Forced-air gas

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A photo of a forced-air gas furnace, circa 1991.
A forced-air gas furnace, of the older, non-condensing type.

Forced-air gas heating systems are used in central air heating/cooling systems for houses. Sometimes the system is referred to as "Forced hot air".


Older furnaces sometimes relied on gravity instead of a blower to circulate air.[1]

Generally, they have a furnace heated by natural gas that pushes hot air through duct-work, then through vents to heat a building. Depending on the age of the system, forced air gas furnaces utilize either a pilot light or a solid state ignition system (spark or hot surface ignition) to light the natural gas burner.[2] A climate-control device (e.g., thermostat) regulates and controls the usage of the furnace. A digital thermostat can be programmed to activate the gas furnace at certain times. For example, a resident may want the temperature in their dwelling to rise 15 minutes before returning from work. This will conserve energy, expenses and natural resources. The natural gas is fed to buildings from a main gas line submerged a few feet beneath the earth, most likely running under and parallel to a street. The duct work supplying the hot air (and sometimes cool air if an AC unit tied into the system) can be insulated, or not insulated.

Simple types of gas-fired furnace lose significant amounts of energy in the hot waste gases going up the chimney. Modern high-efficiency condensing furnaces condense the water vapor emitted as one of the products of consumption. This increases the efficiency (energy delivered into the building vs. heating value of gas purchased) to over 90%. An incidental beneficial effect is that the waste gas stack is much smaller and can be made of plastic pipe; since the waste pipe is cooler than the chimney of a non-condensing furnace, it can be easily routed through walls or floors. However, the condensing furnace is more expensive initially because of the extra induced draft fn and condensate pump required, and the extra heat exchanger in the firebox.

Areas of usage[edit]

Residential and commercial buildings located in rural and remote areas do not often use forced hot air systems. This is due to the financial impracticality of running natural gas lines many miles past areas of relative dense habitation. Usually these rural and remote buildings utilize oil heat, which is delivered by a truck. When reading ads for houses for sale in the MLS, this is abbreviated as FAG.


  • Information on forced air pilot lights and maintenance from [1]
  1. ^ Gromicko, Nick. "Gravity Furnace Inspection - InterNACHI". InterNACHI. Retrieved 2014-06-25. 
  2. ^ "How to Light a Furnace Pilot Light". Gas Furnace Guides. Retrieved 4 December 2012. 

External links[edit]

  • From [2]
  • Info about Forced-Air Furnace Heating Systems from [3]