A boiler is a device in a central heating system that is used to heat water. There are different types of boiler available. Combi boilers are a type of boiler that is used to heat both hot water and the central heating in one device. System boilers require a hot water storage tank but can be used to save space, since a cold-water tank is not required. Regular boilers, or traditional boilers require both a hot water and cold water storage tank. Ultimately, boilers all do the same thing, which is to heat water, either for heating, or for hot water supply to the taps in a home.