In forced-air heating systems, where does the air absorb heat?

• A. Inside the burner.
• B. Over the surfaces of the heat exchanger.
• C. In the duct work only.
• D. In the water coil.

Answer: (B)

In forced-air heating, the air gains heat by moving over the hot surfaces of the heat exchanger. The burner heats the exchanger, and the air flowing past those hot metal walls absorbs heat through conduction and convection. The ductwork just carries the warmed air to the rooms; it isn’t the heat source itself. A water coil would belong to a different system (hot-water or steam) and isn’t the mechanism for heating air in a standard forced-air furnace.