Home Improvement

Boiler Overheating: Signs, Causes and Solutions

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An overheating boiler is a serious problem for your household. It can damage parts of the unit and, in the most severe cases, it puts your home at risk of explosion. Before you panic, don’t worry! All modern boilers come with a safety feature that prevents this from happening. Still, that doesn’t mean you should ignore it when your boiler is overheating.

So, in this article, the Fantastic Services team of skilful handymen will explain how to recognise the signs of an overheating boiler, understand the causes, and ultimately present you with the most common solutions.

Table of Contents:

This article is for you if you:

  • Live in a residence with a gas boiler;
  • Are experiencing the problem of an overheated boiler;
  • Wish to know how to prevent this dangerous emergency before it occurs.

Signs your boiler is overheating

There is one main sign that your boiler has overheated, and that is mainly the activation of the safety feature that prevents explosions. It’s called a boiler lockdown and it’s a shutdown procedure that stops your water heater from working. If your boiler has shut down, you know it has become unsafe. It will be able to work again after a specialist finds the problem, fixes it and resets the heater.

What causes boiler overheating?

There are many causes that can make your boiler overheat and, by law, only a Gas Safe registered engineer can determine and fix the problem!Some of these problems might include:

  • High pressure. This is the main cause for an overheated boiler, though it’s usually a result of some other malfunction. The best course of action, in this case, is to simply relieve the pressure.
  • Blockages. There may be central heating sludge blockages and condensate pipe blockages present. Central heating sludge forms in the pipes, valves and radiators and can clog up the whole system, which creates backpressure. Condensate blockages occur in the condensate pipe, which is known to freeze in cold weather and cause an obstruction.
  • Failed thermistor. The thermistor’s job is to let the PCB know if the demand for temperature is more or less than is currently being produced. If the thermistor fails, it will provide more fuel, the water will get too hot and the boiler will overheat.
  • Limescale on the heat exchanger. Limescale can form everywhere there’s water and is especially common in hard water areas. As it builds upon the heat exchanger, it creates hotspots and boils up the water to temperatures higher than it should be. This overheats the boiler.
  • Overheated pump. Pumps should be warm, but not hot. If a pump is too hot to touch, then the internal components are seized. A gentle tap can sometimes stop a seizure, but this is not a fix. The pump might need to be replaced.

Has your boiler overheated?

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How to fix a boiler overheating problem

If you have an overheated boiler, DO NOT under any circumstances try to fix it yourself! Call a Gas Safe engineer instead. Depending on the cause of the overheating, they will do the following:

  • If it’s limescale on the heat exchanger, the engineer will clean it up with a special descaler for boiler heat exchangers. The procedure is not simple, it requires a lot of measuring and water draining.
  • If it’s a blockage, the engineer will remove it. If it’s a central heating build-up, they will most probably powerflush your heating system. If it’s a condensate blockage, they would defrost the condensate pipe. However, this isn’t a long-term solution. The best countermeasure for a condensate obstruction is to winter-proof your condensate pipe.
  • If it’s a faulty pump, the reliable boiler repairman will test it to see if it’s still able to work, or if it’s broken beyond repair. If it’s the latter, they will find you a replacement.
  • If it’s a thermistor issue, the Gas Safe engineer will try to see if it needs fixing, but in most cases, it just needs to be replaced.
  • You might have to consider a new boiler. In the end, if your water heating appliance is more than 10 years old, it’s advisable to replace your boiler with a newer model. Modern boilers withstand higher pressures and will also alert you in times when you need to call a heating engineer before the situation becomes hazardous.
  • Boiler and central heating cover is another option you can benefit. You can invest in a boiler cover plan that can help you keep your boiler running safely and get annual services by Gas Safe experts.

How to prevent your boiler from overheating

Every member of your household should know how to prevent boiler overheating. As you’ve seen, the most common causes come from hard water issues and faulty boiler parts. So, installing a magnetic filter is a very effective preventative solution. This gadget will catch any debris circulating around the system before it has a chance to cause a blockage. Many heating engineers will recommend having a magnetic filter installed as a standard along with a new central heating system.

For properties with an older central heating system, the best way to prevent the unit from overheating is to simply purchase a new boiler. Other options are to powerflush the heating system regularly and use a central heating inhibitor.

Do you need a professional boiler repairman?

Is your boiler overheating? Do you notice one or more of the signs we mentioned? Is your boiler more than 10 years old? If there is any issue you notice around your gas boiler and heating system, don’t hesitate to call a professional to check out the problem. It could seem small now, but it could have great consequences. The professional boiler experts from Fantastic Services are always ready to help you out!

Visit the main website for price rates for our professional boiler technicians!


  • Modern boilers have a safety feature that could shut it down if it overheats;
  • The most common causes of an overheated boiler are faulty parts, blockages in the system, or high pressure;
  • Finding the problem that caused your boiler to overheat requires a thorough inspection of every part and may also involve replacements, or even buying a brand new boiler;
  • You can prevent a boiler from overheating if you install a magnetic filter that averts blockages and powerflush the system regularly;
  • Every boiler repair procedure should be performed by a certified Gas Safe technician!


Has your boiler ever overheated? What was the problem? How did you fix it? Share your experience with us in the comments!

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