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. (Heating appliances that use steam or hot water as the fluid are normally referred to as a residential steam boiler or residential hot water boiler.) 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.
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 PVC/Metal pipes that can be vented through the side or roof of the house. Fuel efficiency in a gas furnace is measured in AFUE (Annual Fuel Utilization Efficiency).
Central heating systems have a primary heating appliance, such as a furnace, typically located in your basement or garage. All furnaces consist of four main components: 1) burners that deliver and burn fuel, 2) heat exchangers, 3) a blower and 4) a flue that acts as an exhaust for gaseous by-products. Depending on your situation, region and needs, you can choose from heating systems running on either gas or oil as fuel, or a hybrid packaged system that can use both fuel types.
Combustion gases are generated by the burners in your furnace and passed through a heat exchanger. Air from your home blows across the heat exchanger to be warmed. It is then blown through a system of ducts to distribute around your home. During warm seasons your heating system works with your central air conditioning. Air is cooled as it’s blown over your air conditioning unit’s cooling coil, often attached to the air circulating fan of the furnace, and then sent through the same air ducts throughout your home.