Measuring Airflow, Electric Heat Temperature Rise Method

Use this method with electric resistance heat.  You can also use a velometer or the gas heat method.  See Diognostics catagory.

  • With power to the air handler heater section on, call for heat.  Make sure all heating stages are energised.
  • Measure the supply voltage to the air handler and record the value.
  • Measure the total amperage being drawn by the heaters and record the value.
  • Multiply the measured voltage by the measured amperage.  multiply the result by 3.414.  This is your total BTUH output.  Record this value.
  • When the supply air temperature with all heat on has stabilized, measure the temperature of the supply air 36″ from the plenum.  Record this value.
  • Measure the temperature of the return air at the filter rack.  Record this value.
  • Subtract the return air temperature from the supply air temperature to obtain the difference (delta t).  Record this vallue.
  • Multiply the delta t by 1.08.  Record this value.
  • Divide your BTUH output by the value above .  The answer is your actual CFM.


  • Voltage = 235
  • Amps = 75
  • Supply air temp = 110F
  • Return air temp = 74F
  • 235V x 75a x 3.414 = 60171 BTUH
  • 110F -74F =36F
  • 60171 / 1.08 x 36 = 1548 CFM