The main parts of a heating system are a source of warmed air, the part that distributes the warmed air and a part that regulates the temperature of the air. These parts include a heat pump, furnace or boiler and duct work.
In a forced-air heating system, a heat pump sucks in air from the outside and sends it through the duct work to the heating area. At this area, the outside air is warmed by the burning of gas or oil. After the air is warmed, the forced-air system pushes it through duct work to each room of the home. The colder air being replaced is sucked into duct work and sent back to the warming area, continuing a cyclical design of heating.
A radiant heating system uses the existing air in the room and the heat source, such as radiators, to warm the air. The radiators are warmed by hot water, and the air is circulated throughout the house by a circulator pump or gravity.
A gravity heating system uses the physics of warm air rising and cold air sinking to cycle air through duct work to the warming area and then into each room of the home. The furnace in this system must always be below the rooms being warmed.