what is a combi boiler

What is a Combi Boiler? The Complete A-Z Guide for 2022

A combi boiler is a single unit that heats your central heating system when you put the heating on, and hot water when you run the hot tap or mixer shower. Combi boilers are the most common type of boiler in UK homes.

Short for combination boiler, a combi boiler combines the two processes of heating your radiators and hot water all from one wall hung or floor standing unit.

Combi boilers heat your radiators on a sealed and pressurised system, and heat the mains water as you need it, so no need to heat cylinders full of water that might not even get used.

What is a Combi Boiler System?

There are three main types of gas boiler systems in the UK, combi boiler, system boiler, and heat only boiler (or conventional). A combi boiler system is the simplest and most liked, as there is no need for a cylinder or water tanks to store water. The water is heated instantaneously as you need it.

A combi boiler is the largest of the three boiler types as they house the most parts internally, but there are some very small combi boilers on the market. A heat only boiler is the smallest with the least amount of parts.

A combi boiler will need repressurising if the pressure drops, the same as with a system boiler. Changing to a combi boiler might be the best option unless you have a very high hot water demand,

How Does a Combi Boiler Work?

Hot Water

A gas combi boiler works by heating the hot water, via a heat exchanger, when a hot tap or mixer shower is used.

When you call for hot water, the gas burner fires up and heats the primary heat exchanger, this heated water circulates in the combi boiler and goes through a secondary plate heat exchanger on a mini circuit.

This then heats the cold mains water that is entering the other side of the plate heat exchanger, which will come out hot and travel along the pipe to the hot tap you are using.


When the central heating is turned on, the gas burner heats the primary heat exchanger the same as when the hot water is turned on. Then, with the help of a diverter valve and a pump, the water circulates around all the radiators and back to the boiler via the flow and return pipes.

This will keep circulating around the sealed central heating system and through the radiators until the room thermostat turns the heating off when it reaches temperature, or is manually turned off.

If someone opens a hot water tap, the hot water takes priority and the diverter valve will divert the water away from the radiators and back to the plate heat exchanger to heat the hot water.

What is a Condensing Combi Boiler?

All combi boilers fitted now are condensing combi boilers. A condensing combi boiler is a boiler that uses condensing technology to reuse the heat from the flue gases, so you use less gas.

They’re better for the environment and better for your bank account, as your gas bills should be lowered when you have your non-condensing combi boiler replaced. All new boiler installations have to be condensing boilers.

Whilst condensing combi boilers are better, they do add the problem of the condensate pipe getting blocked or frozen in the winter, this causes the boiler to cut out, and it will remain cut out until the problem is found and fixed, normally with the help of a heating engineer.

When Did Combi Boilers come Out?

Combi boilers first came out in Europe in the 60s, and first came out in the UK in the 70s. Combi boilers have come a long way since then, and had many problems along the way. Breaking down too often and not enough power to heat our hot water in the cold winters were pretty serious problems.

Now, combi boilers are very reliable and the most fitted and preferred gas boilers in the UK.

How Long Should a Combi Boiler Last?

Depending on how well it’s maintained and the quality of the boiler and installation, a modern combi boiler should last around 10 to 15 years, maybe a bit longer if you’re lucky.

In my experience as a boiler installer and repairer, they tend to break down not long after the warranty has ended. Fitting a magnetic system filter when the boiler is installed will help expand the lifespan of your boiler, as the number one cause of boiler breakdowns is sludge problems.

Combi Boiler Servicing

Having your combi boiler serviced regularly can play a big part in how long it lasts. With annual boiler servicing, your engineer will clean any parts that need cleaning and hopefully spot any leaks that are not on show but could be potentially damaging your boiler.

Also, any problems with the flue gases or problems with the way it was installed should be spotted and recommendations will be given for new parts I needed. All playing a part in helping your combi boiler last longer.

What is a Storage Combi Boiler?

A storage combi boiler is a combi boiler with a hot water cylinder inside. They are larger than a normal combi boiler and are better for homes with higher hot water demand and maybe not enough room for a system boiler with a separate hot water cylinder.

So, if the largest combi boiler is not powerful enough for your home’s hot water demands, then a storage combi boiler might be the best option for you.

A system boiler with a separate hot water cylinder can supply you with pretty much any high hot water demand for your home. A combi boiler can supply enough hot water for one or two bathrooms with no problem, but a storage combi boiler is somewhere in between.

Disadvantages of a Combi Boiler

A couple of disadvantages of a combi boiler in your home would be that they are not suited for large homes with high hot water demand.

If you have multiple mixer showers being used at the same time, a combi boiler might struggle to supply enough hot water to feed all the showers being used. You might not reach a high enough temperature on a combi boiler on the hot water.

This is because the water is heated instantly as you need it. If you regularly have two or three showers running at the same time and two or three bathrooms, then a system boiler with a separate hot water cylinder might be the best option.

Another disadvantage of a combi boiler would be if the boiler breaks down, you will be without heating AND hot water until it is repaired.

If your boiler’s safety device kicks in because of a hot water problem, then you will lose your central heating as well as your hot water. Having an electric shower can be helpful in this situation, as it does not use hot water from the combi boiler.

What is a Combi Boiler FAQs (Frequently Asked Questions)

Are Combi Boilers Cheaper to Run?

Yes, combi boilers are cheaper to run as you only heat the hot water you need as you need it. A system boiler or heat-only boiler has to heat a large cylinder full of water regularly, just in case you need to use some.

This means a lot of hot water is heated unnecessarily and is wasted. Combi boilers are cheaper to run because you use the hot water instantly as it is heated, and only the exact amount that you need is heated.

Can Combi Boilers Heat Water and Radiators at the Same Time?

No, combi boilers can not heat water and radiators at the same time. If your heating is on, and you run the hot water tap or mixer shower, the circuit on the combi boiler will divert to heat the water and the combi boiler will temporarily stop heating the radiators.

Once the hot tap or shower is turned off, the flow of hot water in the boiler will be diverted back to the radiators.

Is it Worth Changing to a Combi Boiler?

If you are replacing your old boiler and your hot water needs are not too high, then changing to a combi boiler is probably worth it for you.

If you have more than two bathrooms, then it is probably not worth changing to a combi boiler. But, every situation and house is different, and the best thing to do is get some boiler installation quotes and ask the heating engineers for their advice.

Do Combi Boilers Run Out of Hot Water?

No, combi boilers do not run out of hot water. This is impossible. The water supply is coming from your water mains feed, the same as your cold water.

Instead of coming straight out of the tap as the cold water does, it is diverted and forced to go through the combi boiler to get heated. It is heated instantly as you use it.

If there is no hot water coming out of your hot taps, then you have a problem somewhere, and you might need to get your combi boiler repaired.

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