With a ‘wet system’ hot water circulates through a system of pipes that connect to radiators throughout a house. At the centre of the system, a boiler burns a fuel – or sometimes there is a ‘heat exchanger’ and this heats the water that feeds the network of pipes. ‘Wet systems’ are the most popular form of heating system in the UK.
Radiators, despite their name, do not just give off radiant heat, in fact they deliver most of their heat through convection; air warmed by the radiator naturally rises, and cool air falls relative to it, as a result the warmed air circulates and the ‘space’ in a room is heated.
The pipework may also be connected to a hot water cylinder (tank), which will provide a supply of hot water for bathing and washing.
Warm air systems were sometimes installed in the sixties and seventies in the UK, but continue to be popular in North America. Air is heated by a boiler, typically fuelled by natural gas, and fed via ducts to rooms around the home. The warm air enters the room via a floor or wall vent.
In commercial buildings, variations on warm air systems are still in widespread use, although they typically also serve as a cooling (air conditioning) system. In most homes, warm air systems have been replaced with ‘wet’ systems, which are generally more comfortable and efficient.