Boiler Overheating? Signs, Causes and Solutions

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Last updated: October 31, 2023
boiler overheating

If you have an overheating boiler, then this can be a serious problem and even means it is at risk of exploding or significantly damaging your appliance. Fortunately, modern boilers are fitted with a safety feature which, when tripped, will lockdown the boiler before it dangerously overheats.

We take a look at what you should do if you suspect your boiler is getting too hot, or if the shutdown is suddenly triggered, and your boiler is overheating.

Why do Doilers Overheat?

A boiler will head into overheating territory when the hot water that is normally sent to your home’s central heating system or hot water taps is prevented for some reason from circulating as it should. This build-up of hot water should trigger a shutdown procedure that has been hardwired into the boiler itself, so the boiler cuts out – this is called a boiler lockout.

If your boiler has been locked out, then there has been a failure in its system which, if left unchecked, would lead it to your boiler overheating dangerously and so the boiler is automatically turned off. An error code is then usually shown on the boiler’s digital display to indicate the problem.

Potential Risks of an Overheating Boiler

Modern boilers are sophisticated pieces of engineering which have been designed to withstand and operate safely under high heats and pressure, with most boilers able to safely work under an operating pressure of 20 PSI. And, with the carefully tuned safety features and shut-off mechanisms built into modern boilers, the likelihood of a boiler exploding is extremely rare. However, there are potential risks associated with a boiler overheating, especially if for some reason the safety features fail, and it becomes what is known as a ‘runaway boiler’.

The potential risks of a boiler overheating include:

Damaged internal components – unchecked, overheating can lead to components inside the gas boiler to become damaged or have their ability to function significantly impaired.

Release of toxic fumes – many internal boiler components are made from plastic, which can melt under high heat, releasing smoke and toxic fumes into your home.

Irreparable internal damage – with heat and pressure a risk to the internal components of your boiler, when a boiler is overheating, it can lead to irreparable damage to the components, and ultimately, to your boiler. And that means a new boiler will be needed.

Worst-case scenario – unresolved build-up of gas and pressure inside a boiler can eventually lead to an explosion. If you suspect a rupture, call the fire brigade and emergency services immediately. However, this situation is rare, thanks to the safety features of today’s boilers.

What Causes A Boiler To Overheat

So, now we know the potential risks when a boiler is overheating, it’s important to also understand the possible causes so you can react accordingly if you suspect your appliance is getting hotter than it should be. The main reasons your boiler may be overheating are:

  • High pressure – this is the main reason why a boiler overheats, and the high pressure is usually the symptom of another system malfunction, including leaking pressure relief valves.
  • Build-up of limescale – limescale can form anywhere there is water, especially if you live in hard water areas that are limescale prone. And if you get a build-up of limescale on the heat exchanger, it can create hotspots by restricting the flow of water which then amplifies the heat of the slowed down water, taking it to temperatures higher than it should. And you could get an overheating boiler as a result. Look out for boiler noise, or noises in your heating systems or water heater – if your boiler starts to make a whistling sound, it could be caused by a limescale build-up on the heat exchanger.
  • A system blockage – if sludge is allowed to develop in your heating system, then it can eventually cause blockages, especially in the pipework, radiators and gas valves. The condensate pipe, which is used to drain away vapours produced by condensation in the boiler, can also freeze in low temperatures, causing condensate blockages in the system.
  • A failed thermistor – if your boiler is operated electronically, then a thermistor lets the circuit board know if there is a need to increase temperature in the boiler. If this thermistor fails, it can indicate an increase in fuel to raise the temperature when it’s not required, leading to an excessive increase in water temperature and ultimately an overheating boiler.
  • Overheated or faulty pump – your boiler’s pump is there to circulate hot water around your central heating system and so if there is a failure in the pump, water can sit in the boiler for too long, and so get overheated, leading to either the safety lockout being triggered or even total boiler failure.
  • Faulty safety feature – a common cause for a boiler to go into the overheating zone if is its safety feature fails and so doesn’t trigger the appropriate lockout response. However, such high, unregulated temperatures will most likely see the boiler shut down altogether and you will see an error code on its display.
  • A bad service – Using an unqualified engineer can actually cause your boiler to overheat if it is not serviced correctly. Plus, by law only qualified and Gas Safe authorised and regulated engineers are able to work on gas boilers and using someone without the appropriate qualifications to fix the problem can actually invalidate your boiler warranty.

How to Fix an Overheating Boiler

The first thing you should do if you suspect your boiler is struggling to regulate its heat is to shut off the water supply immediately. Cutting off the intake of more cold water will prevent a rise in steam as the cold water hits hot internal components and so avoid any dramatic rise in internal pressure which could rupture the whole boiler.  But don’t turn off the electricity supply at this point, or you could prevent the boiler from cooling itself down.

It is important to avoid attempting to fix or remedy the symptoms of an overheating boiler yourself but call out a gas safe registered engineer who will have the expert knowledge and experience to fully assess the situation and make the appropriate repairs or remedial action.

Depending on the identified cause or causes of your overheating boiler, gas engineers will:

Flush Out the System

This can be an effective remedy if your overheating boiler is caused by limescale build up on the heat exchanger or blockages in the central heating system.

  • For limescale build-up in your system – which causes what is known as a ‘kettling’ sound as it resembles a stove-top kettle’s whistle – an engineer will be able to flush out the system to remove the limescale. For specific limescale build-up on the heat exchanger, your heating engineer will clean it up, using a special descaler specifically for boiler heat exchangers.
  • For larger limescale, sludge and debris build-ups – for a significant blockage, a heating engineer can perform a full-scale internal cleanse called a powerflush which, as its name suggests, uses high pressure water and targeted chemicals to blast any blockages clean of your heating system, followed by an inhibitor to help prevent blockages returning.

Resolve a Faulty Pump

Your engineer may well be able to fix the pump in question, but in most cases, a new hot water pump is the best solution.

Tackle a Thermistor Issue

With such an essential piece of fine-tuned equipment in your boiler, only a qualified heating engineer should work on your boiler’s thermistor. And due to its sensitive nature, it’s likely they will replace it with a new one.

Replace Your Whole Boiler

Especially if your boiler is more than 10 years old, a whole replacement may be the best solution. Newer boilers are able to withstand higher pressures and have more sophisticated lockdown and warning systems. So, for long term safety as well as central heating efficiency, it may be time to bite the bullet and get a new boiler installed.

What You Can Do To Prevent Your Boiler Overheating

As with anything in life, prevention is better – and often less financially painful – than the cure. Looking after your boiler and central heating system appropriately, including making those annual boiler service dates, can all help to keep your boiler cool-headed and on track.

For preventing specific limescale and sludge issues which can lead to your boiler overheating, it is also worth considering having a special magnetic filter installed. These filters are added internally and uses a magnet to catch sludge and debris before it creates a high-pressure causing blockage. Ask your gas safe registered engineer the next time he comes out to service your boiler if he thinks adding a magnetic filter to your system is the right idea.

And if your boiler is on the older side, then considering a new boiler replacement and upgrading to one of the latest models will take off the pressure of your boiler developing an over-heating fault.

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