Where is heat transferred in a furnace's heat exchanger?

In a furnace's heat exchanger, heat is transferred from the combustion gas or electric heating elements to the circulating indoor air. The primary function of a heat exchanger is to facilitate the efficient transfer of heat generated from burning fuel or electrical resistance into the air that circulates throughout the building.

When the furnace operates, combustion gases are produced, which are typically very hot. The heat exchanger receives these gases, allowing the heat to be absorbed by the surrounding metal of the exchanger. As the exchanger warms up, it subsequently heats the indoor air that is drawn over or through it. This process ensures that the air distributed throughout the home is warmed before it exits the furnace and enters the ductwork.

This mechanism of transferring heat facilitates maintaining a comfortable indoor temperature, making it a critical component of the heating process in forced-air heating systems.