How Gas Ducted Heating works ?

To understand how a high-efficiency furnace works, consider these basics. In principle, a forced-air furnace is a relatively simple device, somewhat like a gas oven that’s hooked up to a fan.
Natural gas is piped to a burner inside a combustion chamber where the gas is mixed with air and ignited by a pilot light, a spark or a related device at the request of a thermostat.

A blower in the furnace pulls cool air in from rooms through air ducts, passes it through a metal “heat exchanger” where it’s heated by the burner, and blows the warm air back into rooms through ductwork.

Exhaust gasses from the burners are vented outside through a flue Furnaces can be fueled by natural gas, oil, propane, coal, wood, or electricity.
Today, most use gas because it is clean-burning, commonly available, and relatively inexpensive. A gas forced-air heating system goes into action when a room’s air temperature drops below a preset level on the thermostat.

The pilot light ignites a burner in the furnace’s heat exchanger, a metal chamber around which air flows and is then heated.

The warmed air moves into the hot-air plenum and into the rooms through ducts.
The combustion gasses are vented through a flue in the roof or, in some newer homes, through a wall.