Why Your Boiler Is Overheating And How To Fix It

Boilers of all makes and model can overheat from time to time. It is a common problem but still hugely frustrating. In this guide, we will explain the main reasons why a boiler can overheat and detail how you can fix the problem.

Overheating boilers are a major annoyance because the problem tends to arise when you are halfway through a shower or running a bath and the hot water just cuts off.

There are a variety of different issues that can cause a boiler to overheat. In this guide, we will examine five of the most common problems that can lead to overheating, explain the cause, and also outline how to fix it.

It will only take you five minutes to read this guide, but it could save you a lot of time, effort, and money, as well as let you enjoy that bath you were halfway through running.


Why Does A Boiler Overheat Then Lock Out?

When a boiler overheats, it will automatically shut itself down. This is known as a lockout and is a common failsafe measure that is built into all boilers.

Overheating can result in serious damage to important components in the boiler as well as potentially be dangerous for you. A lockdown is designed to protect the boiler and also ensure your own safety too.

When your boiler does lockout, you can usually reset it and get things going again. But it is invariably an indication of a bigger problem and it is likely to continue to lockout intermittently until the issue has been fixed.

Regular Maintenance That Can Stop Your Boiler Overheating 

As you will see, the most common cause of boilers overheating is a build-up of sludge or limescale in the system. In addition to the specific fixes we detail below, there are also some general maintenance methods that can help stop these problems occurring. These include:

  1. Hot Flush: If your engineer carries out a hot flush, it will help to clear limescale and heating debris from your boiler.
  2. Central Heating Inhibitor – Adding a central heating inhibitor to your heating system can help limit overheating.
  3. Install a scale reducerA scale reducer is designed to help prevent limescale build-up and is particularly recommended in hard water areas.
  4. Magnetic system filter – Installing a magnetic system filter can help stop debris building up in your boiler and central heating system.

What Causes A Boiler To Overheat And Lockout?

In order to stop your boiler from overheating and going into lockout, you need to identify the root cause. This can be a number of things but there are some causes that are far more common than others.

In this section, we will detail the five most common issues that can cause this fault in all boilers and explain the best way to fix them.

#1 – A Blockage

If there is a blockage somewhere in your boiler or heating system, this can lead to overheating and lockdown.

There are two common causes of blockages. One is heating sludge building up in the pipes or around key components. The other is a blockage in the condensation pipe.

How to Fix it:

If your blockage is caused by heating sludge, this means that debris from inside pipes, valves, and radiators is obstructing the flow of water around your system. This, in turn, creates an increase in pressure and can sometimes blow the seals around your central heating pump.

If these seals blow, it is likely to lead to your boiler overheating and going into lockout. You will need to get a qualified engineer out to replace the seals. Sometimes they may recommend replacing the entire central heating pump. This is a much bigger job and could cost you several hundred pounds.

If the blockage is in the condensation pipe, this is a simpler issue to fix. Simply locate the condensation pipe and clear any blockages. If it is cold weather, the blockage could be caused by frozen water. If this is the case, defrost it with some warm water, check for blockages, and then lag the pipe to prevent freezing from reoccurring.

You should then be able to reset the boiler and your overheating problem should be resolved.

#2 – Limescale On The Heat Exchanger

Limescale is another common cause of overheating. If limescale builds up on the boiler’s heat exchanger it can not only cause boiler noise but also create hotspots.

These cause a whistling noise often referred to as kettling but this is as a result of water boiling at higher temperatures than it should. This, in turn, can lead the boiler to think it is overheating and so lockout.

Limescale issues are far more common in hard water areas, but they can occur in any boiler.

How to Fix it:

You will need to get a Heating Engineer to examine your boiler and identify where the limescale build-up is. Sometimes a simple clean can resolve the issue but if the build-up is too severe, any affected components can need replacing.

If this is the case, costs can vary, but a new heat exchanger is likely to set you back several hundred pounds.

#3 – Central Heating Pump Overheating

One possible cause of overheating which a lot of people overlook, but which actually occurs quite frequently, is the central heating pump.

Pumps always tend to be warm as they are motorized components but if they get too hot, this can be an indication that some parts of them have become seized up. If this occurs, your boiler is likely to go into lockout as a safety measure.

How to Fix it:

Seized-up components inside your central heating pump can often be released by giving the pump a gentle tap. You can do this yourself as the pump is invariably located just behind the boiler casing.

If this doesn’t work, you will need to get an engineer in to dismantle and clean the pump and fix anything that has broken.

#4 – Removing Pressure Release Valves

When your boiler gets too hot, it will release pressure through its aptly named Pressure Release Valves (PRVs).

But these valves can go wrong or leak from time to time and this means they are unable to release the pressure. When this happens you boiler can easily overheat and if it doesn’t go into lockout, there is even a risk of the boiler exploding.

How to Fix it:

You will need to get a qualified engineer out and they should replace any damaged PRVs. We have seen some examples of bad engineers removing or even sealing the PRV. This is very dangerous and you must ensure this doesn’t happen to yours.

#5 – A Failed Thermistor

The thermistor is the component within your boiler that measures the boiler temperature and communicates it to the printed circuit board (PCB). The information provided by the thermistor is used by the boiler to determine how much fuel (gas, oil, or LPG) to release into the system.

But if the thermistor is broken, this can result in too much fuel being provided. This will mean the water heating up by the boiler gets too hot and the boiler can then overheat.

How to Fix it:

If there is a problem with your thermistor, you will need to get a qualified engineer to take a look at your boiler. Usually, it is possible to replace a faulty thermistor, although this can be a fairly privacy fix.

However, it is also possible that the fault could be with the PCB. If this is the case, the price is likely to be in excess of £500 for a replacement.

Where to buy a new boiler

Most of the problems that result in overheating can be dealt with either by yourself or by an engineer. But if a major component is beyond repair, you may have to shell out large sums to fix them.

If your boiler is old and out of warranty, it often makes more sense to buy an entirely new boiler than patch up your old one. New boilers are more energy-efficient, so they will save you money on your utility bills. They also come with a long warranty so you shouldn’t have to worry about maintenance costs for a while.

We always recommend you get several quotes before buying a new boiler and we strongly suggest that one of those quotes comes from WarmZilla.

WarmZilla is an online boiler installation company that offers rock-bottom prices on a huge range of boilers from all the main manufacturers. They have plenty of exclusive special offers available too.

In our experience, WarmZilla offers some of the most competitive prices around and we recommend you take a look before you buy your new boiler too.


There are a number of reasons why your boiler might be overheating. In this short guide, we have highlighted some of the most common ones and explained how to fix them.

Many of these fixes are simple, but if there is a more major problem, you may decide you would be better off buying a whole new boiler. If you do, make sure you get a quote from WarmZilla to see how it compares.

Have you had issues with your boiler overheating? How did you solve the issue? Was this guide helpful? Do you have any more tips or advice for our readers? Are there any questions you have that we haven’t addressed in this guide?

We always love to hear thoughts and questions from all our readers, so why not share yours with us today using the comment box below.

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