A Combi Boiler provides both heat and hot water upon demand. The Boiler is connected to the mains water supply and is able to send mains pressurised hot water to your central heating system (radiators) and hot water outlets (baths, taps and showers). If you have a Combi Boiler you will not have a hot water cylinder or tank.
Regular Boilers (also referred to as "heat only" or "conventional" boilers) only generate the heat for your central heating system (radiators) and hot water via cylinder. The hot water is stored in the hot water cylinder until it is needed, giving you a ready supply of hot water needed for your baths, taps and showers. If you have a Regular Boiler you will also have a cylinder, cold water cistern and feed & expansion tank (commonly found in the loft area).
Back Boilers which are commonly found behind fireplaces in chimney cavities are typically regular boilers.
A System Boiler is very similar to a Regular Boiler, generating the heat for your central heating system (radiators) and hot water via cylinder. This hot water is stored in the hot water cylinder until it is needed, giving you a ready supply of hot water needed for your baths, taps and showers. Unlike a Regular Boiler, the System Boiler does not require a feed & expansion tank as the system is sedent and the water in it is raised above atmospheric pressure. This helps to save on space. If you have System Boiler you will have a hot water cylinder and may also have a cold water cistern. You will not have a feed & expansion vessel.
