A furnace has four main parts: the burner or combustion chamber, where natural gas or another fuel is burned; the heat exchanger, where the heat from the burner—but not the potentially dangerous gases produced by combustion—transfers to the home’s air supply; the blower, which forces the heated air through ducts into the home; and the flue or vent, which carries combustion gases such as carbon monoxide to the outside.
Some furnaces heat water, not air, for use in radiators or steam pipes. These furnaces are usually referred to as “boilers.”
Air handlers work in conjunction with a heat pump, a device that uses refrigerants to move heat energy from one place to another. The handler has a blower that forces air through the house, and the refrigerant lines run through the air stream just “downwind” from the blower. During the summer, the refrigerant pulls heat out of the air steam and moves it outside; the result is that the air handler blows cool air. In the winter, the refrigerant pulls heat energy from the outside air—yes, there is heat energy even in cold winter air—and moves it inside, with the result being that the air handler blows warm air. The air handler is not heating (or cooling) the air. It’s just blowing the air past the refrigerant lines, which are changing the temperature. That’s what makes an air handler different from a furnace.
For homes with both a furnace and central air conditioning, the furnace’s blower acts as the air handler for the air conditioning system.