Combi boilers common faults  

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Combi boilers common faults 
27 Jul, 2023

Combi boilers are one of the most popular options for people to install in recent years in the UK. If you are living in an average sized home, then combi boilers are your best choice. However, they do tend to be troublesome on certain occasions. On this blog we will take a look at combi boilers parts and how they can break down. We will also try to cover combi boilers common breakdowns. But before doing that let’s take a look at what a combi boiler is. 

What is a combi boiler? 

The word Combi stands for combination boiler. This single unit both provides hot water and heating. That’s why they are better than older systems that consist of an immersion heater which uses water from a tank way up. With the older system, hot water wasn’t always on-demand. To have hot water, you need to turn on the water heater in advance. Not to mention the high cost and the amount of money of operating these older systems. Furthermore, an older system is much larger and needs a lot of space. 

How do Combi Boilers work? 

Cold water runs through the combi boilers directly from the main water source. In order to produce heat, the gas fuel is burned. The heat is then transformed from the combustion chamber to the water through a heat exchanger. 

Your thermostat would know when the temperatures drop in your house. Subsequently, it will send a signal to the combi boiler which would fire up and heat your home. 

The minute the required heat is reached, your combi boiler would decrease the produced heat and keep the status quo. As a result, it will reduce your energy waste as well as keep your house warm. 

A Combi boiler directs hot water to any outlet you are using such as a hot tap, a shower or a bath. When you turn off the tap, the central heating duties will be back in action. Keep in mind that your Combi boiler directs hot water to the water outlets or the central heating, but not both at the same time. 

Is a Combi Boiler the perfect choice for your home? 

Combi Boilers take water directly from the mains water supply in your home. To provide an appropriate water flow, they rely on the pressure coming from the mains. So if the pressure from the mains is not strong enough, a combi boiler is not, therefore, suitable for your home. 

A combi Boiler comes with two outputs; one for how water and the other is for home heating. Hot water for central heating takes less effort than on-demand hot water. That’s why it’s crucial to choose a boiler that corresponds with your hot water demands. Another key point is that if you live in a large home with many bathrooms, then a system boiler is what you need. 

Does a combi boiler use a lot of electricity? 

Although electric combi boilers are more expensive to run than gas boilers, they’re much more efficient. While most gas boilers have an efficiency rating of around 93%, electric combi boilers typically have a 99%–100% efficiency rating. 

Combi boilers parts and breakdowns:

1- Air Pressure Switches 

What makes a combi boiler work safely is the boiler’s air pressure switch. This switch ensures that the fan is able to eject the waste gases through the flue before the process of firing up the boiler.  

Harmful gases are released from the boiler via the flue by using a fan that forms a draft for that specific purpose. 

The air pressure switch wouldn’t send a signal to fire up the boiler before recognising that the fan is working as it should. 

There are two possible signals. A signal to confirm that the fan is working and that the boiler is safe to ignite. The other one is sent when the fan is not working. This will mean that the boiler is not safe to ignite. 

Why does a boiler air pressure switch fail? 

It’s hard to exactly pinpoint why a boiler air pressure switch would fail. There are many reasons. These might include: 

  • A faulty boiler fan 
  • Blocked flue is blocked 
  • A broken PCB 

How do I know if I have a faulty air pressure switch? 

There are some signs to indicate a faulty air pressure switch: 

  • Your boiler will keep turning on and off at random times 
  • The fan is working but your boiler is not 
  • Your boiler is displaying fault codes which are related to faulty air pressure switch 

Is trapped air a problem? 

When air is trapped up in the heat exchanger, it has the ability to restrict the flow of the energy transfer.  

It is important that the water is touching all the surfaces in the hydraulic or chilled water system. The air must be out of the system.  

Air can get trapped in different parts and places in your heating system. It is important to have a device that can get the air out of your heating system. That is why an auto air vent is important to your system. 

2- Auto Air-Vents (AAVs) 

Loud noises coming from your boiler can be the result of air being trapped in a sealed heating system. This can make your boiler only heats some parts of your home and, in some severe cases, it can result in boiler breakdown. To avoid this problem, we recommend regular maintenance of your boiler’s air vent. This can ensure that your boiler wouldn’t stop giving heat when it is in demand.  

It is installed in the heating system, mostly within cylinder cupboards. It will release the trapped air from your heating system automatically. 

Trapped air in your boiler can cause pump cavitation or boiling and overheat within the heat exchanger. 

Why does a boiler Auto Air-Vent fail? 

The pressure would move to the auto-air vent when your boiler is working at high pressure. Auto-air vents are there to help release pressure when it is too high for your boiler to deal with. In some cases, these valves can get stuck. When that happens they’ll not only get the air out but water, too.  

How do I know if I have a faulty auto-air vent? 

  • Water leaking from your boiler 
  • A fault code is being displayed by your boiler .

3- Boiler Expansion Vessel

A combi boiler has an expansion vessel inside. The aim of this vessel is to keep the pressure consistent and prevent sudden changes to your boiler’s pressure. 

If there is too little air in your expansion vessel, the pressure may become too high. This can result in the water coming out of your pressure relief valve. Too much air may cause your system to be over-pressurised. This can also result in water coming out of the pressure relief valve. 

How do I know if I have a faulty boiler Expansion Vessel? 

  • Poor flow rate from your taps 
  • PRV is dripping 
  • A low-pressure fault code will be displayed  
  • Your boiler will not be able to meet your hot water demand 

4- Boiler Fan 

The aim of a boiler fan is to drive toxic gases out of your home.  

The burning fuel in your boiler’s combustion chamber releases harmful gases. The flue pipe is there to take these toxic gases far away from your home and keep it safe. 

The flue pipe can be an escape route for these harmful gases. However, the fan helps push these gases further and prevent them from accumulating or moving slowly in your flue pipe. 

What can cause a boiler fan to fail? 

  • The connections and wiring might come loose due to the vibration of the fan. As a result, the signals going to the boiler’s PCB could be blocked 
  • Fan losing speed over time. A heating engineer could easily fix this problem 
  • Signal issues can also be caused by damaged connection or wiring. 
  • A complete failure of the fan due to wear and tear. The fan should be replaced 

How do I know if I have a faulty boiler fan? 

  • Boiler is making noises 
  • Boiler is not turning on 
  • A boiler displaying a faulty code 

5- Boiler Gas Valve 

A boiler gas valve is responsible for the gas flow to the pilot light and burner as well. An electrical signal from the printed circuit board (PCB) controls this valve. 

What problems are common with Boiler Gas Valve? 

  • The gas valve can fail to get the signal to work due to connection or wiring problems coming from the PCB to the gas valve.  
  • Your boiler gas valve can get stuck  
  • Faulty boiler gas valve 

You can tell that your have a a faulty Gas Valve if your boiler is displaying a fault code or if your boiler is not turning on.  

6- Boiler Printed Circuit Board (PCB) 

A Printed Circuit Board (PCB) is an essential  part of your boiler.  

To put it simply, the PCB is the backbone of your boiler. It connects the electrical components (pump, timer, thermostat etc) together. 

If your PCB isn’t working as it should, it can lose power, lockout and display and error code or completely shut down.  

PCB problems 

A gas engineer would need to check your PCB to see if there is anything wrong with it. It will either be fixed if possible or replaced. Replacing your PCB might cost you as much as ÂŁ500. So you should consider if it’s worth replacing or is it better to invest in a new boiler. 

How do I know if I have a faulty PCB? 

  • Your boiler is leaking 
  • Boiler has no power 
  • Burning smell coming from your boiler  
  • Intermittent display panel operation  

7- Circulating Pumps 

Circulation pumps are also known as the central heating pumps. Circulation pumps push hot water around a heating system. They supply a constant flow of heated water from your boiler to the radiators. 

The heated water moves from your boiler and through the pipes that lead to your radiators, hot water cylinder and towel rails. After that, the heated water will flow back to your boiler in order to keep a constant flow of heated water running around your home. 

What are the signs of a faulty Circulating Pump? 

  • The pipe is leaking 
  • The water flow pipe is not hot despite the fact that the pump is running 
  • The pump casing is too hot 
  • You have no hot water 
  • You radiators aren’t heating  
  • Strange noises are coming out of your pipe 

8- Diverter Valve 

A boiler diverter valve decides where hot water is sent throughout your home. A diverter valve, over time, can become faulty and as a result cause issues with sending hot water to your radiators or taps. A diverter valve works by opening or closing itself in order to direct hot water either to radiators or taps and showers.  

Diverter valves are only used with systems where water is heated on demand such as a combi boiler. System boilers or conventional boilers store hot water in a storage cylinder so this valve is not needed. 

When a tap is turned on, your combi boiler will heat water and the valve will direct the hot water to that specific tap. The same process takes place when heating is turned on; hot water will be sent to your radiators. If both the shower and heating are turned on at the same time, the valve will give priority to the shower.  

What are the signs of a faulty Diverter Valve? 

  • Only lukewarm water is coming out of your taps 
  • You have hot showers but your radiators are cold 
  • You have hot water only when your central heating is turned on.

9- Heat Exchanger 

To begin with, a heat exchanger enables heat to be exchanged between two fluids or substances, mostly gas or water, without letting the substances mix together. When you turn on the heating in your home using a combi boiler, water will be supplied from the mains and circulated around your radiators.  

When combi gas boilers burn gas, the heated gas rises towards the heat exchanger. Cold water will pass through the heat exchanger. As the water circulates, the heat is transferred from the gas to the water, which then heats up to heat water for your tank.  

or warm your radiators. 

In short, the process starts with hot gas and cold water supplied from the mains. With the help of the heat exchanger, you will have cooler gas and hotter water, without the two separate fluids having to meet at all. 

What are the causes of heat exchanger failure? 

  • Sludge in your central heating 
  • A build-up of limescale  

What are the signs of a faulty Heat Exchanger? 

  • Your boiler is leaking 
  • Whistling and kettling noises coming out of your boiler 

Any more questions?

If you have any further questions about Combi boilers, other Boilers, or anything else, please see our FAQ Page because you might find the answers there or on our News Blog.

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What is the average lifespan of a combi boiler? 

Modern combi boilers should last up to 15 years with their high efficiency and fewer pressure issues when compared to a conventional boiler. Without a doubt, this all depends on the quality of your chosen model and how well you look after your heating system. 

Does a combi boiler need to be serviced every year? 

Whether you have an oil boiler, gas boiler, or combi boiler, boilers need to be serviced annually by a qualified heating engineer.

What is included in a combi boiler service?

Pipework and seals will be examined for leaks. What is more, the heating engineer will check the pressure and inspect the boiler flue to ensure safety. Other things to be inspected include the burner and the heat exchanger as they might need cleaning.