• Blog
  • Combi boilers common faults  

  • Blog
  • Combi boilers common faults  

    Updated: April 16, 2024

    Combi boilers have become increasingly popular among UK homeowners in recent years, especially for those living in average-sized homes. While combi boilers offer convenience and space-saving benefits, they can encounter issues from time to time. In this blog, we’ll delve into the components of combi boilers and explore potential breakdowns. We’ll also address common problems associated with combi boilers. But first, let’s understand what a combi boiler is.

    What is a combi boiler? 

    The term “Combi” is short for combination boiler, a type of boiler that serves dual purposes by providing both hot water and heating from a single unit. This modern system offers several advantages over older immersion heater systems, which relied on water tanks positioned high up. With older systems, hot water wasn’t always readily available on-demand, requiring advance planning to heat water. Additionally, operating older systems incurred high costs and expenses. Furthermore, older systems typically occupy more space due to their larger size.

    How do Combi Boilers work? 

    In a combi boiler system, cold water is supplied directly from the main water source and heated by burning gas fuel. The heat generated in the combustion chamber is transferred to the water through a heat exchanger.

    When the thermostat detects a drop in temperature inside your home, it sends a signal to the combi boiler to ignite and start heating. Once the desired temperature is reached, the boiler modulates its heat output to maintain a consistent temperature, reducing energy waste while keeping your home warm.

    Combi boilers deliver hot water to any outlet you’re using, whether it’s a hot tap, shower, or bath. When the tap is turned off, the boiler switches back to providing central heating. It’s important to note that a combi boiler cannot supply hot water to multiple outlets simultaneously; it prioritizes either hot water or central heating depending on demand.

    Read more about:

    types of Gas Boilers 

    Best combi boilers & boiler brands

    Is a Combi Boiler the perfect choice for your home? 

    Combi boilers draw water directly from your home’s mains water supply, relying on the pressure from the mains to provide adequate water flow. If the mains pressure is insufficient, a combi boiler may not be suitable for your home.

    Combi boilers have two outputs: one for hot water and the other for home heating. Providing hot water for central heating requires less effort than delivering on-demand hot water. It’s essential to select a boiler that meets your hot water demands accordingly.

    Furthermore, if you reside in a large home with multiple bathrooms, a system boiler is more suitable for your needs.

    The Best Brands of System Boilers & Pricing

    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. 

    Read more:

    New Boiler Cost

    Top UK Combi Boilers

    Best Boiler Replacement Cost

    Combi boilers parts and breakdowns:

    1- Air Pressure Switches 

    The safety of a combi boiler is ensured by its air pressure switch. This critical component ensures that the fan effectively removes waste gases through the flue before the boiler ignites.

    A fan is employed to create the necessary draft to expel harmful gases from the boiler through the flue.

    Before initiating the boiler’s firing process, the air pressure switch verifies that the fan is functioning correctly. It only sends a signal to ignite the boiler when the fan is confirmed to be operational.

    There are two potential signals from the air pressure switch: one confirming the fan’s proper operation and signaling the boiler’s safety for ignition, and another indicating that the fan is malfunctioning, rendering the boiler unsafe for ignition.

    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 becomes trapped in the heat exchanger, it can impede the transfer of energy, limiting the system’s efficiency.

    Ensuring that water makes full contact with all surfaces within the hydraulic or chilled water system is crucial. Any air must be expelled from the system to optimize its performance.

    Air can become trapped in various components and locations within your heating system. Therefore, having a mechanism to effectively remove air from the system is vital. This is why an automatic air vent plays a significant role in maintaining the efficiency of your system.

    2- Auto Air-Vents (AAVs) 

    Loud noises emanating from your boiler may indicate trapped air within a sealed heating system. This can lead to uneven heating in your home and, in severe cases, result in a boiler breakdown. To prevent such issues, regular maintenance of your boiler’s air vent is highly recommended. This ensures uninterrupted heat supply when needed.

    Typically installed within cylinder cupboards, the air vent automatically releases trapped air from your heating system.

    Trapped air in your boiler can cause pump cavitation or overheating within the heat exchanger, potentially leading to performance issues or system damage.

    Why does a boiler Auto Air-Vent fail? 

    During operation at high pressure, excess pressure within your boiler system is directed to the auto-air vent. These vents are designed to alleviate pressure levels that exceed the boiler’s capacity. However, there are instances when these valves may become stuck. When this occurs, they may not only release trapped air but also water.

    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

    Inside a combi boiler, there is an expansion vessel whose purpose is to maintain consistent pressure and prevent abrupt fluctuations in your boiler’s pressure.

    Insufficient air in the expansion vessel can lead to excessive pressure, causing water to be expelled from the pressure relief valve. Conversely, an excess of air can lead to over-pressurization of the system, also causing water to be discharged from 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 primary function of a boiler fan is to expel toxic gases from your home.

    When fuel burns in the boiler’s combustion chamber, it releases harmful gases. The flue pipe is responsible for directing these toxic gases away from your home to ensure safety.

    While the flue pipe provides an escape route for these gases, the fan plays a crucial role in pushing them further away, preventing their accumulation or slow movement within the 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. 

    Read about:

    The right Combi Boilers size for you.

    Best Condensing Boilers

    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 will inspect your Printed Circuit Board (PCB) to identify any issues. If possible, they will repair it; otherwise, it may need to be replaced. The cost of replacing a PCB can be as high as £500. Therefore, it’s essential to consider whether it’s more cost-effective to replace the PCB or invest in a new boiler.

    Read more about: All You Need to know About Electric Boilers

    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, also referred to as central heating pumps, play a vital role in heating systems. They are responsible for circulating hot water throughout the system.

    These pumps ensure a continuous flow of heated water from the boiler to the radiators. The heated water travels through pipes to reach various components such as radiators, hot water cylinders, and towel rails. Subsequently, the water returns to the boiler to maintain a consistent flow of warmth throughout your home.

    Read about:

    Ideal Boilers in Cardiff

    Find the Best Boiler for your home 

    Combi Boiler Reviews

    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 plays a crucial role in determining the destination of hot water in your home. Over time, it may develop faults, leading to issues with hot water distribution to either radiators or taps.

    Functioning by opening or closing, the diverter valve directs hot water to either radiators or taps/showers as needed. It’s primarily utilized in systems where water is heated on demand, such as combi boilers. System boilers or conventional boilers, which store hot water in a cylinder, do not require this valve.

    When a tap is opened, the combi boiler heats water and the valve redirects the hot water to that specific tap. Similarly, when heating is activated, hot water is sent to the radiators. If both the shower and heating are in use simultaneously, the valve prioritizes 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 

    A heat exchanger facilitates the transfer of heat between two fluids or substances, typically gas and water, without allowing them to mix. When you activate the heating in your home with a combi boiler, water is sourced from the mains and circulated through your radiators.

    During operation, the combi gas boiler burns gas, generating heated gas that rises toward the heat exchanger. Simultaneously, cold water from the mains passes through the heat exchanger. As the water circulates, heat is transferred from the gas to the water, causing it to heat up for use in your tank or to warm your radiators.

    In summary, the process involves hot gas and cold water supplied from the mains. Through the heat exchanger, the result is cooler gas and hotter water, with the two fluids remaining separate throughout the exchange.

    Read about:

    New Boiler Buying Tips

    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 

    Read more about:

    Condensing VS Non-Condensing Boilers

    best boilers in uk

    Step by step guide to boiler installation


    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.