Combi Boiler Parts – What’s Inside Your Combi? Re May 13, 2022

Combi Boiler Parts – What’s Inside Your Combi?

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What combi boiler parts are inside my combi boiler?

When we talk about buying a new boiler for heating and hot water, we’re invariably talking about a “combi”, or combination boiler. The type is so ubiquitous in the UK that it’s often taken for granted that a home would have a gas-fired combi sitting in the kitchen or in an airing cupboard. But although the term is widely known, there’s plenty going on under the hood that you might not be aware of in terms of operation, cost, installation and repairs. So let’s take a look at all things combi…

What’s a combi boiler?

Combi boilers are the most common type throughout the United Kingdom, providing a combination (hence ‘combi’) of both hot water and central heating from within one singular unit. Their popularity lies in their comparatively compact size, high efficiency and overall affordability when compared with other boiler types and models.

how does a combi boiler work?

We’ll not go too in-depth here as to the workings and machinations of your average combi boiler but rather give a quick, simple overview. 

To put it very simply, with a combi boiler, water is supplied directly from the mains, a gas burner is ignited when the tap is turned on, the heat exchanger transfers the energy to the water which heats it up, ready for you to use. 

Combi boilers are the best boilers for smaller homes with limited space and for those who need hot water and central heating on demand. Modern boilers are designed with a built-in pre-heat system, which helps to deliver hot water even more quickly. This is a small reservoir of water that is kept at a certain temperature, meaning you will have instant hot water as soon as you turn the tap on. 

If the temperature falls below a set point, a combi boiler’s external room thermostat will send a signal to the boiler to fire up so that heat can be pumped into the central heating system. Once the optimum temperature, that you have set, is reached again, the boiler will then turn itself off. This helps to maintain the correct temperature and also helps you save energy.

Combi boiler parts

Before getting into the nitty-gritty, it’s important to understand that it’s not safe (or legal) to open up a boiler and carry out work on the part within the combustion chamber without a valid Gas Safe accreditation, so don’t try to DIY a boiler repair!

Air Pressure Switches

The boiler’s air pressure switch is what makes the boiler work safely.

It ensures the fan ejects the waste gases through the flue before letting the boiler fire up, and this is how it works;

  • The fan forms a draft which allows harmful gases to be released from the boiler via the flue.
  • The switch needs to recognise that the fan is running before sending a signal to the circuit boiler to allow your boiler to fire up. These signals could be one of two;
    • Yes – the fan is working – which is translated in the circuit board as “the boiler is safe to ignite
    • No – the fan is not working – which is translated in the circuit board as “the boiler is not safe to ignite

Why does a boiler air pressure switch fail?

The boiler air pressure switch is part of integrated components and processes. That’s why it’s slightly complicated to diagnose why it’d stopped working. There could be many reasons, such as;

  • Blocked flue is blocked
  • Faulty boiler fan
  • Broken PCB

Signs you have a faulty air pressure switch

  • Boiler keeps turning on and off at random
  • Boiler won’t work (although the fan is working)
  • Fault codes related to faulty air pressure

Costs of replacing a boiler’s air pressure switch

It’s a bit tricky to set a price for replacing a boiler’s air pressure switch, but here’s a rough estimate of what to expect;


The costs will vary depending on boiler type and if the heating engineer needs to replace the air pressure switch or the ​​complete unit with hoses.

Your location will also affect the cost of installation.

Auto Air-Vents (AAVs)

When air gets trapped in a sealed heating system, it can cause the boiler to make loud noises, heat some parts of your home but not others, and in some cases, completely break down.

Maintaining your boiler’s air vents can ensure that your boiler doesn’t stop working when you need it most. 

The Automatic Air Vent is installed in the heating system, typically within cylinder cupboards, to release trapped air from the heating system automatically.

If air is trapped in the boiler, it may cause pump cavitation or boiling and overheat within the heat exchanger.

But, why is air a problem in a heating system?

Air is known as an insulator, so if we get air trapped up in the boiler’s heat exchanger, it will restrict the flow of the energy transfer.

We want to have water touching all the surfaces in the hydraulic or chilled water system, so the key is to get the air out of the system. 

There are different places where the air gets trapped in the system, and that’s why the boiler needs a device to get the air out of the system; hence ‘AAV’ is a vital part of a boiler.

Why does a boiler Auto Air-Vent fail?

When your boiler is working at a pressure that’s too high, the pressure moves to the auto-air vent.

Auto Air-Vent (AAVs) release pressure when it gets too high for the boiler to handle. 

One of the most common problems is these valves can get stuck, they’ll not just get the air out, but water too.

Signs you have a faulty Auto Air-Vent (AAV)

  • Boiler displaying a fault code ‘boiler pressure too high’
  • Water leaking from the boiler

Costs of replacing a boiler’s Auto Air-Vent (AAV)

The prices will vary depending on the make of the boiler and location, but here’s a rough guide;


Boiler Expansion Vessels

Your combi boiler will have an expansion vessel inside. This part is responsible for keeping the pressure consistent and preventing sudden changes to your boiler’s pressure.

Too little air in your expansion vessel, and the pressure may become too high, resulting in the water coming out of your pressure relief valve. 

Too much air could result in an over-pressurised system which could also lead to water coming out from the pressure relief valve.

Signs you have a faulty boiler Expansion Vessel

  • Your PRV is dripping
  • You have a poor flow rate from your taps
  • A combi boiler won’t meet your hot-water demands
  • Your boiler displaying a low-pressure fault code

Costs of replacing a boiler Expansion Vessel


Boiler Fan

Unlike your laptop’s fan, your boiler ban isn’t there to cool the parts off. 

Instead, the boiler fan is designed to drive toxic gases out of your home. 

Burning fuel in the boiler’s combustion chamber releases harmful gases, and that’s why you need a flue pipe to take these toxic gases away from home.

Although the flue (the white pipe that’s fitted to your boiler) can offer an escape route for the gases, it can’t just move the toxic gases out without the help of the boiler fan. The fan creates a small draught that propels flue gases up and out of the flue.

Why does a boiler fan fail?

  • The fan could lose speed over time – This is an easy problem to fix, by a heating engineer of course. The heating engineer can change the speed of the fan and then restart the boiler. 
  • The fan causes vibration which (over time) could cause the connections and wiring to come loose – this could block the signals from getting through to the boiler’s PCB.
  • Damaged wiring/connection – this also can cause signal issues.
  • The fan has completely failed (wear and tear) – so you’ll need a new one. 

Signs you have a faulty boiler fan

  • Noises coming from the boiler
  • Boiler displaying a fault code
  • Boiler not turning on

Costs of replacing a boiler fan

As you can imagine, no two boilers are the same, and not two fans are the same! Some boiler fans could start at £100, and some brands charge around £200 for the fan.


We’d recommend replacing your boiler fan instead of repairing it, as it will cost the same.

Boiler Gas Valve

A boiler gas valve is in charge of the gas flow to the pilot light and burner.

It’s controlled by an electrical signal from the printed circuit board (PCB).

Problems with your Boiler Gas Valve?

The problems we’ve seen with the Gas Valve are;

  • Stuck boiler gas valve
  • Wiring or connection problems coming from the PCB to the Gas Valve, therefore the gas valve fails to get the signal to work.
  • Faulty boiler gas valve

Signs you have a faulty Gas Valve

  • Boiler displaying a fault code
  • Boiler not turning on

Costs of replacing a Gas Valve


Boiler Printed Circuit Board (PCB)

A Printed Circuit Board (PCB) is a vital part of your boiler. 

Think of it as the motherboard of a computer. In simple terms, the Printed Circuit Board (PCB) is the backbone that ties the boiler’s electrical components (such as the pump, thermostat, timer etc.) together in one spot and lets them talk to each other.

If your boiler has a PCB error, it could lose power, completely switch off, or lockout and display error codes relevant to a PCB fault which we will cover later in this blog.

PCB failure is a common issue for boilers not switching on.

Having PCB problems?

Unluckily, PCB failure is not a thing your can fix by yourself. 

It is a complicated issue that requires the knowledge and expertise of a Gas Safe engineer.

A Gas Safe engineer will examine your PCB by running an electrical test. If the engineer finds fault with the PCB, they may be able to fix it, or they may need to replace it. Depending on the supplier and quality of the PCB, this could cost anywhere up to £500, plus installation costs.

At this point, you should weigh your option, do you want to spend this huge chunk of money, or would you rather invest in a new boiler with a better efficiency rate, which can save you money on your heating bills, and a longer warranty.

Signs you have a faulty PCB

  • No power to boiler
  • Intermittent Display Panel Operation
  • Burning smell coming out of the boiler
  • Leaking boiler

Signs you have a faulty PCB


Circulating Pumps

Circulation pumps (also known as the central heating pumps) push hot water around a heating system. They supply a constant flow of heated water from your boiler to the radiators, and here’s how;

  • The heated water moves from your boiler through the pipes that feed your radiators, towel rails, and hot water cylinder. 
  • The water then flows back to your boiler to keep a constant flow of heated water around your home.

Signs you have a faulty Circulating Pump

  • The water flow pipe isn’t hot despite the pump running
  • The pump is leaking
  • The pump is making strange noises
  • The pump casing feels too hot
  • Some/all of your radiators aren’t heating up
  • You have no hot water

Costs of replacing a Circulating Pump


Diverter Valve

A boiler diverter valve is an essential part of a combi boiler heating system, deciding where hot water is sent throughout the home. 

Over time a diverter valve can become faulty and consequently cause issues with sending hot water to your taps or radiators.

A diverter valve is a mechanism in a combi boiler that opens or closes to direct hot water either to radiators or taps and showers. 

Diverter valves are not found in system or conventional boilers because the valve is only needed in boilers where water is heated on demand, whereas system and conventional boilers store hot water in a storage cylinder.

If a specific hot water tap is turned on, the combi boiler will heat water and the valve will direct the hot water to the tap rather than to the home’s radiators or other taps. If the heating is turned on, the combi boiler will do the same for the radiators. If the heating is on and someone turns on the hot tap in the shower, the diverter valve will prioritise sending hot water to the shower instead of the radiators until the shower is turned off.

Signs you have a faulty Diverter Valve

  • Hot showers but cold radiators
  • You can only get lukewarm water from your taps
  • Your hot water only turns on when central heating is on

Costs of replacing a Diverter Valve



Heat Exchanger

A heat exchanger allows heat to be exchanged between two fluids or substances, usually water or gas, without letting the substances mix together.

When the heating is turned on in your home, with a combi boiler, water is supplied from the mains and circulated around the radiators.

Combi gas boilers burn gas, the heated gas rises towards the heat exchanger that cold water passes through. As the water circulates, the heat is transferred from the gas to the water, which then heats up to effectively warm your radiators or heat water for your tank. 

In simple terms, you start with hot gas and cold water supplied from the mains. Thanks to the heat exchanger, you end up with a cooler gas and hotter water, without the two separate fluids having to meet.

Why causes Heat Exchanger to fail?

  • Limescale build-up
  • Central heating sludge

Signs you have a faulty Heat Exchanger

The cost of replacing a Heat Exchanger


How much does a combi boiler cost to buy (and install)?

The cheapest combi boilers will set you back under £500 (without installation), however, their power output will be significantly lower than more expensive models, meaning they’re less likely to be suitable for larger homes.

As a rough guide, these are the kind of prices you can expect to pay for different boiler installation types from WarmZilla, inclusive of VAT:

  • Combi to combi boiler swap – from £1,529
  • System/Standard boiler to combi conversion – from £2,249
  • Back boiler to combi conversion – from £2,499
  • System to system – from £1,995 
  • Standard to standard – from £1,649 

A new boiler from WarmZilla contains everything you need to get your boiler up and running. Here’s a complete list of what’s included with a WarmZilla boiler installation – based on a Worcester Bosch 30i boiler.

  • Worcester Bosch 30i Boiler with a 10-year guarantee – £1200
  • Gas Safe Installation – £450 (minimum)
  • System Cleanse – £30
  • System Filter – £120
  • Wireless controls – £80
  • Flue – £80
  • Plume Kit – £100 (if required)
  • Moving Boiler – £150 – £500 (if required)
  • Gas pipe upgrade – £300 – £500 (if required/included in your fixed price quote)

So when you look at the price of a boiler and see the headline price of £1229.99 at Screwfix on a Worcester Bosch 30i boiler, you now know the reasons why WarmZilla charges £1949 for the same boiler.

Signs you need a new boiler

Your boiler could last between 10-15 years, or even longer if it is well maintained.

If your boiler stops working entirely and can’t be fixed, you’ll have to replace it immediately, but bear in mind that many other less obvious signs let you know it’s time to say goodbye! 

Here are the top 6 warning signs;

  • An unexplained rise in energy bills – naturally, boilers lose efficiency over time, which means your energy bills will go up regardless, but if your energy bills are creeping up out of control, this might be a sign that your boiler needs replacing.
  • Your boiler makes odd noises such as banging, whistling, popping or hissing sounds.
  • Your boiler is leaking – while leaking doesn’t mean you have to replace your boiler, it is a sign that there are issues in one of the internal components. We recommend calling a heating engineer to check it out before it becomes more serious.
  • Frequent breakdowns – this happens a lot towards the end of the boiler’s lifespan and it may be time to start shopping around for a new boiler.
  • The boiler doesn’t heat your home as it used to and doesn’t provide hot water either.
  • Finding replacement parts for your boiler is a nightmare.

Before you commit to the big purchase, ask a heating engineer to check that there isn’t a build-up in your heating system preventing your boiler from working as it should.

Should I repair or replace my boiler?

If your boiler keeps breaking down, it may be cheaper, in the long run, to cut your losses and get a new one, just come to us at WarmZilla and we’ll sort you out!

When shopping around for a new boiler, we’d advise using the WarmZilla boiler comparison tool. Just select up to 3 boilers you want to compare and you’ll be able to compare key stats side by side to see which boiler is best for your home.

Not prepared to waste money on fixing your knackered boiler?

Then it’s time for a new boiler! When you buy a new or replacement boiler with WarmZilla, your boiler package will also include;

  • Your chosen boiler, along with the necessary flue kit needed for your central heating system
  • Installation by a qualified Gas Safe Engineer 
  • Removal of your old boiler and parts
  • A FREE system cleanse of your central heating system
  • Chemical Inhibitor for added protection for your new boiler
  • 10-years guarantee, which will be registered by WarmZilla after installation
  • A FREE magnetic system filter for ongoing protection
  • The Neomitis RF Digital Wireless Room Thermostat

*Average boiler prices were taken from an online heating supplier on 16/5/22. Prices are for the charging unit alone and may be subject to change.

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