Combi Boiler Parts – What’s Inside Your Combi?

What combi boiler parts are inside my combi boiler?

When we talk about buying a new boiler for heating and hot water, we invariably talk 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, plenty is 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. They provide a combination (hence ‘combi’) of hot water and central heating from within one unit.

Their popularity lies in their comparatively compact size, high efficiency, and overall affordability 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.

Simply put, with a combi boiler, water is supplied directly from the mains. When the tap is turned on, a gas burner ignites, and the heat exchanger transfers the energy to the water, heating it up and making it ready for 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 small water reservoir 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 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 – 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 the 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;

The 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 the boiler type and if the heating engineer needs to replace the air pressure switch or the entire 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 it continues working when you need it most.

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

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

Why is air a problem in a heating system?

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

Air air gets trapped in the system in different places, and that’s why the boiler needs a device to remove it; hence, ‘AAV’ is a vital part of a boiler.

Why does a boiler Auto Air-Vent fail?

When your boiler works 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 that these valves can get stuck. If they do, they’ll let air out and water in.

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.

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

Too much air could result in an overpressurised system, which could also cause water to escape 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 fan 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, so you need a flue pipe to remove these toxic gases from your home.

Although the flue (the white pipe 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, of course, by a heating engineer. The heating engineer can change the fan’s speed and then restart the boiler.

The fan causes vibration, which (over time) could cause the connections and wiring to come loose, blocking 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 displays 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 controls 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 allows them to 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 you 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 replace it. Depending on the supplier and quality of the PCB, this could cost up to £500, plus installation costs.

At this point, you should weigh your options. 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 the 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 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 essential to a combi boiler heating system. It decides where hot water is sent throughout the home.

Over time, a diverter valve can become faulty, consequently causing 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 to radiators, taps, or 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, a combi boiler supplies water from the mains and circulates it around the radiators.

Combi gas boilers burn gas, and 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

Boiler making noises – Whistling and kettling noises

Your boiler is leaking

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 that of 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 10-15 years or even longer if well maintained.

If your boiler stops working entirely and can’t be fixed, you’ll have to replace it immediately, but many other less obvious signs will 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 properly.

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!

We’d advise using the WarmZilla boiler comparison tool when shopping around for a new boiler. Just select up to 3 boilers 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-year guarantee, which will be registered by WarmZilla after installation

A FREE magnetic system filter for ongoing protection

The Neomitis RF Digital Wireless Room Thermostat