Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
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. Also see INCREASING RETURN AIR. 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]
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved.
How do we measure air flow in CFM (cubic feet per minute) in an HVAC system such as air conditioners or warm air heaters?
How is CFM measured? - Anon.
Reply: Definition of Air Flow Rate & Flow Rate Measurement
Air flow rates for HVAC systems are expressed as a volume of air being delivered at some rate, typically cubic feet per minute (CFM) or m/sec (meters per second), ft/sec (feet per second), or ft/min (feet per minute).
A nice clear technical answer of how we measure flow rate is provided by 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).
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.
Vane or Fan Blade Anemometers, for Fan type air flow measurement: these are the most commonly used lower-cost CFM measurement devices used by home inspectors and HVAC technicians. At 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).
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.
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.
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.
Fans such as a blower assembly, are rated at a cubic feet per minute of air that the fan can move, presuming a particular rotating speed.
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.
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.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Questions & answers or comments about how to improve heating or cooling air supply by improving the return air into the HVAC system.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.