On furnaces with ECM motors, it is pretty easy to determine what CFM the blower is producing since there is an LED on the board that flashes 1 time for every 100 CFM that the motor is producing.  But how can you determine the CFM of a blower when there is no LED to show the CFM?

This goes back to Basic Heating 101.  In school we learned a simple formula called the SENSIBLE AIR EQUATION.  That equation is:


Since the manufacturer calculates the BTU OUTPUT based on proper manifold pressure, with a fossil fuel furnace, determining the BTU’s Per Hour is as easy as reading the name plate OUTPUT on the furnace.   So, the BTU output of the furnace = BTU’s Per Hour for our formula.

1.08  is a constant used in the formula and represents the specific weight of a cubic foot of air.

The main variable in the formula is the temperature rise across the heat exchanger.  Of course, when taking a temperature rise you need to let the furnace run at least 15 minutes before taking any readings.  The supply air temperature should be taken in a main trunk so it is not in direct line of sight of the heat exchanger or “A” coil to prevent radiant heat from affecting the reading.  The return air temperature can be taken in the return duct.

So, let’s look at an example.  If a furnace has a name plate rating of 80,000 BTU OUTPUT; a supply air temperature of 122 degrees; and a return air temperature of 70 degrees, then the CFM would be: 

80,000 BTU Output / 1.08 X (122 – 70)  =  80,000 BTU Output / 1.08 X 52 degree rise =  

80,000 Btu Output / 56.16  = 1424 CFM 

You can also verify the CFM for an air conditioner using the same formula, you just have to be sure that you take the temperature rise with the fan on the COOLING SPEED of the blower.  

If you read my previous BLOG on Air Flow Effects on Air Conditioning back on7/25/11, you can see how this could help on some problem cooling jobs as well.



About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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  1. Julian Diaz says:

    Congrats on retirement, I quit my job and retired as an engineer because I couldn’t take
    the stress . Now playing stock market, lots more stress, may unretire lol.
    I have a question about 80,000 btu furnace, 85% efficicient The fan in furnace, doesn’t blow hard enough and questioning if the fan is the correct size since it is not original or if blockage in ducts some where since this was a forclosure property I purchased. My home is approx 1000 sq ft on main floor, but also has full basement. I believe a 1/4 horse power fan that can blow 1240 CFM is needed. I live in illinois

    • What you need to check id the “temperature rise” through the furnace. On the rating plate, it will list a range. I you are at the high end of the range, you fan is just barely moving enough air. If you are at the low end of the range, the fan is moving almost too much. If you are OUT OF THE LISTED RANGE, then the fan is definitely not moving the correct amount of air. If in the correct range — look for blockages in the duct — closed dampers, etc.

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