Combi – short for "combination" – boilers are the most popular form of boiler in the United Kingdom, accounting for more than 50% of new condensing boiler sales. A combi boiler is, as the name suggests, a highly efficient water heater and central heating boiler combined in a single unit.

Water is heated on demand from within the unit itself, so a combi boiler (unlike a conventional boiler -- which typically requires a copper cylinder in an airing cupboard or elsewhere to store hot water) can be a practical solution if space within your home is at a premium. Combi boilers are typically compact; they can be hung conveniently on a kitchen wall, for example ; and their installation is less disruptive and less expensive than that of conventional boilers.