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What is a combi boiler?

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If you’re looking to install or upgrade your heating system, a combi boiler is the most common option. Combi boilers are one of the most popular heating systems on the market. In this blog we will be taking a look at combi boilers and why it’s important to decide whether they’re right for your home.

How does a combi boiler work?

A combi boiler instantly starts heating water directly from the mains, so you’ll have hot water within seconds of turning on a tap. Combi boilers only heat the amount of water you need when you need it. And because its under pressure from the mains already, water flows naturally resulting in not needing a pump. They also don’t use a hot water storage tank and can provide you with both heating and hot water in a single appliance. As a result of that, they are much more energy efficient giving potential for lower energy bills.

What size combi boiler do i need?

Boiler size is measured in kilowatts (kw). When choosing your combi boiler there’s a few things to consider such as the amount of bedrooms, radiators, and baths and showers in your property. In a large property where there’s a high demand for heating and hot water, you would need a large boiler. They range in sizes up to 40kW. Whereas a smaller boiler 26kW would provide plenty of power for your hot water and heating for a smaller property like a one bedroom flat.

The larger the boiler the more energy it used while its working.

Pros and Cons of Combi Boilers


  • The most efficient advantage of a combi boiler is that they only heat the water you need. As a result of this combi boilers should cost less to run that other models, as well as being more environmentally friendly. Unless you have a large family or use an excessive amount of hot water.
  • People that have a conventional or system boiler often save money by keeping their heating on a timer. With a combi boiler there’s no need to do that as you’ll have hot water 24/7.
  • Combi boilers are a great idea if you life in a flat or don’t have a loft space. They don’t take up too much space and don’t require a seperate hot or cold water tank. They’re mostly installed in the kitchen or the utility room.
  • If you’re installing a boiler from scratch, the combi boiler will be cheaper than others as there’s less equipment and pipe work to fit.


  • Combi boilers have a limited flow when it comes to boiling water. The central heating will switch off when you turn on a tap. Its not usually something that gets noticed, even if you turn a bath on for ten minutes as the radiators stay warm. Although, if you turn two taps on simultaneously there will be a noticeable drop in pressure and/or temperature.
  • A larger tank will be better for you if you have a large family or have more than one bathroom. As you could have multiple hot taps in use at once. If you’re smart about the way you heat it, it doesn’t have to cost more to run.
  • When a hot water tank breaks down with a fitted immersion heater, you can still heat water. If a combi boiler breaks down you’ll have to use the kettle.
  • Combi boilers don’t tend to work well if you have a low mains water pressure. As they may not have enough flow rate to supply both hot and cold water at the same time.
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Other heating options

Just as a guide, a combi boiler will most likely be the best option if you live in a smaller home.

When a household gets bigger the number of bathrooms grow, and the need for a conventional boiler gets stronger.

Heat only boilers

A heat only boiler is the type of boiler that was standard before the combi. It forms of a cold water tank, a main boiler, and a hot water storage tank. Cold water is fed to the boiler and then is heated up and sent to the hot water tank and central heating system until its needed.

When you run a bath, the water will need to be replaced. So you will have to wait for it to be heated before you can run another.

System boilers

System boilers feed the central heating and the hot water storage tank without the need for a cold water tank. They also work at a higher pressure than conventional boilers.

System boilers work the same way as conventional boilers, once the hot water in the tank is used you’ll have to wait for it to reheat so it can be used again

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