A forced-air heating system draws room air through ductwork and a filter into a furnace, where the air is heated. The warmed air is then blown back to rooms through ductwork.

With older "gravity" furnaces, the heated air is delivered by natural convection, not by a blower.

Most furnaces are gas-fired, but other fuels include oil, coal, wood and electricity. With a conventional furnace, natural gas is piped to a burner that's located inside a combustion chamber.

The flame heats up a metal box-the heat exchanger- where room air is heated as it flows through. Exhaust gasses given off by burners vent outside through a flue that goes up through the roof or, with newer high-efficiency models, out through a wall.

An electric forced-air furnace (shown below) uses heating elements rather than burners to heat in the heat exchanger.

A forced-air heating system can be combined with air conditioning for cooling, a humidifier for maintaining proper moisture balance and an air filter. Ductwork is generally metal, wrapped with insulation to help keep heat in. In some cases, flexible insulation-style ductwork is preferred.

With a forced-air system, a blower draws cool air in from rooms through ductwork.

Inside the furnace, the air passes through a filter into a heat exchanger where the burner heats it, then it's blown back to rooms through ducts and registers.

The thermostat signals furnace to turn on when air temperature dips below a set level. When this happens, a gas valve regulates and delivers fuel to burners and-if there is one-the pilot light.

The burner is ignited inside combustion chamber, creating heat that is transferred to the heat exchanger.

Combustion fumes are vented out through a flue out the rooftop or, with high-efficiency models, through the wall.

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