Figure 1. A labelled diagram showing the main components of a traditional household furnace.[1]

A furnace is part of the HVAC system inside of buildings that provides heating to the building by warming air and sending it through the ductwork. They can also be known as boilers or heaters (although both of these are more general terms). Furnaces can run on a number of different fuels, but mainly they are run on natural gas or oil.[2] Furnaces and heating take up a large portion of a homes energy bill, about 29% in the US.[3] In Canada, this value is even higher. Estimates from 2009 showed that home heating made up approximately 63% of energy costs in a home.[4] This value can also vary widely across the country as climates and heating needs differ vastly. As well, the way in which people across Canada choose to heat their home varies depending on location, thus the efficiency of different heating methods is unique across the country. For more information on the differences in household energy usage across Canada, see the Survey of Household Energy Use from Natural Resources Canada. Due to this high percentage of energy use from furnaces it is important to have proper settings on the thermostat to ensure money is being saved.


Most furnaces consist of a few main parts:[2]

  • Ignition Source: Ignites the fuel/air mixture when triggered
  • Burners: Burns a mix of fuel and air when triggered
  • Heat exchangers: A series of metal tubes warmed by flames
  • Blower: Blows air over the warmed heat exchangers throughout the ductwork
  • Flue: Vents harmful gases
  • Plenum: Recaptures cold air and filters out particulates

The operation of furnaces begins with the thermostat, which triggers when a home gets below its ideal set temperature. When this occurs, a signal is sent to the burners in the furnace. A flame is then lit in two ways; older furnaces use a small pilot light as an ignition source for gases whereas newer furnaces use a stick of silicone nitride that ignites a flame when electricity is run through it. Fuel for the furnace then mixes with air inside of the burner and is ignited by the silicone nitride or the pilot light, which burns as a controlled fire. Then, the heat exchanger absorbs heat from the combustion process in the burners. Once the air reaches a certain temperature, the blower then forces the heated air into the duct work of the home and out through the registers.[5]

This burning process creates harmful byproducts, such as carbon monoxide. These byproducts are vented away from the home using a flue pipe. As well, some fuel is left unburned in this first combustion. Some high-efficiency furnaces capture these unburned fuels and re-ignite them in a second combustion chamber. The final step in the heating process is to to return the cold air to a box known as a plenum through the ducts. Air going to the plenum is first filtered, as it contains many dust particles. The process of warming this air can then begin again.[5]

Furnaces can also include humidifiers, which reintroduce moisture into the air of the home to improve home air quality.


  1. "Condensing furnace diagram" Licensed under Public Domain via Wikimedia Commons -
  2. 2.0 2.1 Genteq Comfort. (March 21, 2015). How does my furnace work? [Online]. Available:
  3. Genteq Comfort. (March 21, 2015). Go Green and Save [Online]. Available:
  4. Office of Energy Efficiency. (March 25, 2015). Energy Efficiency Trends in Canada [Online]. Available:
  5. 5.0 5.1 eHow. (March 21, 2015). How Does a Gas Furnace Work? [Online]. Available: