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.
