Using a Gas Meter to Calculate the CFM of a Natural Gas Furnace

The process of calculating the Cubic Feet Per Minute (CFM) output of a furnace can be accomplished in a number of ways. 

You can use an air balancing hood (Make sure that the hood has been calibrated properly)

  1. You can traverse the airflow in the duct
  2. You can use the performance table provided with the furnace in conjunction with external static pressure measurements.
  3. You can clock a natural gas meter.
  4. Let's focus on clocking the gas meter.

In this example, we are going to use the 2 Cubic Foot dial on the gas meter to calculate the CFM of airflow when the furnace is firing. 

The first step is to make sure that all natural gas appliances are shut off including all natural gas pilots.  Turn on the furnace and make sure that it is firing on high fire.  Using the 2 cuft dial on the gas meter, and a stop watch, record how long (in seconds) it takes for the dial to complete (1) revolution.  Divide the time by 2, and that will give you the time for 1 cubic foot of natural gas.  Use the following formula to calculate the firing rate

BTU Output = (BTU's in 1 cubic foot of Natural Gas) x (3600 seconds per hour) 

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BTU's in 1 cubic foot of Natural Gas in New York State = 1032

 

Let's use the following example to illustrate how to calculate the CFM.  We clock the 2 cubic foot dial at 3 minutes and 8 seconds to complete one revolution.  This is equivalent to 188 seconds of time.  Divide by 2 to get 94 seconds for 1 cubic foot.

 

BTU Output = (1032x3600)/94 seconds

BTU Output = 39,523 BTU's

 

We can now use the following formula to calculate the CFM of the furnace.

 

CFM = (BTU Output) / ((Supply Air Temperature-Return Air Tempurature) x 1.08)

 

You measure the following supply and return air temperatures:

Supply Air - 120 F

Return Air - 70 F

CFM = (39,523 BTU) / ((120-70)*1.08))

CFM = 731

The above example can then be used to verify the expected CFM of a R92PA041317MSA furnace.  If you refer to the blower performance chart for the furnace, you will also have to measure the total external static pressure of the system.  We measured the external static pressure to be .60" WC, and found the blower speed to be set at Medium-Low.  The corresponding CFM is 725 CFM.