If you’re in the market for a new boiler, you might have come across the term ‘combi boiler’. But what exactly is one?

What is a combi boiler?

A combi boiler is a type of system that can provide hot water on demand, rather than storing it in a tank or cylinder, as with a conventional boiler. This hot water can be used for your heating, appliances and taps. It is called a combi boiler because it is a single unit that serves two ‘combined’ functions: heating and hot water.

These kinds of systems have become increasingly popular for smaller properties, because you don’t need a cold water feed tank or hot water storage tank. 

How does a combi boiler work?

A combi boiler works by taking water directly from the mains and heats it as and when you need it.

When you require hot water, the boiler will begin to burn gas in the combustion chamber. The hot gases rise and begin to move through a series of pipes and into the heat exchanger. The heat exchanged is a chamber that the hot gas runs through in order to heat the surrounding cold water.

In a regular boiler, this process is done according to a programme and the water is stored in a cylinder until it’s needed. With a combi, this cycle starts only when you’ve programmed your heating or turned on a hot water tap, and so the hot water is instant.

To help deliver hot water even more quickly, modern boilers also have a built-in pre-heat system. This is a small reservoir of water that is always kept at a certain temperature and means that you will instantly have hot water when you turn on a tap. Most boilers will turn on the burner around once an hour to keep this reservoir hot, which is why you may notice your boiler turning on even when no hot water is being used.

It is possible to increase your boiler’s efficiency even more by turning off this function, however in most domestic environments this is not recommended.

What is condensing technology?

Building regulations in the UK now require all gas combi boilers to incorporate condensing technology. These rules were introduced to help homeowners benefit from dramatically increased efficiency. In older boilers, as much as 50% of heat energy was lost as the combustion gasses escaped into the flue, taking their heat with them. In a condensing boiler, on the other hand, the heat contained within the combustion gasses is captured within the system and used to preheat cold water as it enters the system.

Condensing technology helps to protect the environment as it reduces your home’s carbon footprint by reducing CO2 emissions. It can also save you a lot of money on your fuel bills by making better use of your energy.

What are the advantages of a combi boiler?

Gas condensing combi boilers are by far the most popular choice of boiler in the UK. They are usually recommended because they are the most efficient way to provide hot water and heating for the average household.

Below, you can find a list of the advantages of a combi boiler, to help you decide whether this type of heating system is right for you.


Combi boilers are ideal for smaller homes, or homes without a sufficient loft space, because they don’t require external hot and cold water tanks. The boiler can do everything itself, including taking in cold mains water and pumping hot water around the heating system. Ultra compact versions are available that will fit in your average kitchen cupboard, too.

(H3) No running out of hot water

As combis produce hot water on demand, there’s no risk of running out of hot water, like there is with a regular or system boiler.


New combi boilers can be up to 95% efficient, thanks to their additional condensing technology and their ability to only heat the water that is needed, instead of filling up a very large tank.

Cost effective

When you have a regular boiler system installed, you may need to replace the hot water tank, change the cold tank and have the boiler installed. Combis can be more cost effective because you don’t need to worry about all these extra features. This also means that installation is easier, and therefore cheaper, too. 

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Why are gas combi boilers more efficient?

The main reason that combi boilers are more efficient is that water is only heated when you need it. They are designed to maximise the amount of energy that is directly used to heat the water, and there is no need to heat up and store more water than is required. This helps to reduce your carbon footprint and also cut down on your energy bills.

What size combi boiler do I need?

There are several factors to consider when deciding which size gas combi boiler to buy. The most important factors are generally how much demand there is likely to be for hot water and how many radiators you will be heating.

As a rule, a 20-35kW combi boiler will generally be sufficient for an average 3-4 bedroom house, whilst those with ensuite bathrooms may find that a 30-35kW boiler is more suitable. Much bigger houses will typically need a larger 35-42kW boiler, although it may be worth considering a system boiler as an alternative.

How much does a combi boiler cost?

A combi boiler can vary in price depending on the size, model, manufacturer and installer you choose. The cost of the boiler itself could range from £600 to £900 for a budget model and £900 to £1,500 for a mid-range model. Installation could range from £1,500 to £2,000 as well, so remember to take this into account when budgeting for a new boiler. 

What else should I consider when choosing a combi boiler?

n choosing a gas combi boiler, it is also worth considering what options are available to control your heating. Many of the latest boilers have computerised controls and new generation thermostats that can help you maximise efficiency and save money on your heating bills.

Also think about whether you’d like to have access to smart functionality, which allows you to control your heating remotely, usually from an app on your smartphone.

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