Air Flow Rate In HVAC Systems CFM & Fpm Air Flow Speed Data For Building Air Ducts, Air Handlers, Air Conditioners & Heating Furnaces
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Air Flow Rate in HVAC Systems CFM & fpm air flow speed data for building air ducts, air handlers, air conditioners & furnaces - POST a QUESTION or COMMENT about air flow rate or CFM rates in buildings and in HVAC systems
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: our inspection client points to an unsafe building air return taken right at the heating furnace. In this location the air return risks drawing combustion dangerous combustion gases into the building's indoor environment and also causes improper and unsafe burner operation.
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Air Flow Rates (CFM) in Buildings
Discussed here: 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 SpecificationsHVAC 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 Unconditioned attic, ducts very well insulated |
| Air Ducts - typical | 600 - 750 fpm | Residential Unconditioned 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
- Bansal, Vikas, Rohit Misra, Ghanshyam Das Agrawal, and Jyotirmay Mathur. Performance analysis of earth–pipe–air heat exchanger for summer cooling. [PDF] Energy and buildings 42, no. 5 (2010): 645-648. Abstract excerpt Earth–pipe–air heat exchanger (EPAHE) systems can be used to reduce the cooling load of buildings in summer. ... Velocity of air through the pipe is found to greatly affect the performance of EPAHE system. The COP of the EPAHE system discussed in this paper varies from 1.9 to 2.9 for increase in velocity from 2.0 to 5.0 m/s. Article excerpt: The increase in temperature of the air at the exit of pipe due to the increment in air velocity occurs because when the air velocity is increased from 2.0 to 5.0 m/s, the convective heat transfer coefficient is increased by 2.3 times while the time to which the air remains in contact with the ground is reduced by 2.5 times. Thus the later effect is dominant and therefore, less drop in temperature is obtained at air velocity 5.0 m/s than the 2.0 m/s. At higher velocities though the drop in temperature of air is less yet the total cooling effect achieved per unit time is much more. It can be seen that the maximum drop in the temperature occurs at air velocity 2 m/s. It can be seen that the maximum rise in the temperature occurs at air velocity 2 m/s for both PVC and steel pipes. The maximum rises in temperature for PVC and steel pipes are 10.3 and 12.7 °C respectively.
HVAC Duct Size Rules of Thumb - Cooling
- Cooling mode: Air flow required: 400 CFM of air flow per Ton of A/C (1 Ton equals 12,000 BTU)
- Cooling mode: CFM of air delivery per square foot: 1 CFM of air is required to heat or cool 1 to 1.25 sq. ft. of floor area.
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.
What is the industry standard CFM for a residential HVAC system?
I just had my 20 yr old HVAC system replaced
I live in a 4 story urban town home Vac 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?
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.
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).
- In an unconditioned attic with exposed ductwork and little duct insulation, the air speed may typically be around 600-750 cubic feet per minute
- In an unconditioned 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.
Energyvanguard.com has a nice discussion of this, The Best Velocity for Moving Air Through Ducts [Web page] 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
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.
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ADVERTISEMENTReader 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.
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Effectiveness of UV Light in Disinfecting Air depends on How far will air have moved in the HVAC system in five seconds
in a typical residential 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. [email protected] Thank you! - On 2022-08-12 by Scott Makrauer
Reply by InspectApedia-911 (mod) - Can UV Lights Disinfect the Air Moving Through Ductwork - a cubic foot of air has moved 37.5 feet in one second - Not enough contact time!
@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.
How do I calculate air changes per hour or ACH if I have hot water baseboard heat?
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 sq.ft. of floor space seems like hand waving 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 2021-09-19 by Carl:
Reply by danjoefriedman (mod) - ACH is an indoor air quality measurement not a hot water heat 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
Followup 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?
Reply 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 unwanted heat loss.
What is the air velocity in small, high velocity HVAC duct systems
What is the maximum velocity of air coming out of home split A/Cs and from high velocity HVAC systems - by sinan
Photo: multiple, small diameter flexible air ducts indicate a high velocity HVAC system in this home's attic. You can see the air handler at upper right in the photo at the end of the main air supply trunk.
Reply by danjoefriedman (mod) - Velocity 2000 - 3500 ft/min (10 - 18 m/s) 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)
Can I connect air ducts to my portable air conditioner?
I have a portable air conditioner. It supplies cold air through 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 A/C conditioner? Thanks by Scott Burnside
Reply by (mod) - no
Adding ducts to a portable air conditioner will certainly restrict the airflow and thus the performance of the unit.
How to measure the air velocity delivered at the furnace or air conditioner air handler
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 2018-02-21 by mechman
Reply 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.
How to measure the air flow rate in CFM of a ceiling fan or table fan in my home?
How to measure the air flow rate of ceiling/table fan? - On 2017-11-30 by BISWAS
Reply by (mod) - How to measure the air flow rate of ceiling/table fan
Biswas 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 eddies 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.
How can we calculate CFM by anemometer and how do we actually measure air flow in the ductwork?
How can we calculate CFM by anemometer ? - by VIKRAM NEGI
Reply by (mod) - 9 ways to find air flow rate in CFM in an HVAC system
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 9 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.
What's the right supply air speed in air ducts?
What is the ideal speed air should exit a vent? by Mark -
Reply 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 unconditioned attic?
Are the air ducts in an in-slab air duct (ugh!)
Are the air ducts located in a conditioned space (probably the least unwanted heat loss during heating or heat gain during cooling)
3. Other duct parameters:
duct material (smooth sheet metal vs. not-very-expanded flex duct, 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
...
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
Recommended Articles
- AIR HANDLER / BLOWER UNITS - home
- AIR FILTERS for HVAC SYSTEMS
- AIR HANDLER / BLOWER UNITS - home
- AIR FLOW IMPROVEMENT, HVAC
- AIR FLOW RATES in HVAC SYSTEMS
- AIR MOVEMENT in BUILDINGS - factors affecting the direction and amount of air movement in buildings.
- DUCT SYSTEM DESIGN SIZE & DEFECTS - home - duct sizing tables
- FRESH AIR VENTILATION RATES & STANDARDS
- HIGH VELOCITY HVAC DUCTS
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AIR FLOW RATES in HVAC SYSTEMS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
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Citations & References
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- [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 22161 USA ph: (800) 553-6847, fax: (703) 605-6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm
- In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested CONTINUE READING or RECOMMENDED ARTICLES.
- Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: [email protected]. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The HOME REFERENCE BOOK - the Encyclopedia of Homes and to use illustrations from The ILLUSTRATED HOME . Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.
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