In this blog post, we’ll discuss the top 10 common problems with combi boilers and what you can do about them.

1.      Low Water Pressure

One of the most common problems with combi boilers is low water pressure. This can be caused by several factors such as leaks in the system, airlocks or faulty pressure relief valves. If your boiler has low water pressure, it may not function properly or at all.


To fix the problem of low water pressure in boilers, check for any visible leaks in the system, bleed radiators to release trapped air, or call a professional boiler repair service. Additionally, ensure that the expansion vessel is properly inflated and that all components such as radiators and valves are tightly secured. It’s also important to regularly check the pressure gauge on your boiler and adjust it accordingly if necessary. Finally, make sure you keep an eye out for any signs of corrosion or wear and tear on components as these can weaken seals and cause water leaks.

2.      No Hot Water

Another common issue with combi boilers is when there is no hot water coming out of the taps. This could be caused by a variety of reasons, such as a broken diaphragm or valve failure. A malfunctioning diaphragm can prevent the boiler from receiving the signal to begin heating the incoming cold water. Valve failure can also prevent the flow of hot water, either due to a worn-out valve or a loose connection between the boiler and the taps.


If this happens, try resetting your boiler or checking if there are any error codes displayed on it. Then you can check the diaphragm and valve for any visible signs of damage. If these components are functioning correctly, then it may be necessary to inspect the connections between the boiler and taps, as well as check for blockages or airlocks in the system. Fix any broken valves or loose connections, it will usually require a qualified engineer to repair it correctly.

3.      Leaking Boiler

A leaking boiler is another problem that needs immediate attention as it could lead to further damage to your home and even cause injury if left unchecked. The cause of the leak could be due to corrosion in older models or faulty components in newer ones.


If you notice any signs of leakage such as puddles around the base of your boiler or damp patches on walls or ceilings near it, turn off your gas supply immediately and call a professional plumber.

In most cases, the issue can be resolved by replacing faulty components or adding new seals, however, if the corrosion has caused extensive damage, it may be necessary to replace the entire unit. Whichever option you opt for, make sure to use a qualified technician to complete the job and avoid any further damage.

4.      Strange Noises Coming from Your Boiler

If you hear strange noises coming from your combi boiler like whistling sounds or banging noises, it could indicate various problems such as kittling (when limescale accumulates inside), trapped air in the system or pump failure among others.


Start by checking the water pressure of your boiler. If it is too low, you should adjust it according to the manufacturer’s instructions. You can also check for any trapped air in your system or look out for any build-up of limescale inside the heat exchanger.

If the problem persists, it could be an indication of a more serious issue and you should call a qualified engineer to investigate and repair any parts that may need replacing. An annual service is also recommended to ensure your boiler remains in top working condition throughout the year.

5.      Frozen Condensate Pipe

During winter months when temperatures drop below freezing point, condensate pipes leading outside from your combi-boiler can freeze up causing blockages in the system which can lead to shutdowns or malfunctions altogether.


To fix this problem pour warm (not boiling) water over the pipe until the ice melts away. Installing a frost protection device to the condensate pipe is recommended in order to prevent it from freezing in the future. These devices will warm up the pipes, keeping them at an appropriate temperature so that they do not freeze. Additionally, insulating the pipes can also help manage heat loss in colder environments and reduce any chances of a frozen condensate pipe.

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6.      Thermostat Issues

If your thermostat isn’t working properly then it might not be communicating effectively with your boiler which means that heating won’t come on when it should do so causing cold spots throughout rooms. This could indicate a faulty thermostat or something more serious such as an issue with your boiler. Common issues with thermostats include:

-Thermostat not turning on/off correctly

-Inaccurate temperature readings

-Thermostat not responding to changes in temperature settings

-Faulty wiring or connection issues

-Incorrectly installed thermostats

-Battery failure or low batteries


If you think your thermostat not working correctly, it’s important to identify the cause of the problem and take action to rectify this as soon as possible. Always check the batteries first and if these are fine then inspect any wiring or connections for signs of damage. If the issue is more serious then call a professional to come out and diagnose and repair the problem. If all else fails, replace your thermostat as this is usually the cheapest and quickest solution. A professional can also advise you on which type of thermostat will be best suited to your needs and home.

7.      Pilot Light Goes Out

Combi boilers use pilot lights as ignition sources that keep burners running smoothly but sometimes, they go out unexpectedly leaving homeowners without heat & hot water. Causes include blocked flues, faulty thermocouples, and wind drafts affecting flames.


Regularly cleaning the flues and thermocouples of a combi boiler is essential to ensure optimal operation and prevent pilot light failure. This can be done either by the homeowner or by a professional heating engineer. Inspecting the flues for any blockages, such as built-up soot, and replacing any faulty thermocouples will help to reduce the risk of pilot light failure.

8.      Faulty Diverter Valve

Diverter valves direct hot water either towards radiators for heating purposes OR towards taps/showers for domestic use. When they break down homeowners find themselves unable to get hot water unless they switch the heating on – this wastes energy & money! Additionally, homes with diverter valves that have malfunctioned may be at risk of accidents due to the sudden release of hot water.

Signs that a diverter valve may be faulty include:

– Unusual noises coming from the valves when they are switched on/off, or when hot water is running through them.

– Unexplained changes in temperature when running hot water through taps & showers.


Homeowners should prioritize replacing faulty diverter valves as soon as they become aware of them. This is because the failure of these valves means that homeowners are unable to use hot water, and instead are forced to switch on the heating, wasting energy and money.

9.      Ignition Failure

Ignition failures occur when sparks aren’t generated during the startup process thereby preventing burners from lighting up. Causes include faulty spark generators, and loose wiring connections between components – have electrical systems checked by qualified electricians before attempting DIY repairs!


Call in a professional to troubleshoot and fix the problem.

10. Overheating

Overheating occurs when boilers produce too much heat which leads to safety mechanisms shutting them down automatically. Causes include blocked vents/flues, and malfunctioning thermostats/valves amongst other things.


Call in a professional to troubleshoot and fix the problem. If needed, replace any malfunctioning parts. Ensure regular maintenance checks are carried out by professionals who know what they’re doing!

Get Help from a Gas Safe Engineer

In conclusion, owning a combi-boiler comes with its own set of challenges but regular maintenance checks can prevent many issues from occurring altogether while prompt action taken once faults arise ensures minimal disruption & expenses incurred over time. Always take advantage of North Glasgow Boiler Repair’s services whenever necessary!

Our Gas Safe Engineers are qualified and certified to carry out all maintenance & repair services for combi boilers in Merchant City, Glasgow– get in touch today for a free, no-obligation quote!