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Why Is My Boiler Overheating?


Your boiler overheating is no joke considering you’ll have to go with no hot water or heating after it automatically shut itself down. The consequences can be even scarier as it can put your boiler at the risk of an explosion in more severe cases.

However, as luck would have it, most modern boilers are fitted with a safety feature that will initiate a boiler lockout when the boiler begins to overheat.  Still, you never know when this safety mechanism will fail, and you must learn how to recognise the signs of an overheating boiler and what must be causing it.

How can I recognise when my boiler is overheating?

Different boiler models have different ways of letting you know when there is an issue of overheating. Most boiler units will initiate an automatic shutdown process when it overheats. In addition to locking you out, new models display an error code indicating that the lockout is due to the unit overheating.

In some cases, the boiler will not shut down but will provide you with an error code so that you can manually shut down the unit before the overheating causes extensive damage.

What are the risks associated with an overheating boiler?

Thanks to the new technology and safety features installed in modern boilers, they can now withstand operating pressures of up to 20 PSI and high water temperatures without overheating and exploding. That is to say, the risks associated with overheating aren’t as concerning as they used to be.

However, prolonged overheating or high temperatures in your boiler can cause several problems, including:

  • The heating system shutting down on its own, leaving you with no heat or hot water supply
  • If the system doesn’t shut down, the overheating and high pressure will damage some boiler components causing them to malfunction.
  • Some plastic components in the boiler will begin to melt and release smoke and toxic fumes into your home.
  • In the worst-case scenario, the overheated boiler can explode. However, this is rare and mainly happens when unqualified personnel handle or repair the boiler.

What are the common causes of boiler overheating?

There are several reasons why your boiler might be overheating. These reasons range from physical problems such as limescale build-up to issues with the pump or thermistor components.

Limescale build-up in the heat exchanger

The heat exchanger burns the gas to generate heat that warms the cold water circulating your radiators. As the water from the mains passes over the heat exchanger, it deposits limescale on the heat exchanger. With time, the limescale builds up, restricting the flow of water. As cold water now flows slowly over the heat exchanger, it gets heated to higher temperatures.

Faulty safety feature

Most boilers are fitted with a safety feature that cuts out the heating when temperatures reach some desired levels. If this mechanism fails, the boiler will continue operating even after this designated safe temperature has been surpassed. The boiler temperatures will then reach high levels, which can be damaging to components.

Blockage in the boiler or pipes

Sludge and other debris accumulate in the boiler and pipework with time. These accumulations can cause blockages that slow down the flow of hot water around your heating system, resulting in too hot. If you hear strange sounds like knocking coming from your boiler or pipework, then a blockage is the most likely culprit.

Faulty pump

Your boiler has a pump that is responsible for circulating hot water around your heating system. If this pump fails or malfunctions, then the hot water in the boiler will be stuck and keep on heating up. The continuous heating of the trapped water will eventually result in your boiler’s safety feature kicking in or your boiler completely failing.

Faulty thermistor

Your boiler is fitted with a thermistor whose role is to monitor the temperature of the hot water flowing out of your boiler. When the water temperature is low, the thermistor will communicate with the PCB, which will instruct the boiler to raise the heating. If the temperature of the water leaving the boiler is too high, the thermistor will inform the PCB, which will then regulate the boiler heating.

However, if the thermistor has an issue, it can fail to detect when the hot water temperature leaving the boiler has reached the desired level. The boiler will then continue to heat the water to even higher temperatures resulting in the boiler overheating.

Boiler not shutting off

Ideally, your modern boiler should automatically power down when your home’s hot water and heating demands have been met. If the boiler has not been fitted with this shutdown feature, you can power it down yourself when its services are no longer needed. However, if the boiler refuses to power down, it can lead to overheating.

If your boiler refuses to power off, shut off the gas and water supply immediately and contact a certified heating engineer to resolve the issue.

What should I do if my boiler is overheating?

When you suspect your boiler might be overheating, the first thing you should do is shut off the water supply. Shutting off the water supply will prevent cold water from being introduced into the system and converted to steam as it makes contact with the hot internal components. If this is not done, the steam generated in the boiler will lead to a dramatic rise in pressure which could cause your unit to explode.

Secondly, you should avoid any attempts to repair or cool the boiler on your own. This is because you could worsen the problem and cause extensive damage to the boiler or even injure yourself. Instead, reach out to a fully qualified heating engineer, preferably a Gas Safe registered one and follow any advice they give you.

Once the engineer arrives on-site, they will inspect the boiler to identify the root cause of the overheating issue and solve it. If they trace it to a blockage in the heating system, they will carry out a power flush on the system to clear the blockage.

If the issue is down to a blockage in the condensate pipe, they will defrost it and insulate it to prevent the same problem from recurring. On the other hand, the heating engineer will clean the system with a descaler if the overheating is due to limescale build-up in the heat exchanger. And if the problem lies with the pump or thermistor, they will determine whether it can be repaired or if a replacement is necessary.

How can I prevent my boiler from overheating?

When it comes to your boiler overheating, the mantra prevention is better than cure holds. But how can you protect your boiler from overheating? Here are some tips which if implemented, will drastically reduce the chances of your boiler overheating.

Carry out annual servicing for your boiler

Arranging for your boiler to be serviced annually goes a long way to minimising the chances of your boiler developing overheating issues.  During the annual servicing, the heating engineer will be able to identify any minor problems with your boiler and fix them before they result in your boiler overheating.

Hot flush or power flush your boiler.

If you suspect a sludge accumulation in your central heating system, you can hot flush it to remove the sludge before it accumulates and results in blockages. Hot flushing your boiler is simple and way less expensive than waiting for sludge to accumulate and arranging for a power flush.

Add a central heating inhibitor into your heating system

This is a liquid chemical that you add to your heating system to break down sludge in the system and prevent it from building up again.

Install a scale reducer

A scale reducer or scale inhibitor helps reduce the amount of limescale build-up on your heat exchanger. The scale reducer is helpful, especially if you live in a region having hard water.

Install magnetic system filters

These filters will filter out rust, sludge, and debris in the water circulating through your central heating system. This will prevent the debris from settling and accumulating in the system and causing blockages that’ll lead to your boiler overheating.

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