How do I stop my boiler from overheating?

How do i stop my boiler overheating

An overheating boiler is definitely something you should seek immediate professional help for.

This article is intended as an explanation about why your boiler could be overheating, and is not a complete instruction manual. This is due to the possibility of a hazard once a boiler may be reaching the point of overheating.

As soon as you know your boiler is too hot or suspect it of overheating, get in touch with a boiler engineer and, if you know how, switch your boiler off or turn your thermostats to minimum and turn off the water supply to the boiler.

How do I know my boiler is overheating?

The most straightforward way to see if your boiler is overheating is to look out for error messages on the display.

They’re often presented as a code, so look up what it means in your manual, or if you can’t find it, just Google the name of your boiler and the phrase “error messages” and you should find an answer.

Some older boilers don’t have digital displays – look out for lights that switch on or flash, as they could indicate a fault. The boiler should trip out, too. You may also notice water escaping from the pressure relief valve outside. If any of this happens, switch it off.

In the absence of error signals or tripping out, a telltale sign could be that the radiators or hot taps are not fully warming up even when the boiler is clearly firing. Where is that heat going? It’s probably building up in the boiler.

Probable causes of boiler overheating

Overheating can be caused by a number of electronic or physical factors. Here are some likely causes.

Failed thermistor

The thermistor is a component that measures water temperature in the boiler, and uses the measurement to increase or decrease flow of hot water to the heat exchanger.

It’s similar to the thermostat that controls your radiators, except a thermostat is a simple on/off switch, while the thermistor gradually adjusts flow to achieve the right temperature. If the thermistor fails, it could be wrongly telling the boiler that the water is too cool and it will never stop heating up.


If the flow of the circulating water is restricted, the flame will be heating slow-moving water much more than it should be, leading to overheating.

Blockages can occur anywhere, and are usually caused by build-ups of sludge, rust or limescale in the pipework. The most vulnerable part is the heat exchanger, however, as that’s where the water passes through much narrower channels, which are easier to block.

Failed pump

If the pump fails, water will not be moved around the system, but it will still be heating up, which will inevitably cause overheating. Even a working pump that’s on its last legs can have the same effect.

Electronic failure

The final possible reason is that there’s some issue with the electronics of the boiler, whether that’s a digital system or some physical electronic component failure. It could lead to the wrong signals being transferred from system to pump, and overheating the boiler.

In all cases, seek help

None of the above problems is serviceable by a regular householder – they all need the immediate attention of a professional boiler engineer.

Generally, the risk is very low, as the working electronic and physical systems like electricity trips and relief valves should kick in to stop pressure building up. However, on very rare occasions, overheating can lead to boiler damage or even an explosion.

Depending on its age, it could be time for a new boiler if it’s overheating. Your engineer will be able to do a proper diagnosis and advise on repair or replacement.

Published: 27 April 2021