An HVAC system heats a house by using a furnace to generate heat. The furnace takes in cold air from the outside, heats it up, and then distributes the warm air throughout the house via ductwork. The furnace is powered by either natural gas, propane, or electricity. The thermostat is used to control the temperature of the house, and when the temperature drops below the desired level, the furnace will kick on and heat the house until the desired temperature is reached.