The temperature rise method of determining air volume can be used on:

• A. Electric resistive heat air handlers
• B. Gas furnaces
• C. Water-cooled coils
• D. Central air handlers

Answer: (A)

The temperature rise method relies on the heat added to the moving air and the resulting rise in air temperature to determine how much air is moving. The key relationship is Q̇ = ṁ Cp ΔT, where Q̇ is the heat input, ṁ is the air mass flow, Cp is the specific heat of air, and ΔT is the temperature rise of the air. If you know the heat input to the airstream and you measure the temperature rise, you can solve for the air flow rate (V̇) using ṁ = ρ V̇ and V̇ = Q̇ / (ρ Cp ΔT).

Electric resistive heat air handlers fit this method best because the heat input is known and stable—the electric wattage directly translates to a known amount of heat added to the passing air. This makes the ΔT a straightforward indicator of how much air is moving.

Gas furnaces complicate things because a portion of the heat goes into exhaust gases and flue losses, so the heat actually transferred to the air is not simply the total energy input. Water-cooled coils transfer heat differently, with the heating occurring via a liquid and coil effectiveness and water flow influencing the air temperature rise, not a direct, easily measured heat input to the air. Central air handlers can use electric heat, but the method relies on a known, controlled heat input, which is characteristic of electric resistance elements.