What is a Combi Boiler?

Over 70% of UK homes choose a Combi Boiler over other types. Discover what makes them so popular, how they’re unique by design and the types available to homeowners today.

Did you know? As of 2018 all Gas Combi Boilers installed in the UK are legally required to be at least 92% efficient.

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what is a combi boiler illustration, person pondering the question 'what is a combi boiler?'.

What is a Combi Boiler?

A Combination Boiler (or Combi Boiler for short) is both a central heating boiler and a high-efficiency water heater in a single compact unit. It provides heat to radiators and limitless hot water on demand, by heating water directly from the mains—this makes them highly economical as they only heat the water used.

They’re also incredible space friendly as there is no requirement for an external cold-water-storage tank or a hot-water cylinder; a compact footprint that makes them ideal for smaller homes where space is limited.

combi boiler internals illustration, detailing the flow of cold to warm water via the boiler and plate heat exchanger.

How does a Combi Boiler work?

Combi Boilers don’t require a storage cylinder or a cold-water-storage tank to operate, instead they heat water directly from the mains. They use a ‘plate heat exchanger’ to heat water, and thanks to their high-heat-recovery rate enables them to transfer heat from one body of water to another very quickly.

The process to heat water begins with the Combi Boiler feeding hot water to the plate heat exchanger. Parallel to this the cold mains runs into and down the plate heat exchanger enabling heat to transfer from the pre-heated water to the cold water, thanks to conduction—this results in hot water being readily available as soon as you turn your taps.

consending combi boiler illustration, detailing how  a condensing combi boiler captures lost heat to reuse for pre-heating incoming cold water.

What is a Condensing Combi Boiler?

The majority of modern Combi Boilers installed today (Gas, LPG and Oil) are Condensing Boilers. These types of boilers are highly efficient this is thanks to how they recycle energy, which is pumped back into the system to help pre-heat water.

To understand how high levels of efficiency are obtained we first must explain how a Non-Condensing Combi Boiler heats water. A Non-Condensing Combi Boiler heats water by combustion (burning fuel); the heat produced hits a plate heat exchanger which warms the cold water before it leaves the system, however during this process a lot of extra heat escapes, and is sucked out of the exhaust flue, with as much as 30% of energy being wasted.

A Condensing Combi Boiler has two heat exchangers and follows the same process when heating water but with the exception of a second heat exchanger; this captures the exhaust heat which otherwise would be lost to the atmosphere. The heat is then recycled and used to preheat cool water as it enters the boiler, leaving the primary heat exchanger with less work to do when reaching a desired temperature.

A Condensing Combi Boiler is a smart choice for homeowners looking for a greener and more efficient boiler; their high efficiency rating ensures that the fuel burned to heat your home goes further.

illustration showcasing the types of combi boilers available: Gas, Electric, L.P.G., Oil.

Types of Combi Boilers

Combi Boilers are available in a range of sizes and operate using different energy sources including gas, electric, LPG and oil.


Gas is the most common fuel type of heating across the UK. Since 2005, all Gas Combi Boilers must also be Condensing Boilers. Gas is a comparatively cheap option and is 3-4 times less than electric heating. However, you must be connected to the gas grid for this to be an option.


This option is a cleaner solution than gas or oil as it doesn’t burn fuel to heat water and alternatively uses an electrical element. Electric Combi Boilers are also easy to install and significantly quieter than the typical Gas Combi Boiler.


A Liquefied Petroleum Gas Combi Boiler works similarly to both oil and gas Combi Boilers. LPG is cheaper than an Oil Combi Boiler and provides a high level of efficiency but still requires a tank to store the gas at your home. If you don’t already have a tank at your home this can be quite an expensive investment.


If there is no connection to the gas grid in your home, you could choose an Oil Combi Boiler. Oil Combi Boilers are supplied by a large tank which needs to be refilled when the fuel starts to run low. However oil is also a great deal more expensive than gas and typically needs to be paid in bulk.

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