What is a combi boiler? 

Combi boilers are one of the most popular types of boiler in the UK. Also known as combination boilers, they act as both water heaters and a central heating boiler without the need for additional water storage systems.
Their compact nature means they’re perfect for small to medium-sized homes, and this factor, combined with their simplicity makes them an ideal choice for many households.
Combi boiler with a blue background

How does a combi boiler work? 

Functioning as both a water heater and a central heating boiler, combi boilers can heat water directly from the mains when you turn on any hot tap in the system. They have two independent heat exchangers – one for the radiators and the other for the hot water supply. The primary heat exchanger focuses on your radiators and carries hot water around your home. The secondary exchanger is for heating the water that comes out of your taps. This heats an external clean water pipe that runs through the system, feeding it out to the taps in your home.
Diagram of a house with a combi boiler setup

Condensing combi boilers

Condensing combi boilers are far more energy efficient than non-condensing, which are believed to account for around 16% of the world’s CO2 emissions. The heat exchangers in a condensing boiler recover more heat from the burning gas, which is then used to heat the water. As a result, they can be up to 25% more energy and cost efficient than their non-condensing counter-parts.

It is now compulsory that all new boilers must be condensing.


Pros and cons of a combi boiler

Benefits of a combi boiler

  • They're generally very economical and energy efficient, as the water for your home is heated instantly, meaning you can have a hot shower without having to pre-heat the water.
  • They’re compact, so perfect for smaller homes with little or no loft space – many other boilers require hot water storage tanks.
  • Installation is often cheaper, as less pipework is required to feed hot water and warmth around your home.
  • They’ll be easier to get fitted - most boiler engineers are trained to repair them as they’re so common.

Disadvantages of a combi boiler

  • You may suffer from reduced water pressure if you’re using multiple taps and/or the shower at the same time; which could be an issue for larger households or with homes that have to cater for several people.
  • There are several moving parts in the boiler unit, meaning there’s more chances for something to go wrong than with a conventional boiler.
  • You’re entirely reliant upon your boiler for both hot water and heating – if this breaks down you will lose both. Other systems with hot water tanks can still rely on these even when the boiler is experiencing issues.
  • Hot water is delivered at mains pressure so you’re unlikely to be able to support a power shower.

Potential problems with a combi boiler  

In areas with very hard water and a high scale content, the heat exchanger can ‘fur up’ with limescale if and water flow can slow. Most savvy engineers will use limescale reducer to prevent the build-up in the first place, however, once the heat exchanger plate is covered in limescale, you’ll probably have to get it replaced.
Engineer repairing a boiler

Different fuel types for a combi boiler

Whilst there’s a range of different fuel types that can be used to power a combi boiler, the three most popular are gas, electric and oil-fired. Find out the pros and cons of each below:

Fuel type Description Pros Cons
Gas Combi Boilers Gas boilers are the most popular fuel type.

Gas is supplied from the mains and feeds directly into the boiler, which burns the gas to make hot jets that play on the heat exchanger to heat up the water.
  • The most widely-used and cheapest heating fuel available to consumers.
  • Gas powered boilers are usually cheaper.
  • Emissions are half that of oil and a third of coal.
  • Not all homes are on the gas network.
  • Can be noisier than electric alternatives.
  • Whilst greener than other fossil fuels like coal, gas is not as clean as electric.
Electric Combi Boilers The second most popular fuel type.

Rather than burning fuel to heat the water that powers the central heating system, electricity is used to heat elements within the boiler, which then heats water.
  • A very energy efficient way to heat both home and water.
  • No need for a mains gas supply.
  • They are generally much quieter than gas boilers.
  • No fuels are burned, so no gas is emitted.
  • No flue is required – this makes electric boilers more space efficient and allows for more flexibility when placing it in the home.
  • No need to service as regularly as a gas or oil powered boiler.
  • The process of creating electricity in a power plant can generate the same amount of pollution as gas and oil.
  • Electricity is more expensive than gas, so prices will often be higher.
  • This boiler wouldn’t work during a power cut, meaning no heating or hot water. Larger homes may struggle as they can’t heat a large amount of water at once.
Oil Combi Boilers Oil boilers work in very much the same way as gas or electricity boilers, except they will be powered by a supply of oil, which will need to be delivered and stored in a tank somewhere in your home or garden.
  • Just as energy efficient as gas powered systems.
  • Ideal for homes with no direct access to the gas mains grid.
  • Running cost can depend on the price of oil, which can fluctuate with demand, weather conditions and political unrest. Currently oil is cheaper than electricity but more expensive than gas.
  • Reliant on delivery supplies – you need to be careful not to run out before the next batch is delivered.
  • Can be slower to work than gas.
  • Requires space for the storage tank.
  • As with the boiler, the storage tanks will also require yearly servicing.
LPG Combi Boilers LPG combi boilers are similar to oil combi boilers but instead of using oil as a fuel supply, they use LPG (Liquid Propane Gas) instead.

Similar to an oil boiler, you’ll need it delivered to your home and a storage tank to store your gas.

You’ll most likely have seen caravans running their cooking appliances off smaller portable LPG gas tanks but larger storage tanks are available to rent or buy for your home.
  • Ideal for homes with no direct access to the gas mains grid.
  • Living off-grid means you won’t be impacted by national outages.
  • As it a gas, it burns cleaner than oil. LPG has the lower carbon emissions than oil.
  • Price of LPG gas can fluctuate but you can save money by shopping around for a cheaper supplier and buying at the right time of year.
  • LPG compatible appliances can be more expensive.
  • Initial installation can be costly.
  • The gas will be delivered by road, so you could run out of gas before your next delivery. However, a bit of planning can prevent costly emergency deliveries.
  • If there’s a leak it requires LPG clean up specialists to come round.

How much does a combi boiler cost?

Prices* will range across different brands, but the following provides a rough guideline:

Boiler type Price range
Gas combi £499 - £1400
Oil combi £1,107 - £3,770
*Prices correct as of May 2017 according to Which?
Whether you decide to get a combi boiler or conventional boiler, make sure get it serviced regularly. if you feel you want peace of mind you can insure your boiler against breakdowns too.