Category - Boilers
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Will Scholfield, Engineer

12 Apr : Updated 16 Apr ● 10 min read

Common combi boiler problems and how to fix them

Combi boilers, sometimes known as combination boilers, are among the most popular home heating systems in the UK. They’re energy-efficient, affordable, and compact, making them ideal for large numbers of households nationwide.  

Combi boilers offer a wealth of advantages, but like any appliance, they are susceptible to the odd mishap.

If your home's boiler or central heating system isn’t working as it should, it can feel like the end of the world, but don’t worry—help is at hand.

In this guide, we’ll help you troubleshoot some of the most common combi boiler problems so that you or your engineer can get things back up and running as soon as possible.

The most common combi boiler problems

Below are some of the combi boiler problems our engineers often encounter. 

In most cases you will need to contact a Gas Safe registered engineer who’ll be able to come and investigate.

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No hot water

Finding yourself without hot water is one of the most common (but most irritating) combi boiler issues people encounter. This can present itself in a few different ways, which we'll go through below.

1. There is no hot water, but the central heating is working

Sometimes, your home's central heating will work as usual, but hot water won’t come from your taps. 

In this scenario, The two things listed below, amongst many others, could be causing this problem. 

  • Faulty diverter valve: In a combi boiler, the diverter valve directs hot water to the taps or the central heating system. If the valve isn’t working as it should, it may not switch to the hot water mode when required.
  • Scale/magnetite buildup: Over time, the accumulation of scale or magnetite in a combi boiler’s plate heat exchanger could restrict or even block the flow of water through it, which would cause the boiler not to produce hot water and potentially overheat.

For any of the potential causes listed above, you must contact a Gas Safe registered engineer to come and take a look.

2. There is no hot water and no central heating

Here are some likely causes of no hot water or central heating in your home:

  • Loss of water pressure: A considerable loss of water pressure in the system can prevent the boiler from operating for both hot water and central heating. If there isn’t enough water, it won’t be circulating in the home and safety devices in modern boilers will prevent it from lighting to protect the boiler.
  • Faulty programmer or timer switch: A malfunctioning programmer or timer switch may not signal the thermostats or boiler to produce hot water or activate the central heating.
  • Ignition issue: Problems with the boiler's ignition system or electronic controls can prevent it from firing up for hot water or heating.

To resolve these issues, you can try a manual reset by following the manufacturer’s instructions or repressurising your boiler. If neither of these fixes gets things back up and running, contact a Gas Safe registered engineer, who can investigate further.

3. There is hot water, but only when the heating is on

If your combi boiler only produces hot water when the heating is on simultaneously, there will likely be an error.

Here are the main ones to look out for:

  • Faulty diverter valve: As mentioned, this valve directs hot water to the heating system or the domestic hot water supply, depending on the user's selection. If the diverter valve is faulty or stuck, it may not move into the hot water position when required.
  • Faulty thermistor: If it is defective or malfunctioning, it can impact the boiler's ability to produce and deliver hot water on demand.

Unfortunately, neither of these are DIY fixes. Contact a Gas Safe registered engineer to ensure your combi boiler’s fault is investigated and repaired safely. 

Water leaking

If your combi boiler leaks, you might first notice a drop in the water pressure in your system. When you look at the combi boiler, you might see a puddle forming or a stain beneath the appliance where water has gathered and dried.

Leaking water can indicate that internal components within the boiler are damaged and need replacing, such as the boiler's hydraulic blocks, O rings, seals and gaskets. Alongside this, leaking water out of your blow off pipe outside could mean you have a faulty pressure relief valve or issues with your boiler’s expansion vessel.

If water is leaking from your boiler, the whole system must be inspected. These repairs are technical and can require specialist equipment, meaning you must contact a Gas Safe registered engineer rather than trying to carry them out yourself.

No pilot light

If you have an older combi boiler that fails to ignite or shows signs of intermittent heating, there may be an issue with the pilot light. You can sometimes see your pilot light from outside your combi boiler via a sight glass. 

The boiler's pilot light could be struggling to light for several reasons, including an issue with the gas supply or a blocked/dirty pilot burner assembly.

A Gas Safe registered engineer must investigate any combi boiler issues relating to gas supplies or faulty ignitions. 

Thermostat error

If your combi boiler isn't responding to changes you're making on your room thermostat, there could be an error with your home's wireless network, or if you have a hard-wired thermostat, the thermostat itself could be faulty. Another telltale sign of a thermostat error is inaccurate temperature readings from the system and uneven heat distribution around the home.

The causes of a thermostat error can be things like incorrect wiring, signal loss, or boiler control board issues.

Here are some simple solutions that might help:

  • Check the batteries if the thermostat is wireless
  • Reset the thermostat settings
  • Update the software

If you’ve tried the above DIY fixes and are still experiencing issues, get in touch with a Gas Safe registered engineer. During a callout, they'll check your whole system and get to the bottom of any issues.

Loss of pressure

A loss of pressure within your combi boiler can cause your entire system not to heat up as it should and, in some cases, lead to what’s known as a boiler lockout. Low water pressure can be caused by a system leak or other issues within the appliance, such as issues with the expansion vessels or pressure relief valve.

If you think low boiler pressure could be the cause of your boiler issue, you can check the pressure gauge on the front of the system. The needle should be sitting in the green zone between 1 and 2 bar. If your pressure is low, you can try repressurising your boiler

If this doesn’t work, you’ll need to contact an engineer to come and take a look. They may need to repair or replace faulty components or carry out more complex maintenance on your home’s central heating system.

Strange noises

Household appliances can make a selection of weird and wonderful noises as they serve your home, but sometimes, rumbles and gurgles can shed light on a more significant issue with your combi boiler.

If you hear bubbling and gurgling from your combi boiler, air could be trapped in the system, limescale buildup, a damaged pump, or low water pressure.

Because a number of different things can cause these strange noises, there are several DIY fixes you can try before calling an engineer, including bleeding the radiators, checking the water pressure or adjusting the temperature settings.

Read our full guide on why your boiler might be making a noise, or watch our troubleshooting video below for more information.

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Frozen condensate pipe

If you have a modern condensing gas combi boiler, it will have a condensate pipe and flue. Sections of these pipes sit outside your property, leaving any liquids inside at risk of freezing in sub-zero temperatures. Frozen condensate pipes are a common boiler issue in the UK during the winter months. 

Symptoms can include gushing sounds from the boiler, frequent lockouts and visible ice on the outside of the pipe itself.

If you suspect your condensate pipe is frozen, you can try to reset the system. Or, if your condensate pipe runs externally, you could try thawing it by pouring hot (not boiling) water onto it. If neither of these fixes works, contact an engineer. 

Here, you can read our complete guide on how to fix a frozen condensate pipe.


Kettling is when water begins to boil inside the boiler. Even though this might not sound like a problem, the water inside your boiler (whether it’s combi, system or standard) should never actually reach boiling point. If it does begin to kettle, it will generate steam inside the system and, as the name suggests, cause a loud rumbling or whistling noise, just like a kettle. 

In terms of symptoms, you might notice a decreased efficiency with your system. This means it may cost more to heat your home, or your boiler won't be heating up as it should. It could also mean a potential fluctuation in boiler pressure.

The causes for kettling can vary, but some leading causes are limescale buildup, debris or sludge in the system, incorrect pump settings or a failure causing overheating.

If you're experiencing kettling in your combi boiler, don’t try and fix it yourself. Contact a Gas Safe registered engineer to come and service or repair the system.

Cold radiators

Everyone knows that sinking feeling when you think the heating is on, but your radiators aren't warming up as they should. Maybe you can feel cold patches on the bottoms of one or more of your radiators, or perhaps certain areas of your home feel warmer than the others.

If your radiators are cold, it can mean many things. If you have cold spots towards the bottom of your radiators, then there is more than likely magnetite or sludge in them, which has settled in the bottom, preventing correct heat transfer. This will require a system flush to remove the sludge and resolve the issue.

If they are cold at the top, this will more than likely be due to air trapped in them, and they need bleeding. 

If air is trapped in your radiators this can be solved by bleeding your radiators. You can learn how to bleed a radiator by reading our expert’s guide. 

If you've bled your radiators and they still aren't functioning as expected, then call an engineer to take a closer look.

Repeated lockouts

Boiler manufacturers use the term boiler lockout when a boiler has shut down. When a boiler has a fault, the 'lockout' function shuts the system down to prevent further damage. 

Depending on what kind of boiler you own, the interface may have a flashing light or an error code.

Combi boiler lockouts are usually because of one of the following reasons:

  • Fan issues
  • Pump failure
  • Power cut
  • An ignition problem
  • A system blockage

To fix a combi boiler lockout, you can try resetting the system and see if anything changes. Beyond this, frequent lockouts are a sign that your system needs to be examined, serviced or repaired by a qualified boiler engineer.

Explore your heating options with BOXT

While an issue with your boiler might seem like the end of the world, many can be solved by a Gas Safe registered engineer in a single visit.

Here at BOXT, we strive to provide prompt, high-quality services nationwide. If you ever think your boiler needs fixing, contact our team, and we can arrange a callout.

If your combi boiler is over 15 years old, it might be time for a more energy-efficient replacement. Tell us a few details about your home and its heating needs, and we’ll recommend top-performing boiler models at fair, fixed prices.