room consideration for the amount of airflow into a closed space, like a bedroom with a closed door within a forced air system.

A particular CFM input into that room translates to a certain amount of warmed or chilled air flow into that room and an equal flow out through the return air duct, assuming that the supply and return ducts are well matched. But can't well matched ducts still be inadequate for heating or cooling that room?

On 2021-09-19 by danjoefriedman (mod) - ACH is an indoor air quality measurement

@Carl,

Thank you for the helpful question.

ACH or Air Changes per Hour data for a building generally refers to the fresh air or ventilation air makeup rate.

ACH is an indoor air quality measurement describing rated which Fresh Air is supplied to a building interior.

ACH is not generally used to describe a heating system's adequacy, at least certainly not directly. I will explain.

Airflow for heating systems is only pertinent if you were talking about a gravity type or forced warm air heating systems typically referred to as a heating furnace.

In that case the heating systems capacity is often described as warm air flow or delivery rate measured in cubic feet per minute (CFM) into the heated area. It's not expressed as air change rates. (ACH)

But your thinking isn't entirely off base in that for forced air heating systems we must have a balance between the supply of warm air to the heated space and the return of cool air from the heated space.

If your heat is from hydraulic systems such as hot water baseboard or radiator, or Steam Heat and steam radiators, The sir change rate is not given and has nothing to do with your heating system.

Instead the heating adequacy is usually defined as the square feet of radiating area or the btu input rate of the heating system and the heat loss rate of the home.

However there is an indirect relationship in that the more leaky your house is or the more drafts it has and therefore the greater air change rate, even if accidental, the harder it is to heat the house.

That'll be true regardless of the type of heat source

On 2021-09-19 by danjoefriedman (mod)

Re posting

AUTHOR: Carl (no email)

COMMENT: I am really confused about ACH. By using air to deliver or remove BTUs, a ducted HVAC system seems to acquire a parameter I don't understand. If I use hydronic baseboard or radiant floor heat, do I get 0 ACH? If I use a mini-split for cooling a room, do I get 0 ACH?

The use of a rule of thumb such as 1 CFM per sqft of floor space seems like handwaving around the issue of how many ACH I need to move enough heat into or out of my rooms. How do I determine how many ACH I need in a room?

On 2020-12-23 by danjoefriedman (mod) - air velocity in small, high velocity HVAC duct systems

Sinan

That toolbox air velocity figure is consistent with what I read at other HVAC maintenance sources for small high velocity duct systems, that is, the air is exiting at 1000 to 2000 fpm or a bit more in commercial or larger applications.

The same cfm and velocity rates apply for a split system A/C unit - based on tonnage, given in the table above on this page.

A small-diameter high velocity A/C system duct will run around 200 250 CFM per ton.

The point is that to meet a building's heating or cooling requirements we need the same effective air delivery to the occupied space, whether it's coming out of a small high velocity duct or a larger, low velocity duct. The air speed will increase through the smaller duct system but the CFM requirement remains the same.

From there you'll need to apply D'Arcy or another equation converting CFM to velocity. I'll post more on this.

The much-admired Engineering Toolbox gives us, for High Pressure Loss Ducts

Maximum friction rate less than 0.4 inches W.G./100 ft

Velocity 2000 - 3500 ft/min (10 - 18 m/s)

On 2020-12-23 by sinan

what is the maximum velocity of air coming out of home split ACs and from high velocity HVAC systems

On 2020-10-22 by Paul

I have 120,000 BTU furnace I have to run 50 ft of pipe spiral the building is 20x60x12 ft high what would be the correct piping and correct registers size the furnace sits at one far end of the building and pipe goes straight across from the furnace to the other far side down the middle

On 2018-10-22 by Kesh Nair

We are after a sensor (i.e. laser or camera), that can measure air particles inside a contained area. By measure, I mean calculate the percentage of particles inside the contained area.

We have a recycling machine that crushes cartridges and would like to know the amount of airborne toner inside the machine.

On 2018-04-24 - by (mod) -

Adding ducts to a portable air conditioner will certainly restrict the airflow and thus the performance of the unit.

On 2018-04-18 by Scott Burnside

I have a portable air conditioner. It supplies cold air thru two 12 inch openings. My question. If I try to put a length of 25 feet of 12 inch duct on both will the CFM be the same if, I hook an increaser ( 12" to 16") and add 25 feet of 16 inch duct. Which duct would have the greater CFM given the same air flow from the portable Aic conditioner?

Thanks
Scott

On 2018-02-21 - by (mod) - standard measurement points for air delivery

Mech

I think there are two different types of measurements in different objectives here. Measuring air flow right at the supply register will give you an idea of the air flow into the room, although even those measurements can be quite variable depending on exactly how the airflow measurement is made and of course on the equipment used.

Measuring at other locations in the system have diagnostic use such as assessing the impact of filters were looking for defects in the return or supply duct system.

Finally, there are standard measurement points that we use in order to at least attempt to be more consistent across systems and situations.

An example of a more standard measurement is measuring the drop in temperature across a cooling coil or across a heating plenum for those measurements we measure close to the heating or cooling Source but not so close that radiant heat for example would throw off her actual measurement.

On 2018-02-21 by mechman

Hi
what is the proposed method to verify the air delivery of an AHU measuring the velocity at the intake plenum with anemometer or in front of filters some distance away or direct against the filter pads?

On 2017-11-30 - by (mod) - How to measure the air flow rate of ceiling/table fan

Hi

I don't believe that you can make a highly accurate measurement of a ceiling or table fan installed in a normal home. You'd need to construct a test chamber.

That's because I think that there are non-uniform air currents and eddys around a fan operating in the open air. Those would confound a question about the accuracy of simply making an in-situ measurement.

However you can get a reasonable approximation by using one of the air flow measuring devices described in the article above on this page, holding the air flow sensor in the air path for the fan.

On 2017-11-30 by BISWAS

How to measure the air flow rate of ceiling/table fan?

On 2017-05-25 by (mod) - How can we calculate CFM by anemometer ?

Vikram

I think you could get an imprecise estimate by considering the cfm given by your anemometer inserted into an air duct along with the cross sectional area of the duct. More accuracy would require considering that air doesn't flow uniformly in a rectangular duct - e.g. in the corners its velocity is probably less. And precise accuracy would require eliminating the effects of inserting a large tool into an air duct.

In fact there are at least 8 methods for measuring air flow in an HVAC system: a Rotating Vane Anemometer, measuring the pressure drop across a dry cooling coil in the air handler, using a Wilson Grid or "Trueflow Grid" - we've illustrated some of these products in this article series), using manufacturer's specifications and actual RPM of the air handler's blower fan, looking at temperature drop or temperature rise in the system and sensible heat, using a hot wire anemometer (less interference effects) also called a "velocity stick", a total external static pressure method, a pitot tube and digital anemometer (aircraft).

Using an anemometer that gives airflow in feet per minute or fpm, here is an example airflow calculation whose source I'll cite below:

A (sq. ft. / 144 sq. in.) x V = Q (1)

where

A = duct cross sectional area (sq.in.)
q = air flow rate (cfm)
v= air speed (fpm)

Example
A = 10 x 6 grille opening = 10” x 6” = 60” / 144” = 0.42
V = air speed or (fpm) = 198 fpm to 351 fpm reading from cross sectional of grille with anemometer,
average this reading by adding together and dividing in half 198 + 351 = 549 / 2 = 274.5 fpm averaged.
A = (0.42) x V (274.5) = Q (115)CFM
Q = air flow rate in cfm

Source:
Mark Bowman, "Airflow quick test from anemometer," Florida State College (Jacksonville), USA, course handout material, retrieved 2017/05/25, original source: http://web.fscj.edu/mark.bowman/handouts/Airflow%20quick%20test%20from%20anemometer.pdf
This site is here to give the students, past and present, from the Northeast Florida Builders Association and FSCJ, access to some of the materials and handouts used in the second year class for Heating, Air Conditioning, and Refrigeration.

On 2017-05-25 by VIKRAM NEGI

How can we calculate CFM by anemometer ?

On 2020-04-02 by Mark - what's the right supply air speed?

What is the ideal speed air should exit a vent?

This Q&A were posted originally at AIR FLOW TOO WEAK FAQs

On 2020-04-03 - by (mod) - ideal air speed in HVAC ducts?

Mark,

There is no one "Right answer" to the question: What is the ideal air speed in HVAC ducts?

Because we need to know more about the system such as

1. Are we heating or cooling the building?

The optimum blower fan speed and thus air speed for heating and cooling are different.

2. Where are the HVAC ducts through which air is moving?

The optimum air speed in cfm or "feet per minute" through an air duct depends.

Are the air ducts in a hot or cold un-conditioned attic?

Are the air ducts in an in-slab air duct (ugh!)

Are the air ducts located in a conditioned space (probably the least un-wanted heat loss during heating or heat gain during cooling)

3. Other duct parameters:

duct material (smooth sheet metal vs. not-very-expanded flexduct, duct size, shape, cross-sectional area, length, obstructions, elbows and restrictions, etc. become critical in a real-world heating or air conditioning duct system installation and design.

Bottom line

To see typical heating and cooling system air flow rates in CFM, and also to see what instruments you can use to measure air flow rates - what's actually going on at individual supply registers, return registers and ducts please

see AIR FLOW RATES in HVAC SYSTEMS

Basic Concepts about air flow rates in HVAC ductwork

1. the more slowly air moves through ductwork, the greater will be the heat gain or loss

through the duct material

2. the less insulation on ducts, the greater will be the heat gain or loss

through the duct material

3. the greater the temperature difference between the ducts and the surrounding air along the duct routing, the greater will be the heat gain or loss through the duct material

4. the faster the rate of air movement through ducts the noisier

is the HVAC system and at greater velocities, occupants may complain of being in an uncomfortable supply-air draft, particularly in cooling or air conditioning systems

5. For a fixed-capacity air handler or blower unit, the larger the duct size

the more-slowly air will move through the ductwork. Bigger HVAC ducts perform better in most applications.

6. The actual supply register location, size, design, and the boot

that supplies the register have a big effect on the rate at which supply air actually enters the room and is distributed in the conditioned space).

Energyvanguard.com has a nice discussion of this, more-useful than some of the engineering websites that give ideal calculation formulas but don't know a darn thing about the actual on-site conditions in your home. www.energyvanguard.com/blog/best-velocity-moving-air-through-ducts

That article points out a topic that we've discussed here ad-nauseum:

7. the greater the temperature difference between materials the faster or greater is the heat transfer

from the cooler to the warmer material or area. I've also discussed this for hot water heating systems, claiming that the heat transfer efficiency is greater in a hot water heating system when it's run at higher temperatures.

The heat transfer rate is exponentially greater as the temperature difference increases. Put in five dollar words, we're discussing the second law of thermodynamics: "The second law of thermodynamics states that the entropy of any isolated system always increases." and

as livescience.com states it nicely, "... as energy is transferred or transformed, more and more of it is wasted. The Second Law also states that there is a natural tendency of any isolated system to degenerate into a more disordered state. "

If your concern is with heating your home you'll want to see

WARM AIR SUPPLY TEMPERATURE & IMPROVEMENT

For both heating and cooling air flow you should be sure also to review the most-common problem in air heating and cooling systems:

RETURN AIR, INCREASE

at the end of that article are more links to advice on improving air flow for heating and air conditioning.

And at service time on older air handlers you can make a big difference in air flow and a big reduction in heating and cooling costs by cleaning the blower: see

BLOWER FAN ASSEMBLY CLEANING


...

Continue reading at AIR FLOW MEASUREMENT CFM or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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