what is a combi boiler

What is a Combi Boiler? and Other FAQs Answered

Last updated on January 29th, 2024

What is a combi boiler? How do they work?

Find out the answers, and lots of other frequently asked questions, from a full-time combi boiler installer and repairer.

Gas combi boilers are the most common type of boiler in UK homes.

Here are my answers to some of the most searched for questions about combi boilers.

What is a combi boiler?

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.

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 get used.

What is a condensing combi boiler?

All new combi boiler installations have to be condensing 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.

Whilst condensing combi boilers are better, they do add the problem of blocked and frozen boiler condensate pipes in the winter.

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.

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 inside.

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.

What is the smallest combi boiler?

Intergas ECO RF smallest combi boiler

The smallest combi boiler on the market in the UK is an Intergas Compact ECO RF.

Whether you need it to fit in a tight cupboard or just want it to take up as little room as possible, there are some small combi boilers out there but the Intergas Compact ECO RF is the smallest combi boiler I know of.

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.

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 having 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 does a combi boiler last?

Depending on how well it’s maintained and the quality of the boiler and installation, a modern combi boiler typically lasts around 10 to 15 years.

Fitting a central heating magnetic filter when the boiler is installed will help expand the lifespan of your boiler.

The number one cause of boiler breakdowns is sludge problems.

Having your combi boiler serviced regularly can play a big part in how long it lasts.

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.

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.

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.

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.

Combi boiler pipes explained

A (condensing) combi boiler has 7 pipes.

An older (non condensing) combi boiler should have 6 pipes.

Comb boiler pipes explained
Comb boiler pipes explained

  • Flow
  • Hot water outlet
  • Gas supply
  • Cold water inlet
  • Return
  • Blow off
  • Condensate pipe

(In order from left to right)

This is a standard layout for most combi boilers but some have the pipes in a different order, Vokera and Alpha for example.


The flow pipe is the pipe that feeds all the radiators.

This pipe is usually 22mm copper coming from the boiler for at least 1 meter. It is connecting one radiator valve on every radiator.


The return pipe is the pipe that comes back from all the radiators.

This pipe is usually 22mm copper pipe at the boiler. It is connecting the other valve on every radiator together.

Gas Supply

The gas pipe is supplying the gas from the gas meter.

It is usually 15mm or 22mm copper pipe.

Hot Water Outlet

The hot water pipe is supplying the hot water that is heated in the boilers plate heat exchanger and to the hot water taps and mixer showers.

It is usually in 15mm copper pipe at the boiler.

Cold Water Inlet

The cold water inlet pipe is supplying the boiler with water to heat for the hot water and water to fill the heating system.

This pipe is usually 15mm copper at the boiler.

Blow Off Pipe

The blow off pipe is the pipe that blows the water off outside when boiler pressure gets too high after the pressure relief valve opens.

This is usually 15mm copper at the boiler.

Condensate Pipe

The condensate pipe carries the condensate wastewater from condensing boilers to the drain.

It is usually 21.5mm at the boiler and must increase to at least 30mm diameter when ran outside to help prevent the condensate pipe from freezing.

It must be in plastic pipe as the acidic condensate water will eat through any metal pipes and fittings.


Whether you want to fit a combi boiler in a bedroom or more commonly in a kitchen, If your hot water demands aren’t too high, then a combi boiler will probably be the best solution.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

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