There’s nothing quite as awful as being able to see your breath on a cold winter’s night from your own living-room couch. Unfortunately, heating systems have been known to break down, often when the thermometer reaches its lowest depths. Although mechanics train for years to be able to fix faulty furnaces, the forced-air system itself is relatively straightforward, consisting of only four main parts: the thermostat, the burner, the heat exchanger, and the blower.