Modern heating systems use various methods to create heat; electricity, a variety of fossil fuels and several techniques involving solar power. All of these methods, however, use either forced air or forced water circulation to heat the actual living spaces.
Hot Air Heating Systems
A forced air system uses air as the heat exchange medium. This means that air is drawn from the space needing heat, directed into an enclosed space where one of the above methods is used to heat it, and then circulated back to the original space. Typically, the heating equipment used in this method is referred to as furnace.
Hot Water Heating Systems
Similarly, forced water systems use water as the medium of heat exchange. However, these heating systems can work in one of two ways.
The first method uses a hydronic, or water heating appliance, to heat the water in an isolated area. Hot air is blown past the heated pipes and the air is subsequently sent to the space needing heat in a process like those above.
The second method also uses a boiler but involves the transfer of the water through a series of pipes directly to the space and into radiators/baseboards. The ambient in the water contained in the radiators is then directly radiated into the space. The cooled water is then circulated back to the boiler and reheated.
Both systems have advantages and drawbacks that can be readily explained by a reputable HVAC contractor. One final note, both hot air and hot water heating systems often rely on the combustion of fossil fuels. It should be noted that in no case is the recirculating air mixed with the combustion air. These heating systems are safe, economical and highly efficient.