The temperature rise method of determining air volume is applicable to which equipment?

• A. Electric resistive heat air handlers
• B. Gas furnaces
• C. Water-cooled chillers
• D. Ice machines

Answer: (A)

The temperature rise method uses the known heat input to the airstream and the resulting rise in air temperature to calculate the airflow rate. It works best when the heater is electric resistance with a fixed, known wattage, because you can determine exactly how much heat is being added to the air and relate that to the measured temperature rise. In that case, you can use the relation Q = 1.08 × CFM × ΔT (with Q in BTU/hr and ΔT in °F) to solve for CFM, knowing Q from the heater’s wattage (1 watt ≈ 3.412 BTU/hr). Electric resistive heat air handlers provide a stable, predictable heat input, making the temperature rise method straightforward.

Gas furnaces involve combustion and venting losses, so the heat delivered to the air isn’t as directly tied to a simple, fixed input, which complicates using this method. Water-cooled chillers and ice machines are primarily cooling devices rather than heating sources for conditioning air, so the temperature rise approach isn’t the standard method used for determining their supply-air volume.