How to Measure Airflow, Gas Heat Temperature Rise Method

The best method to measure airflow is with a vane anemometer.  Following is one of the alternate methods if you have gas heat.

  • Disconnect power to the furnace.
  • Set the indoor blower to run on high speed in heat mode.  Restore power.
  • Set to call for heat and confirm heat is running.  Make sure both stages of heat are on.
  • Determine the heating output capacity of the furnace in btuh.  record this value.
  • Run the heating system and allow time for the system to stabilize.  (blower on)
  • Measure the temperature of the return air at the filter.  Record this value.
  • Record the supply air temperature at least 36 inches from the plenum.  Record this value.
  • Disconnect the power and return the blower speed back to its original setting for heating mode.
  • Subtract your measured return air temperature from your supply air temperature.  This is the Delta-t.
  • Multiply your Delta-t by 1.08.  Record this measured BTUH value.
  • Divide your furnace label BTUH output by your measured BTUH value.  This is your CFM.


  • BTUH output label:  100,000
  • Supply air temp:  120F
  • Return air temp:  70F

100,000/1.08 x 50 = 1852 CFM