Boiler Overheating: Causes, Signs & How to Fix It

Boiler overheating is no joke. The annoying part is that it leads to discomfort but the scary part is that it can have lethal consequences. And there are many ways this can happen. Except, there are also some very simple solutions to those problems. Let’s take a look at all of it.

Why Is My Boiler Overheating?

Usually, boiler overheating happens when there is a problem with one of the safety features. You see, these machines are made such that the heating and cooling systems are well balanced.

How to Recognise an Overheating Boiler?

This depends on the model you own. But usually, when the boiler is overheated the unit tends to have an automatic shutdown process. In fact, the newer models have a design feature that makes it possible for the system to ascertain when the unit overheats. If the boiler doesn’t shut down when this happens, you at least get an alert so that you can manually shut it down before there is damage to the unit.

The first thing you must do is to turn off the water so that it does not run into the unit. If there is a safety failure, this can be very dangerous, because the steam will increase the pressure inside the boiler and the unit might even explode. If you’re not sure what to do after switching the water off, call an engineer before you use the boiler again.

What Causes a Boiler to Overheat?

Well, there’s more than a couple of reasons why this might happen. It could be a physical problem with limescale building up on the heat exchanger which leads to several other issues.

It could also be an issue with any of the components like the pump or thermistor. Let’s look at it in detail.

Causes of an Overheating Boiler

Knowing what is causing the problem is a great way to solve it. Let’s see what we can do about your situation.

Build-up of Limescale

If you notice that limescale has been building up on your heat exchanger, act immediately. Because this blocks the flow of cool water into the heating system which must be heated before it goes into the radiator.

If this is blocked, the boiler overheats and eventually, its lifespan gets affected. This problem is also called kettling because you will hear a noise that resembles the one of a kettle whistle. 

Limescale build-up happens because of the high mineral content in the hard water supply. It creates hotspots on the heat exchanger and the water inside gets heated more than it has to.

Blockage in the System

If you are dealing with an older heating system, blockages are not uncommon. And all of them lead to overheating if left unfixed. You will notice that your heating system starts to make an uncomfortable amount of noise.

This might even lead to radiators not working as efficiently as they should or used to. These blockages must be cleared before the overheating gets to a critical point.

Heating Sludge

This is basically the build-up of debris inside the valves and pipes of the central heating system. And when it is cleared, you will notice that it is basically in the boiler, in the form of dirty water.

When this dirty water contains debris like rust, it has the ability to clog the system. This creates pressure and if neglected, it can even blow up the seals of the pump.

And if the pressure release valves or PRVs are not functioning properly, the boiler ends up getting overheated unless you have a mechanism to shut it down automatically.

Condensate Blockages

But the problems are not just on the outside. If you notice that your boiler is overheating and this happens in the dead of winter, it is possible that this is because your condensate pipe is frozen.

This pipe is used to vent gases from the inside of the boiler to the outside. If this is happening throughout the year, then it is likely that the condensate pipe is clogged.

Pump Fault

Now, this is a factor that is actually quite overlooked although it shouldn’t be. Your pump overheats and results in the entire boiler being overheated, usually, if it is seized.

How do you know that? Well, pumps are motorized and they are supposed to be warm, not hot. So, if your pump is so hot that you cannot touch it, it means that internally, there is a seizure.

Sometimes, you just need to tap on it to stop it, but other times, you need to open it and clean it.

Thermistor Issue

This one is a very important component. A thermistor works along with the printed circuit board or PCB of the boiler, which is like the heart of the system. It tells the PCB when the temperature requirements are too much or too little. So, if it fails it is possible that there is too much fuel in the system which is making the water too hot.

Pressure Release Valves

There is the question of the pressure release valves which, as the name indicates, are meant to relieve the pressure when the boiler gets too hot. So, if your boiler is getting overheated, this is one of the first things to check.

However, some of these PRVs are actually infamous for leaks. Make sure that the engineer does not remove its seal to curb the leak and you might just be fine. That only leads to more overheating and possibly an explosion.

Why Does My Boiler Overheat and Then Cut Out?

There are several reasons for this. And here’s roughly how you can identify the problem with your boiler.

  • If the water pressure is low, the pressure gauge will be under one bar.
  • If it is an airlock, you will not get water from hot water taps and radiators might not heat up properly.
  • If it is not getting enough fuel, your meter might be the problem. You might also want to check your fuel supply pipes.
  • If it is a blockage, you will notice whistle noises from the heating system. And the radiators will act out.
  • If it is a faulty pump, you will notice funny noises and your central heating system will not heat up as quickly as it used to.
  • If the condensate pipe is frozen, the outside of the pipe might not have been insulated and hence the cold weather got to it.

Why Is an Overheating Boiler a Problem?

If your system can detect it and shut down on its own, you won’t have a heat supply. If it doesn’t and the overheating continues, the pressure in the boiler increases and the components of the boiler get damaged due to the excess heat. But worse can happen too.

Can an Overheated Boiler Explode?

Indeed. This happens because there is too much pressure inside the boiler and the PRV has also failed. And while modern boilers can handle up to 20 PSI, it can get dangerous if there is no auto-shutdown option.

Explosions are also a possibility if the boiler has been repaired or handled by inexperienced engineers. Wiring problems and handling gas valves manually are just a couple of examples that could cause you some serious trouble.

How to Fix an Overheating Boiler

Ideally, you should get a gas safe engineer to do this for you. And this is what you can expect them to work on.

  • If there is a blockage in the central heating system, they will power flush the system. 
  • If the clogging is in the condensate pipe, they will defrost it. You must make sure you insulate it for the future.
  • If there is limescale on the heat exchanger, it needs to be cleaned with a descaler made for boilers. This one takes time.
  • If your pump or thermistor is the problem, they will check if it can be repaired or needs to be replaced.

How to Prevent a Boiler from Overheating

The first step is to take care of maintenance on a regular basis. This keeps overheating problems at bay to a great extent. If you need help, contact your manufacturer and ask for a gas safe engineer’s input.

This is particularly important for old boilers and ideally, work on replacing them with newer models with safety features if you can. But for specific problems, here’s what you can do.

Hot Flush Your Boiler

This is one of the fastest ways to remove sludge from the boiler.

  • Step 1: Put a dust sheet and the floor to protect it from dirty water.
  • Step 2: Remove one radiator and insert the power flush nozzle.
  • Step 3: Block the cold feed and the expansion pipe if yours is an open-vented system.
  • Step 4: Take a hose and connect one end to the valve and the other to the radiator. Secure it.
  • Step 5: Connect the overflow hose to the boiler and take it to a flush point outside.
  • Step 6: Locate the last valve and connect it to the dump hose.
  • Step 7: Open the valves and switch on the machine. Let it run for about 10 minutes and reverse the water flow regularly with the lever on the boiler.
  • Step 8: Turn the dump lever straight and let the dirty water out of the system. Simultaneously, open the inlet valve and allow clean water inside.
  • Step 9: Leave the water running till the clean water comes out of the overflow hose.
    After this, you shut the dump valve and push the power flush back to circulation mode. Pour in the cleaner recommended by the manufacturer and run it for another 15 minutes while reversing the water flow.
  • Step 10: Turn off all but one radiator valve and run the power flush for five more minutes. Do the same for the next radiator till they have all been flushed.

Central Heating Inhibitor

This is a chemical liquid that you can add to your central heating system to make sure you get tipped off when the boiler is overheating.

Install a Scale Reducer

As the name suggests, this is a scale that helps reduce the amount of limescale build-up on your heat exchanger. This way, you can keep tabs on the build-up before you get to an overheating problem.

Magnetic System Filter

Finally, a magnetic system filter helps stop the dirty water and rust kind of debris from building up in the heating system.

These are all some of the simplest things to do and they save you a ton of money in repairs along with lethal consequences.

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Last Updated on June 4, 2021


Author: Indra Gupta

Indra is an in-house writer with a love of Newcastle United and all things sustainable.

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