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Most common boiler problems

installer shaking a mans hand in the doorway

What you'll learn

  • The top 10 most common boiler problems
  • What should you do if you're having problems with your boiler

Boiler problems can cause a significant headache, particularly during the winter months. From leaks to cold radiators, boilers can suffer from a wide range of issues if not serviced regularly. Fortunately, most of the most common issues are easy to identify and can easily be fixed by a Gas Safe registered engineer.

Below we have listed the top ten most common boiler problems across all types of condensing boilers.

Boiler leaks

One of the most common boiler issues is a boiler leak. Leaks can be a nuisance because not only can they cause water damage to a home, but water can also cause electrical components in and around the boiler to malfunction.

A leaking boiler can be due to a number of things:

  • Excessive boiler pressure
  • Corroded pipes or tanks
  • A faulty Component
  • Leaks from seals

A leak should always be treated as a serious issue. If you find a leak you should call a qualified heating engineer as soon as possible to investigate. Do not attempt to remove the boiler casing yourself to find the source of the leak.

running tap

Hot water but no heating

A common boiler issue people face is when a home has hot water but no central heating. This can happen because of several reasons.

Firstly, if you have a combi boiler, it might accidentally be set to only heat water but not the central heating. This can happen if the boiler is set to this mode during the summer months when radiators do not need to be heated but is forgotten about. It may also be that the boiler is set to turn on at an unexpected time. Check the boiler display to see if this is the case.

Another cause could be that the boiler is working fine but there is trapped air in the radiators stopping them from heating up. The solution there is to bleed the radiators to allow the trapped air to leave the system.

If your central heating system is still not turning on, despite having hot water, it may be that the boiler or heating system is faulty. A Gas Safe registered engineer will be able to investigate the fault and find the source of the issue.

Low boiler pressure

Low boiler pressure can lead to cold radiators and a lack of hot water. Low boiler pressure can be identified on the boiler display panel, where either a digital or physical dial will show the level of pressure in the system. The most typical pressure level should be around 1.5 bars, with slight variance depending on the boiler manufacturer and model.

A boiler’s pressure can drop due to two main reasons:

  • A leak in the system will result in not enough water being present in the system, consequently causing the pressure to drop.
  • A bleeding radiator can also reduce system pressure.

Low boiler pressure can be fixed by repressurising the boiler. This involves opening the boiler filling loop to allow water to re-enter the system.

Check out our guide on repressurising a boiler to see the full list of steps to follow.

Boiler kettling

Another common boiler problem is kettling. Boiler kettling occurs when water is heated too quickly and reaches boiling point, causing steam to be generated and air to become trapped. The trapped air gives off a whistling sound, like a kettle.

Boiler kettling can be caused by limescale build-up, which slows down the flow of water in the system and causes it to heat up excessively. A faulty component on the appliance could cause the boiler to heat up water to higher temperatures than expected.

A boiler engineer will be able to inspect the system and identify the cause of the kettling. There is a solution to this problem which depends on the severity. Sometimes the noisy component can simply be cleaned using different methods, alternatively replacing the parts may be required.

Frozen condensate pipe

A frozen condensate pipe is an issue that many people face during the colder months. Every condensing boiler has a pipe that transports waste water produced during the condensing process outside the property. If temperatures drop below freezing, the water in the condensate pipe can become frozen, forcing the system to shut down automatically as a precautionary measure.

The best way to fix a frozen condensate pipe is to pour warm water onto it to thaw it. This should allow waste water to exit the pipe, meaning the boiler can turn back on. For further detail on this, check out our guide on how to safely resolve a frozen condensate pipe.

woman turning down her thermostat

Boiler not responding to thermostat

There are a wide range of thermostats that homeowners can choose from to help increase the efficiency of their homes. These thermostats can work great, but it can also be frustrating when the boiler stops responding to the thermostat.

If you suspect that your boiler is no longer working with the thermostat, first make sure that the temperature on the thermostat is above the current room temperature. If it is not, then the thermostat is likely working fine.

If the room temperature is below the thermostat temperature, try replacing the batteries in the thermostat or moving it closer to its receiver. If this does not work, it may be that the thermostat is faulty and needs to be replaced.

No heat or hot water

If your boiler has left you without heat or hot water, there is definitely something wrong with the system. A boiler may stop providing central heating or hot water for several reasons:

  • The boiler has become disconnected from the power, gas or water supply. These connections can be switched off accidentally or during a power outage.
  • If the boiler pressure becomes too high or too low the system turns off automatically, causing a lack of heat and hot water
  • Piping issues, such as frozen condensate pipes or airlock, can stop a boiler from functioning.
  • Along with various safety features built into the appliances that would display other faults.

A lack of central heating and hot water can be down to any number of these things, so it’s always best to call a Gas Safe registered engineer to inspect the boiler and resolve the issue.

Boiler keeps switching off

If left unserviced, a boiler may over time begin to randomly switch off. There are a number of potential reasons for this, including:

  • Trapped air in the system
  • Low boiler pressure
  • High boiler pressure
  • A faulty thermostat
  • Debris and sludge building up in the pipes

If you notice that your boiler keeps switching on and off, it may be because of one of these faults. Depending on the severity, a qualified engineer will be able to say whether the problem can be fixed, or whether a new boiler is needed.

man doing work inside a boiler

Strange boiler noises

Beyond boiler kettling, there are a wide range of strange noises that a boiler can make:

  • Gurgling noises - this is a sign of trapped air in the system.
  • Drone noises - this is usually caused by a faulty pump.
  • Humming noises - this usually happens if water arrives from the mains at too high a pressure
  • Whooshing noises - this occurs when debris blocks the air intake pipe or if air filters in the system become blocked by dust.

Strange boiler noises do not usually mean there is an immediate danger with the system. Nonetheless, an engineer should be called as soon as possible to investigate.

Pilot light going off

If you have an older boiler you may have a pilot light that needs to be constantly lit to light the larger boiler burner. However, over time the necessary parts for lighting the pilot switch can become faulty, such as the thermocouple.

If your pilot light keeps turning off, do not try to repair/replace this yourself. This would require taking off the boiler casing, which is illegal unless you are registered with the Gas Safe Register. Instead, first check that your other gas appliances are working correctly. If they are not, the issue might be with the gas supply itself.

If the other gas appliances are working correctly, there may be a faulty part, so call a Gas Safe registered engineer to resolve the problem. Newer boiler models are not built with pilot lights so do not suffer from this issue.


  • Boilers can suffer from many problems, most of which should be resolved by a qualified boiler engineer.
  • Boiler leaks should be treated as a serious issue and resolved as soon as possible.
  • A lack of central heating could mean that the radiators need bleeding.
  • Low boiler pressure can be caused by leaks and can be solved by opening the filling loop.
  • Boiler kettling can make a lot of noise and sometimes requires the system to be cleansed or for a part to be replaced.
  • Frozen condensate pipes can be thawed using warm water.
  • A boiler may not respond to a thermostat if the thermostat runs out of battery or is placed too far away.
  • A lack of hot water and central heating is usually a larger issue that definitely needs the help of a boiler engineer.
  • If a boiler keeps switching off, an engineer will be able to say whether a part needs replacing or if the entire boiler needs to be replaced. Sometimes this is more cost effective.
  • Boilers can make lots of strange and distracting noises, such as gurgling and humming.
  • Never attempt to replace any parts yourself. An engineer is best to fix any issues safely.

Contact your local Vaillant installer for help with your gas boiler