Gas Furnances

A household furnace is a major appliance that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air , steam , or hot water . The most common fuel source for modern furnaces in the United States is natural gas ; other common fuel sources include LPG (liquefied petroleum gas), fuel oil , coal or wood . In some cases electrical resistance heating is used as the source of heat, especially where the cost of electricity is low.

Combustion furnaces always need to be vented to the outside. Traditionally, this was through a chimney , which tends to expel heat along with the exhaust . Modern high-efficiency furnaces can be 98% efficient and operate without a chimney. The small amount of waste gas and heat are mechanically ventilated through a small tube through the side or roof of the house.