heat pumps

How Furnaces Work

Repairing and providing maintenance for furnaces can be daunting and expensive, but with YellowBird Services, you can be assured that your investment will be protected.

A home furnace is a major appliance that provides heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water and is permanently installed in your home. In the United States, the most common fuel source for modern combustion furnaces is natural gas; another common fuel source is propane (LP), then there is fuel oil, coal or wood. Where the cost of electricity is low electric resistance heating is used as the source of heat. Combustion furnaces always need to be vented to the outside which is typically done through a flue, that expels heat along with the exhaust.

Central Warm-Air Furnace: A central warm-air furnace is a type of space-heating device where a central combustor or resistance unit–generally using gas, fuel oil, or electricity–provides warm air that is circulated through ducts leading to the various rooms.

  • A forced-air furnace uses a fan to force the air through the ducts.
  • A gravity furnace relies on the natural flow of warm air up and cold air down; the warm air rises through ducts and the cold air falls through ducts that return it to the furnace to be reheated, thus completing the circulation cycle.

Furnace Components

Typically a furnace has three primary components:
  • The Burner (includes heat exchanger, draft inducer and venting) – Flames originate at the burners and are drawn into the heat exchanger by the negative pressure. The negative pressure is produced by the draft inducer. Then the hot gasses produced by the combustion of the flame pass through the chambers of the heat exchanger and the metal walls of the heat exchanger heat up. On a high efficiency furnace the gasses cool as they transfer the heat to the heat exchanger and are at about 120 degrees as they exit. Cooled gasses then enter the draft inducer blower and are pushed into the venting pipes. Exhaust gasses are then directed out of the house through the ventilation pipes.
  • The Blower  – Air is distributed throughout the home when the blower creates negative pressure on the intake side which draws air into the ductwork return air system and blows the air out through the heat exchanger and then into the supply air ductwork.
  • Controls and Safety Devices – Controls include a gas valve, ignition control, ignitor, flame sensor, transformer, limit control, blower control board, and flame roll out switch. A limit control is a safety device that will open the electrical circuit to the ignition control and stop the gas flow if the furnace over heats. The flame roll out switch does the same thing if the flame was rolling out of the heat exchanger instead of being completely induced into it by the draft inducer.