A gas forced-air heating system goes into action when the thermostat tells it that room temperature has dropped below a preset comfort level. The thermostat sends a low-voltage electric signal to a relay in the furnace, which signals a valve to open and deliver natural gas to the burners and the blower to turn on.

The furnace’s pilot light or electronic ignition lights the burner inside the combustion chamber. This creates heat in the furnace’s heat exchanger, a metal chamber around which the moving air flows.

Once warmed, the air is pushed into the hot-air plenum and then on to the rooms through ductwork. The combustion gases created by burning fuel are vented through a flue in the roof or, with high-efficiency furnaces, through a wall.

One of the benefits of a forced-air system is that it can receive a whole-house air-conditioning unit, a humidifier, and an electronic air filter—all of which can take advantage of the furnace’s air handler and ducts for delivery of conditioned air to rooms. Ductwork is generally metal wrapped with insulation or a special flexible plastic-wrapped insulated material that helps retain heat.