Central heating works by passing air from the home over a heat exchanger, which warms the air, and then moving the air to various rooms throughout the house via a system of ducts. A central heating system works in much the same way as a central air conditioning system, the only difference being that a heating system passes air over a heat exchanger before circulating it throughout the building, while an air conditioning system passes air over a cooling coil.
All central heating systems require some heat source, either a furnace or a heat pump. For systems that use furnaces, fuel burns inside the furnace, generating hot gases. These hot gases then warm the heat exchanger, which in turn warms the air in the home. Because the combustion gases are hazardous, a flue and chimney then safely remove the gases from the home. Heat pumps do not generate waste gases, and these systems do not require flues or chimneys.
While forced air is the most common type of central heating by a large margin, central heating systems that use steam or hot water also exist. Both steam and hot water systems work in much the same way as forced air systems by heating water at a central location and then pumping it throughout the building, but these systems use pipes rather than ducts. While these systems are more efficient than forced air systems, due to water's highly specific heat capacity; they are also more expensive, maintenance intensive and prone to problems.