Boilers were created to heat water, and so it’s unsurprising that sometimes the units themselves can overheat. Luckily, there are plenty of safety features built into your boiler to stop anything too dangerous from happening. However, it may be a good idea to get to the bottom of why your appliance is overheating in the first place if this happens.

Below, we run through some of the signs that could show your boiler is overheating, as well as the causes and how to fix the problem.

How to tell if your boiler is overheating

If your boiler is overheating, it will likely shut itself down, even if it’s mid-programme, and may display an error code on the screen. This is also known as boiler lockout and is an imperative safety feature. Therefore, if your boiler keeps turning itself off, overheating may be the reason why.

When this is the case, the problem must be treated as soon as possible. Extreme temperatures can put a lot of pressure on a boiler and its internal parts and there may be a risk of boiler damage, so this issue mustn’t be taken lightly. For this reason, read on to discover the causes and potential fixes.

Why is my boiler overheating?

There are a few culprits that may be the cause of your overheating boiler. We’ve covered the main ones below.

Why does my boiler keep overheating and cutting out?

  • Limescale

Limescale is probably the most common cause of an overheating boiler, so if you live in a hard water area, this may be your problem. Hard water contains lots of minerals that soft water doesn’t, including magnesium and calcium. While these deposits are very small, they can build up, particularly in systems that handle large quantities of water, like your heating system. Over time, these deposits can get stuck inside pipework and even in the heat exchanger. You’ll probably notice them on your taps and inside your kettle, too, and they’re known as limescale.

When a boiler is impacted by limescale, you might notice that it makes a whistling noise. This is known as kettling and is a sign that the limescale in the system is causing the water inside the boiler to overheat and boil. This, in turn, can cause the boiler to overheat and shut down.

  • Blockage

Heating systems can be prone to blockages, either from sludge or system debris, and so it could be one of these things that are creating issues with your boiler.

Sludge is very common in central heating systems. This is because, over time, the repeated contact with water can result in rusty pipework. The rust becomes dislodged from the pipes and can be carried around the system in the current of water. When the rust becomes bad enough, it can block a pipe or radiator altogether, causing a pressure increase in the whole system.

Increased pressure can normally be handled by the pressure relief valve (PRV), but a faulty valve may mean that the system can’t cope with the additional strain, and your boiler could overheat.

The other most common type of blockage is caused by ice. Your boiler produces excess condensate water and the condensate pipe allows for this liquid to exit the system. However, on a very cold day, this water can turn to ice and block your condensate pipe. A frozen condensate pipe could be the cause of an overheating boiler.

  • Pump fault

Your boiler relies on a pump in order to get the hot water to all of your radiators. If the pump is faulty for any reason, or not working at all, the hot water won’t be able to travel away from the boiler and could continue to be heated long after it is needed.

You may be able to tell if your central heating pump is faulty if it is making banging noises, a heating engineer would help to diagnose this. Sludge and limescale can impact the pump as well as the boiler, so these could be the culprits, once again.

  • Thermistor fault

There are lots of parts in a boiler system so that it can run everything as it should. A programmer is needed to alert the boiler to a set heating schedule and a thermostat is needed to tell the boiler when the correct room temperature has been met. A thermistor is a requirement so that the boiler knows the temperature of the water and whether this needs to increase or decrease. Generally, boilers are set to heat the water for your radiators to homeowner preference but generally the lower the better for efficiency. The thermistor will let it know when this is the case. When the thermistor is faulty, it may not be able to communicate with the boiler correctly, and the boiler will continue to heat the water, resulting in overheating.

How to stop my boiler from overheating


Now that you understand some of the causes of an overheating boiler, what can you do to resolve the problem? We’ve provided some ideas below, but it’s usually a good idea to call out a suitable qualified Gas Safe heating engineer to resolve the problem for you in a safe way.

Power flush the system

For a boiler that’s overheating from limescale build-up, power flushing the system might be the only way to stop this from happening. A power flush involves pushing fresh water through the system at high pressure to drain any rust, limescale and other debris that has built up over time.

Check your condensate pipe

It’s really easy to check your condensate pipe for ice, and if there are icicles on it, you can use warm water to defrost them and unblock the pipe.

Check the PRV (Pressure Release Valve)

A Gas Safe heating engineer may be able to check the PRV to ensure it’s not cracked and that it is working efficiently to remove excess pressure from the system. A replacement part may be required.

Replace the pump

If the issue is caused by a faulty central heating pump, you may need a new one. A heating engineer should be able to advise whether this is the case and fit it for you.

Replace the thermistor

For overheating that is caused by a faulty thermistor, the device may need replacing. This is a job for a qualified engineer and they will be able to advise you on which size and type to go for.

Add a scale reducer

Many of the issues we’ve mentioned in this guide, such as blockages, kettling and faulty central heating pumps can be caused by limescale. If you live in a hard water area, you may want to prevent such issues altogether by introducing a scale inhibitor or water softener to the system as per manufacturer guidelines. 

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