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• POST a QUESTION or COMMENT about air flow rate or CFM rates in buildings and in HVAC systems

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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.

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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]

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Air Flow Rates (CFM) in Buildings

Discussed 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 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 later on this page at

at AIR FLOW RATE TYPICAL CFM

air velocity measured in feet per minute or liters or meters per minute 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.

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

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

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.

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:

• 350 CFM per ton of cooling capacity is required for high-latent-heat HVAC applications. (A latent heat cooling system is one that uses a cooling media that changes state - for example refrigerant that changes state between a liquid and a gas form).
• 400 CFM per ton of cooling capacity is required for typical cooling designs
• 500 CFM per ton of cooling capacity is required for heat pumps and sensible heat designs

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? Stat 18 0 – git integration with finder free.

Reply: rule of thumb calculation of required cooling capacity

For an average climate and building, you need

• 1 CFM/Sq.Ft. of living space, or 1 x 2000 or a total of 2000 CFM of air flow into the total occupied building space.

Now divide the total CFM required by 400 CFM (typical air flow per ton of cooling capacity of an air handler)

• 2000 CFM required / 400 CFM per ton = 5 tons of cooling capacity.

You need 5 tons of air conditioning capacity.

Air Flow Rate References

• ANSI/ASHRAE Standard 62.1-2016, Ventilation for Acceptable Indoor Air Quality, article 6.2 Ventilation Rate Procedure
• ANSI/ASHRAE Standard 111-2008, Measuring, Testing, Adjusting, and Balancing of Building HVAC Systems, PP 5.2.4
• ASHRAE: Handbook, A. S. H. R. A. E. 'Fundamentals.' American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta 111 (2001).
• Duda, Stephen W., P.E., 'Selecting and Specifying Airflow Measurement', , May 2019 pp. 74-80.
  
• Etheridge, David W., and Mats Sandberg. Building ventilation: theory and measurement. Vol. 50. Chichester, UK: John Wiley & Sons, 1996.
• Norton, Tomás, Da-Wen Sun, Jim Grant, Richard Fallon, and Vincent Dodd. 'Applications of computational fluid dynamics (CFD) in the modelling and design of ventilation systems in the agricultural industry: A review.' Bioresource technology 98, no. 12 (2007): 2386-2414.
• Persily, Andrew K. 'Evaluating building IAQ and ventilation with indoor carbon dioxide.' Transactions-American society of heating refrigerating and air conditioning engineers 103 (1997): 193-204.
• Yuan, Xiaoxiong, Qingyan Chen, Leon R. Glicksman, Yongqing Hu, and Xudong Yang. 'Measurements and computations of room airflow with displacement ventilation.' Ashrae Transactions 105 (1999): 340.

Reader Comments & Q&A

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.

http://xxmixpe.xtgem.com/Blog/__xtblog_entry/19307988-how-to-change-steam-password-on-mac#xt_blog. 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 slower 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).

• In an un-conditioned attic with exposed ductwork and little duct insulation, the air speed may typically be around 600-750 cubic feet per minute
• In an un-conditioned attic with ductwork that is well-insulated from the attic air temperature, the ideal air speed will be slower - 400-600 fpm.
• Where the HVAC ducts are moving through conditioned space the air speed can be still lower - around 400 feet per minute.

Hdr photo. 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:

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.

Or see these

HVAC Return Air Improvement Articles

• AIR FLOW IMPROVEMENT, HVAC
  • RETURN AIR REGISTERS & DUCTS - home
• AIR MOVEMENT in BUILDINGS factors affecting the direction and amount of air movement in buildings.

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Technical Reviewers & References

• [8] Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• 'Air Conditioning & Refrigeration I & II', BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• [9] Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• [10] Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates'Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• [11] Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• [13] Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: 363–376. Retrieved 2009-06-19.- Pitot Tubes, Henri Pitot (1732)
• [18] N Lu, YL Xie, Z Huang, 'Air Conditioner Compressor Performance Model', U.S. Department of Energy, August 2008, [copy on file as PNNL-17796.pdf] Available to the public from the National Technical Information Service,U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161ph: (800) 553-6847, fax: (703) 605-6900email: orders@ntis.fedworld.govonline ordering: http://www.ntis.gov/ordering.htm
• .

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Airflow, or air flow, is the movement of air. The primary cause of airflow is the existence of air. Air behaves in a wave like manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric difference|Atmospheric air pressure]] is directly related to altitude, temperature, and composition.^[1]

In engineering, airflow is a measurement of the amount of air per unit of time that flows through a particular device.

The flow of air can be induced through mechanical means (such as by operating an electric or manual fan) or can take place passively, as a function of pressure differentials present in the environment.

Types of airflow[edit]

Like any fluid, air may exhibit both laminar and turbulent flow patterns. Laminar flow occurs when air can flow smoothly, and exhibits a parabolic velocity profile; turbulent flow occurs when there is an irregularity (such as a disruption in the surface across which the fluid is flowing), which alters the direction of movement. Turbulent flow exhibits a flat velocity profile.^[2]

The Reynolds number, a ratio indicating the relationship between viscous and inertial forces in a fluid, can be used to predict the transition from laminar to turbulent flow. This number and related concepts can be applied to studying flow in systems of all scales.

The speed at which a fluid flows past an object varies with distance from the object's surface. The region surrounding an object where the air speed approaches zero is known as the boundary layer.^[3] It is here that surface friction most affects flow; irregularities in surfaces may affect boundary layer thickness, and hence act to disrupt flow.^[2]

Units[edit]

Typical units to express airflow are:^[4]

By volume[edit]

• l/s (litres per second)
• m³/h (cubic metres per hour)
• ft³/hr (cubic feet per hour)

By mass[edit]

• kg/s (kilograms per second)

Airflow can also be described in terms of air changes per hour (ACH), indicating full replacement of the volume of air filling the space in question.

Measurement[edit]

The instrument that measures airflow is called an airflow meter. Anemometers are also used to measure wind speed and indoor airflow.

There are a variety of types, including straight probe anemometers, designed to measure air velocity, differential pressure, temperature, and humidity; rotating vane anemometers, used for measuring air velocity and volumetric flow; and hot-sphere anemometers.

Anemometers may use ultrasound or resistive wire to measure the energy transfer between the measurement device and the passing particles. A hot-wire anemometer, for example, registers decreases in wire temperature, which can be translated into airflow velocity by analyzing the rate of change. Some tools are capable of calculating air flow, wet bulb temperature, dew point, and turbulence.

Simulation[edit]

Air flow can be simulated using Computational Fluid Dynamics (CFD) modeling, or observed experimentally through the operation of a wind tunnel. This may be used to predict airflow patterns around automobiles, aircraft, and marine craft, as well as air penetration of a building envelope.

Control[edit]

One type of equipment that regulates the airflow in ducts is called a damper. The damper can be used to increase, decrease or completely stop the flow of air. A more complex device that can not only regulate the airflow but also has the ability to generate and condition airflow is an air handler.

Uses[edit]

Measuring the airflow is necessary in many applications such as ventilation (to determine how much air is being replaced), pneumatic conveying (to control the air velocity and phase of transport)^[5] and engines (to control the Air–fuel ratio).

Rider mania 2019 day 2. Aerodynamics is the branch of fluid dynamics (physics) that is specifically concerned with the measurement, simulation, and control of airflow.^[3] Managing airflow is of concern to many fields, including meteorology, aeronautics, medicine,^[6]mechanical engineering, civil engineering, environmental engineering and building science.

Airflow in buildings[edit]

In building science, airflow is often addressed in terms of its desirability, for example in contrasting ventilation and infiltration. Ventilation is defined as the desired flow of fresh outdoor supply air to another, typically indoor, space, along with the simultaneous expulsion of exhaust air from indoors to the outdoors. This may be achieved through mechanical means or through passive strategies (also known as natural ventilation). By contrast, air infiltration is characterized as the uncontrolled influx of air through an inadequately-sealed building envelope, usually coupled with unintentional leakage of conditioned air from the interior of a building to the exterior.^[7]

Buildings may be ventilated using mechanical systems, passive systems or strategies, or a combination of the two.^[8]

Airflow in mechanical ventilation systems (HVAC)[edit]

Mechanical ventilation uses fans to induce flow of air into and through a building. Duct configuration and assembly affect air flow rates through the system. Dampers, valves, joints and other geometrical or material changes within a duct can lead to flow losses.^[2]

Airflow 2 4 1 X 42

Passive strategies for maximizing airflow[edit]

Passive ventilation strategies take advantage of inherent characteristics of air, specifically thermal buoyancy and pressure differentials, to evacuate exhaust air from within a building. Stack effect equates to using chimneys or similar tall spaces with openings near the top to passively draw exhaust air up and out of the space, thanks to the fact that air will rise when its temperature increases (as the volume increases and pressure decreases). Wind-driven passive ventilation relies on building configuration, orientation, and aperture distribution to take advantage of outdoor air movement. Cross-ventilation requires strategically-positioned openings aligned with local wind patterns.

Relationship of air movement to thermal comfort and overall Indoor Environmental Quality (IEQ)[edit]

Airflow is a factor of concern when designing to meet occupant thermal comfort standards (such as ASHRAE 55). Varying rates of air movement may positively or negatively impact individuals' perception of warmth or coolness, and hence their comfort.^[9] Air velocity interacts with air temperature, relative humidity, radiant temperature of surrounding surfaces and occupants, and occupant skin conductivity, resulting in particular thermal sensations.

Sufficient, properly-controlled and designed airflow (ventilation) is important for overall Indoor Environmental Quality (IEQ) and Indoor Air Quality (IAQ), in that it provides the necessary supply of fresh air and effectively evacuates exhaust air.^[2]

See also[edit]

Airflow 2 4 1 X 48

• Ventilation (architecture)

References[edit]

1. ^'How Do Air Pressure Differences Cause Winds?'. ThoughtCo. Retrieved 2017-11-09.
2. ^ ^abcdASHRAE, ed. ASHRAE Handbook of Fundamentals 2017. Atlanta, GA: American Society of Heating, Air-Conditioning and Refrigeration Engineers, 2017.
3. ^ ^ab'Aerodynamics - Introduction to the science of air flow'. Explain that Stuff. Retrieved 2017-11-09.
4. ^'Airflow Unit Conversion'. Comairrotron.com. Retrieved 2014-06-10.
5. ^'Air volumetric and mass in pneumatic transport - PowderProcess.net'. powderprocess.net. Retrieved 2019-06-11.
6. ^'Air Flow'. oac.med.jhmi.edu. Retrieved 2017-11-09.
7. ^Axley, James W. 'Residential Passive Ventilation Systems: Evaluation and Design.' Air Infiltration and Ventilation Center, Tech Note 54 (2001).
8. ^Schiavon, S. (2014), Adventitious ventilation: a new definition for an old mode?. Indoor Air, 24: 557–558. doi:10.1111/ina.12155
9. ^Toftum, Jørn. 'Air Movement–good or Bad?' Indoor Air 14 (2004): 40–45. https://doi.org/10.1111/j.1600-0668.2004.00271.x.

Airflow 2 4 1 X 4 Air Filter