Which component is responsible for heating the air in a furnace?

The heat exchanger is the component in a furnace that is specifically designed to heat the air. When the furnace is activated, the burner ignites, creating heat. This heat is transferred to the heat exchanger, which is usually a series of metal pipes or tubes that absorbs the warmth. As air from the home is drawn through the heat exchanger, it picks up this heat and is then distributed throughout the living spaces by the blower motor.

The blower motor, while crucial for moving the heated air throughout the house, does not itself generate heat; it only facilitates the circulation of air. The thermostat regulates the overall functioning of the heating system by sensing the home's temperature and signaling the furnace to turn on or off as needed, but it does not directly heat the air. The expansion valve is a component typically found in refrigeration systems, such as air conditioners, and does not pertain to the heating function of a furnace.

In summary, the heat exchanger is the critical component that actually warms the air before it is circulated, making it the correct answer in this context.