Combi boiler problems, and how to fix them

combi boiler problems solved
Credit: Worcester Bosch

The combi boiler can still malfunction despite its reputation for reliability!

Here, we identify the most common combi boiler problems, and the solutions to these.

We have identified the following eight combi boiler problems as the most likely:

  1. Pressure loss in boiler 
  2. Pump speed too high
  3. Radiator leaks 
  4. Heating, but no hot water
  5. No pilot light on?
  6. Kettling (look here if your boiler is noisy)
  7. The thermostat isn’t working 
  8. The condensate pipe is blocked/frozen

If you realise you’re in need of a boiler replacement, fill in the form anywhere on this page with your information and one of our partner installers will get in touch with you ASAP to help you.

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1. Pressure lost in my combi boiler 

The most common pressure in a combi boiler is 1.2 bar. The pressure as a whole should be somewhere between 1.0 to 1.5 bar. 

The indicator bar should be somewhere in the green section.

If the bar is around 0.5 or below, this is usually a sign you have a water leak. If it goes below 0.4 the boiler might lock down and refuse to start again.

If the indicator bar is very high (as high as 2.75) the combi heating system is being overfilled

Solution for low pressure

Find the filling loop (usually underneath the boiler). Turn the small tap anti-clockwise. The boiler’s pressure should start to rise. Turn the tap back clockwise when the pressure reaches 1.5 bar. 

Solution for high pressure

Bleed the radiators until the pressure has dropped to normal. 

If you find yourself having to constantly repressurise the boiler, then contact the installer or bring in an engineer. 

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2. Pump Speed 

If your combi boiler pressure is still faulty, it can be a sign that the pump speed is too high. 

The pump speed, when too high, can create “cavitation”.

This is when the pump creates hydrogen by separating the water and air. 

You may find the pump isn’t working at all. This could be related to the fuse. 


Turn down the pump speed but whilst maintaining a sufficient speed around your heating system.

This should release any hydrogen and the system’s pressure will eventually drop. An engineer can replace a fuse with ease. 

3. My radiators are leaking 

Another common issue with the combi boiler is leaking radiators.

If the boiler isn’t leaking itself but the heating is ineffective, a radiator leak could be the issue. 

This can be down to corrosion, debris in the system or loose connections. 

Even the smallest leak can cause an entire system to lose pressure and is entirely common with boiler pressure loss.


Finding a leak isn’t easy, but look for water puddling under any radiator.

You can use your hands to feel along the radiator for a leak. Check your radiator valves.

Once you’ve discovered a leak, tighten up the connections as tight as possible. If this doesn’t work, you’ll have to call in someone to look at it.

4. You have heating, but no hot water

An all-too-common combi boiler problem.

A combi boiler contains the heating system’s primary exchanger. When a hot water source is on (say a tap) water is diverted to a secondary exchanger.

This is via a diverter valve. If this is faulty, it could explain why the heating is on but no hot water. 

Maybe your radiator’s aren’t warming up either, even when the thermostat says the heating is on.

This is likely again a problem with the diverter valve.

Other potential culprits are broken airlocks and damaged diaphragms. 


Firstly, check on your thermostat. It may be flashing which will mean a battery change is needed.

Try and replace them within one minute as otherwise, you will have to reset your heating times.

If the problem is a broken part, there is unfortunately not much you can do yourself.

A gas registered engineer can usually diagnose the problem very quickly. This is because they can replace the part for you. 

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5. The pilot light keeps going out

This is something you should try and attend to quickly.

This is because the pilot light is associated with the gas. There could be potentially a carbon buildup which has blocked the pilot light. 

Otherwise, the flame could be blown out by a draught. This would mean a problem with the flue, as the wind is entering it and blowing out the flame.

It could also be just be a limescale build-up or a faulty thermocouple.


Wait for the pilot to cool then use a needle to remove anything blocking the pilot opening.

This will help the pilot flame to heat the thermocouple if anything is blocking it.

We would recommend getting a professional to have a look if you are doubtful though. It is unlikely to be expensive and will give you a safe assessment. 

6. Kettling 

Another very common problem, which results in you hearing unhealthy noises a bit like a kettle.

This means there is probably a limescale build up in your boiler.

The heat exchanger is usually where the limescale and sludge build-up.

This will cause the water supply to bubble as it passes through the heat exchanger, which makes a sound like a kettle.

This can cause the water to be too hot. It could also possibly be trapped air making odd noises in your system.


If you think your heat exchanger is kettling, the best solution is a power flush.

These can be as cheap as £300, or more depending on how strong it needs to be. 

If the cause is trapped air in the system, you’ll have to try bleeding the radiators using a radiator key or flat screwdriver.

Otherwise, fill in your details below and we’ll contact a powerflush engineer for you!

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7. Faulty thermostat

In older combi boiler units this can be a common problem.

Possible signs that can point out a bad thermostat are:

  • Your boiler unit turning off 
  • The temperature misleading 
  • Water being far too hot 


Always check the thermostat hasn’t taken a knock, as you may just need to reset the correct time.

Try slightly tweaking the temperature if your water isn’t hot.

It may just need new batteries (usually standard AA).

But if it is malfunctioning and lost accuracy this can cause it to think water is cooler than it is. 

If such a fault continues you’ll need to get a new replacement.

This can cost anywhere from £20-300, depending on type and labour.

8. Condensate pipe may be causing a faulty boiler 

Most older combi units don’t even have a condensate pipe so this generally applies to newer models. 

The pipe is meant to remove wastewater.

But in winter with colder temperatures, it can become possible for freezes which create blockages.

You may not think this a serious combi boiler problem, but it could cause the boiler to shut down. 


Even if you are a DIY expert, attempting to fix the problem yourself could damage expensive piping.

So instead, locate the condensate and try using a kettle of hot water to thaw it out.

Next, remove the condensate pipe below the boiler and drain it. 

But if this does not work, find an expert who could heat up your pipes instead to melt any blockage. 

Get in touch

Did this article help identify an issue for you?

If your boiler is more than 10 years old, and you keep noticing problems like those in the list above, you may have to consider a combi replacement.

If you leave your details in one of the forms on this page, one of our trusted partner Gas Safe installers will get in touch with you shortly to assist you!

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Archie Everard

Archie is a content writer at Homesage, who has enjoyed freelance writing before and after his time at Oxford Brookes. Obviously he loves boilers most of all, but also enjoys reading, football and going to the pub!