Furnaces come typically as gas or oil fired while electricity is required for certain components like the fan motor, etc.
In the summer the fan motor is used to pull air from the return air ducts and blow this through the evaporator coil where chilled refrigerant from the air conditioner extracts heat. In the winter a furnace provides heat by heating a metal object called a heat exchanger. Indoor air is pulled from the return air ducts and blown across the heat exchanger to warm indoor air.
A variable speed fan equipped furnace is ideal. A variable speed fan is more efficient and it keeps the home more comfortable by constantly moving air at a low rate as opposed to a normal "on or off" furnace fan. When using indoor air quality components, a variable speed fan drastically increases their effectiveness.
Furnace heating efficiency is measured by an Annual Fuel Utilization Efficiency (AFUE) percentage rating. A higher AFUE percentage indicates a more efficient furnace.
The basic components of a furnace are:
- A burner, through which gas (natural or propane) or oil is delivered and burned.
- A heat exchanger, where the heat produced from the burning gas is transferred to the air distribution system.
- A flue or vent pipe, to exhaust byproducts of combustion (such as water vapor and carbon dioxide) to the outside.
An alternative way to heat your home is with a heat pump. Heat pumps are all electric and do not require a fossil fuel source. However a heat pump is not ideal when temperatures dive below 35-40° F. For high efficiency heating, a furnace and heat pump can be combined into a hybrid system. In a hybrid system design, the gas furnace and electric heat pump automatically switch off depending on outside temperature.