What’s Inside a Combi Boiler?

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Combi boilers are by far the most popular type of boiler in the UK. The rise of these efficient and affordable systems has been quite dramatic over the past few years. According to recently published statistics, combi boilers were present in just 1.7% of homes in 2003, but by 2019 they’d been installed in over half (54.8%) of all UK homes. 

Today, combi boilers are so prevalent that when we talk about boilers, you can safely assume that it’s a combi boiler we’re talking about. However, despite their popularity and the ever increasing numbers of them, combi boilers remain a misunderstood breed. 

Few of us know what’s inside our combi boiler, and only a handful of homeowners have a good idea of how their boiler works. So, why don’t we find out what the components in a combi boiler are and what they do? We caught up with a few of our skilled engineers to hear all about what goes on inside a combi boiler. Here’s what they told us. 

What’s in a combi boiler? 

A combi boiler is a unit which takes water straight from the mains, and heats it up in order to heat both your home and any hot water you might need. It does so using a gas burner, which lights whenever the tap is on, and a heat exchanger, which transfers energy to the water in order to raise its temperature. 

Combi boilers are popular because they’re relatively compact, and they can be installed easily within most UK homes. They’re also reliable, and can be used to heat water on demand whenever a household needs it. 

Homes with a combi boiler will often have hot water relatively quickly (usually instantly) as combi boilers have a reservoir of hot water to be used while more is being heated. Combi boilers also react quickly to the environment around them, using thermostats to switch the heating on and off as and when required. This allows the system to maintain a consistent temperature throughout a property, keeping residents comfortable throughout the year. 

Combi boiler components 

If you want to know how a combi boiler really works, and what makes it so efficient, you’ll need to investigate a little further. Let’s take a look at the key components of a combi boiler, and the vital parts that you need to know about. 

The Fan 

The boiler fan has a misleading name, because it isn’t a fan that’s there to keep the boiler components cool. Instead, this fan is tasked with removing toxic gases within the unit, making sure they never make their way into your home. 

Faulty boilers can produce incredibly dangerous gases, such as carbon monoxide. These gases are removed via the flue pipe. The fan is there to move them along the pipe and stop any build-up of harmful gas in your home.

The Air Pressure Switch

Safety is important in the design of any household appliance, but with gas appliances like boilers it really is paramount. One of the main safety features of the combi boiler is the air pressure switch. This switch is designed to stop the boiler igniting if it isn’t safe to do so. 

The air pressure switch checks whether or not the fan is working before the boiler starts. If the fan is working, then waste gases will be successfully ejected through the flue and the boiler will be safe to use. If it isn’t working, then waste gases might be building up within the boiler, meaning it’d be incredibly dangerous if the boiler did ignite. 

The Gas Valve 

Combi boilers are fitted with a gas valve, which helps to control the flow of gas to the pilot light and boiler, igniting the boiler whenever you want it to start. 

The gas valve will open when it receives a signal from the printed circuit board. The circuit board will also tell the valve when to close, using electrical signals. 

Gas valves are prone to a few issues. They can become stuck, and sometimes electrical faults will mean that they don’t work properly. If you have any issues with your gas valve, you’ll need a Gas-safe engineer to carry out any necessary repairs. 

Automatic Air Vents 

Combi boilers rarely experience problems, but when they do they often alert the homeowner with some strange noises. A problem with the air vent is one of the main causes of loud noises coming from the boiler unit. And it’s one that’s guaranteed to get your attention quite quickly. 

The automatic air vent is designed to remove any trapped air from the heating system. Trapped air is problematic for a number of reasons. It can cause the heat exchanger to boil or overheat, and it can also lead to pump cavitation. Neither of these things are good news where your heating is concerned. 

If you hear loud noises originating from your combi boiler, it’s likely that air has become trapped inside. This could mean a problem with your air vents. If left it’ll likely result in a complete breakdown or your heating system. Book in a professional to check the unit and carry out any repairs before the problem gets worse. 

Expansion Vessels 

The expansion vessel carries out an important role in the boiler. It ensures that the pressure within the boiler is consistent, eliminating the risks that come with any quick and dramatic changes of pressure inside the unit. 

Pressure changes are usually caused by either too much or too little air inside the unit. The expansion vessel is there to counteract these changes. 

The Circuit Board

Every combi boiler has what’s known as a printed circuit board, or PCB. This is essentially a computer system within the boiler. It manages the electrical components that control the boiler and heating system, facilitating all the connections the boiler needs to work. 

Circuit boards can sometimes lose power, display error codes or turn off completely, and when this happens the boiler itself will not turn on. So, if your boiler isn’t switching on and you’re unsure as to the cause, it’s possible that an electrical fault within the PCB is to blame. 

Combi boilers are brilliantly efficient, reliable and affordable. And they’re fully capable of meeting the heating and hot water needs of the vast majority of UK households. If you’re considering switching an oil boiler to a combi unit, or you’d like to learn more about the pros and cons of combi boilers, we’re here to help


Get in touch with our team to learn all about our combi boiler installation service. We also carry out all necessary maintenance tasks, and can help with repairs if you ever experience any issues. Give us a call to find out more.