A combination boiler is a gas or oil boiler that heats hot water as it is needed; it therefore needs no separate hot water storage cylinder. A combination (or combi) boiler is like a standard unvented boiler, but with a second heating circuit (the domestic hot water circuit) to provide instant hot water on demand. If the combi is fed directly from the mains water supply to its heat exchanger then, as long as the boiler is firing, there is no limit to the amount of hot water that can be produced.
Combination boilers are especially attractive when space is at a premium, for example in appartments and smaller homes with low/infrequent water consumption, because all water tanks in the loft and even the domestic hot water cylinder is done away with. However, for homes with more than one bathroom it is generally worth considering stored hot water by means of a cylinder.