What is a Combi Boiler and How Does It Work?

Professional plumber checking a boiler control panel

Combi boilers are amongst the most popular boiler types in the UK, and there are some clear reasons why. 

Perfect for homes where space is at a premium, combi boilers heat water on demand, so there’s no need for a hot water tank. They’re brilliantly reliable and energy efficient, which is why so many UK homeowners opt for these boilers for their properties. 

In this article, we’ll explain more about combi boilers, giving you the lowdown on what they are and how they work. Then, we’ll take you through some of the key advantages of combi boilers so that you can make the best decision for your home.

What is a Combi Boiler?

A combi boiler is a space-saving boiler that heats water as and when required. The design of the boiler is such that water can be heated directly from the mains. 

Therefore, no hot water storage cylinders are needed, and there’s no need for any cold water storage either. 

The fact that combi boilers only heat the water that’s actually needed means that they’re incredibly cost-efficient compared to other options. 

Hot water can also be delivered at high-pressure levels, which means powerful, hot showers are available whenever you want them.

How Does a Combi Boiler Work?

A combi boiler works by partnering a water heater and central heating boiler in one unit. 

As with all boilers, the thermostat plays a huge role. It signals to the boiler to turn on whenever the temperature of the property drops below a certain point. When this happens, the boiler fires up, and hot water is moved into the heating system. 

The thermostat will then measure the temperature of the property, signalling to the boiler to turn itself off when an adequate temperature has been reached. 

Combi boilers also provide hot water, of course. If someone turns on a hot water tap or a shower within the home, the hot water that has been going to the heating system is instantly diverted towards the tap. 

In a combi boiler, the valve remains open to allow hot water to travel to the tap while it’s turned on. When the tap is turned off, the valve immediately closes to take hot water back into the heating system. 

Benefits of a Combi Boiler

There are many benefits to choosing a combi boiler for your property, but of course, there are pros and cons to all boiler options. 

We’d recommend speaking to a heating professional about what you need and how you use your heating before taking the plunge and buying a new boiler.

Take a look at some of the top reasons to choose a combi boiler below.


As we’ve mentioned, combi boilers do away with the need for separate hot water storage completely. 

The units themselves are quite compact, and the lack of tanks and storage cylinders makes these boilers the perfect option for smaller homes or properties that need all the storage space they can get.

Increased Efficiency

Combi boilers only heat the hot water that’s actually going to be used, which means they’re incredibly efficient. As the boiler is heating smaller amounts of water when it’s needed, there is no wastage in terms of energy consumption. 

This makes combi boilers a great choice for those looking to reduce energy bills and cut unnecessary spending.

Cost Effective

The efficiency of combi boilers means that they can be run at a much lower monthly cost than other types of boilers. 

Only heating the hot water that you really need means reducing energy bills as much as possible. This can make a dramatic difference to energy bills, particularly during the winter months.

Why Are Combi Boilers More Efficient?

Modern combi boilers are known for their efficiency. They use the minimum amount of energy required to provide heat and hot water, by only heating the water that’s actually being used. 

Bear in mind that the efficiency of modern units far surpasses older boilers. If you’re looking to cut down on heating bills and your current boiler is over ten years old, simply switching to a modern combi boiler could make a huge difference to your energy efficiency and, therefore, your energy bills. 

How to Choose the Best Combi Boiler Size

Choosing the best size boiler for your property is a vital part of the decision making process. 

Opt for a boiler that’s too small, and you might struggle with a lack of hot water for the morning rush. If you choose one that’s too big, you could be overspending for no benefit. 

It all depends on how many radiators you have in your home. If you’re living in a typical three or four-bedroom house, you’ll probably have in the region of ten radiators. A small or medium-sized boiler (around 24-30 kW) should, therefore, be perfectly adequate for your usage. 

Larger homes with far more radiators or homes with more bathrooms might require a more powerful boiler of between 30-35 kW. However, it’s always best to check with an engineer to ensure that your new boiler is adequate for your intended usage.

Get More Advice on Combi Boilers

If you’d like to find out more about the different types of boilers out there, we’re here to help. 

Our team of skilled engineers knows all there is to know about boilers, and we’re on hand to advise you on the best options for your property. 

Talk to us about upgrading your current boiler, and we’ll take you through the options to help you make the best decision for your specific requirements. 

Once you’ve decided on the new boiler you’d like, our team can manage the installation process on your behalf. We’re here to take the stress out of upgrading your boiler. Just get in touch, and we’ll do the rest.

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