Combustion systems produce heat by burning fossil fuels. When burned, the carbon and hydrogen in the fuel combine with oxygen in the air producing heat, carbon dioxide and water. The heat is either used in the space where it is produced, (as in the case of a fireplace) or transferred to the distribution system, (as is the case with a furnace or boiler). The other "by-products" of combustion must be expelled through a chimney or vent pipe.

This section covers heating systems according to the fossil fuels burned, like gas, oil, coal and wood. It also covers them by how heat is distributed to the space, like furnaces, boilers and fireplaces.

Some new, high efficiency systems include: condensing furnaces, pulse combustion, and gas heat pumps.