A combi (or combination boiler) is a compact and highly efficient unit giving all the heating and hot water you need, with significant savings on running and installation costs.

Unlike a conventional heating and hot water system, a combi boiler system does not store domestic hot water. It heats water directly from the cold mains - as you use it. There's no hot water cylinder, no tank in the loft (and so less risk of freezing and flooding), and none of the connecting pipework. So you not only save space, but also reduce hot water costs - which can account for up to 60% of a typical domestic fuel bill.

A combi (or combination boiler) is a compact and highly efficient unit giving all the heating and hot water you need, with significant savings on running and installation costs.

Unlike a conventional heating and hot water system, a combi boiler system does not store domestic hot water. It heats water directly from the cold mains - as you use it. There's no hot water cylinder, no tank in the loft (and so less risk of freezing and flooding), and none of the connecting pipework. So you not only save space, but also reduce hot water costs - which can account for up to 60% of a typical domestic fuel bill.

A combi also supplies hot water at mains pressure, giving you exhilarating power showering without the need for a pump. And as, on average, a shower uses considerably less water than a typical bath, the savings on hot water costs and water consumption can be significant.

A condensing boiler is more efficient due to its ability to extract more heat from the flue gases normally lost to the environment through the flue system.

As the flue gases pass through the heat exchanger this extra surface area cools the flue gases to around 55°C whereupon the latent heat within is released and applied to the system. Heat that would normally be lost to the atmosphere.

It is this ability to extract as much heat as possible from the gas it burns that gives an exceptionally high level of operating efficiency.

This higher efficiency is recognised within section L of the Building Regulations, subsequently achieving a higher SAP or NHER rating.

System boilers work in conjunction with hot water cylinders by heating the water that is to be stored within the cylinder and feeding the central heating system from one energy efficient source.
An expansion tank and pump are built inside the appliance. By building in components, which most other boilers situate outside the unit, system boilers can provide a very neat and attractive installation, and a feed and expansion tank in the roof is no longer needed.

They can be either floor standing or wall mounted with a large range of flueing options. More commonly gas boilers are wall hung with fan-assisted flues.

Advantages
- Cheap to run
- Reliable
- Good hot water flow rate (depending on type of cylinder)
- Longer lifespan