Natural gas forced-air furnaces are by far the most popular method of heating in North America, mainly because they can distribute heated air from a central furnace to every room of the house and they can also be used in conjunction with a central AC unit.

furnace diagram

The basic components of a forced-air system include:

  • An air handler, which may be either a stand-alone blower cabinet used with a heat pump or a forced-air furnace that includes a blower
  • A means of heating air, such as natural gas-fueled burners
  • Ductwork for delivering cold air to the heater via a return
  • Ductwork for sending heated air back to rooms via some vents
  • A thermostat for turning the system on and off

A natural gas furnace reacts to changes in a room’s temperature. When the air temperature in the room drops below the thermostat’s set temperature, it alerts the furnace. Cold air is then drawn from the room’s return through ductwork into the cold air return and through an air filter. The burner ignites to heat the air as it moves through the furnace. The circulating fan helps the warmed air circulate through the ductwork and back into the room via vents. The cycle continues until the set comfort level is reached at which point the thermostat turns the system off until the next time the temperature drops.

Sources:

Wikipedia, “Furnace”
Buzzl, “How does a Gas Furnace Work“