Measuring CFM’s Using Temperature Rise

In heating systems where airflow through the cabinet appears to be a problem, it can be difficult to determine the exact air volume that is moving through the system. In order to test this, you will first need to have verified that the manifold pressure is correct, and that the gas meter has been clocked and found to be accurate.

  1. Start the furnace on high fire and let it run for at least five minutes
  2. Take a temperature reading at the return plenum
  3. Take a temperature reading at the supply plenum
  4. Determine the difference between the two
  5. Verify the input BTU’s listed on the data tag located on the equipment
  6. Follow this formula to determine CFM’s

CFM= (BTUH Input X Efficiency) / (Temperature Difference X 1.08)

Example:
The temperature difference is found to be 40 degrees. Our BTUH input is 80,000, and our efficiency is 80%. Given our above formula, we multiply BTUH input by efficiency: 80,000 X .80 = 64,000 Temperature difference of 40 X 1.08 = 43.2. Next we divide the numbers: 64,000 / 43.2 = 1481. So our CFM’s are 1481.

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