Category - Boilers
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Will Scholfield, Engineer

19 Sep : Updated 8 Nov ● 5 min read

What is boiler kettling?

Do you have a strange noise coming from your boiler? If so, your boiler might be experiencing what is known as ‘boiler kettling’.

We've already covered boiler noises in another blog, but we're going to cover kettling in more detail.

So what is boiler kettling, what causes boilers to kettle, and what can you do to fix it? We're going to walk you through everything you need to know.

What exactly is boiler kettling?

Before determining the cause of the boiler kettling sounds, it's a good idea to know what boiler kettling actually is. Boiler kettling is a whistling, bubbling or popping noise that comes from your boiler whilst it's running.

The hot water inside a boiler should not boil. But if something was to cause the water flow inside the system to slow down, there’s a chance that the water could well boil. This is when the boiler kettling noise occurs - just like a boiling kettle.

Before coming up with a solution, you'll need to know what's causing this boiler kettling noise.

Common causes of boiler kettling

There are three likely causes of boiler kettling: overheating, limescale buildup or a leak. We'll walk you through how to identify the cause of your boiler noise and what can be done to fix the problem.

Cause 1: Overheating

If your boiler keeps cutting out and making strange kettling noises, a likely cause is overheating. Often, an overheating boiler is down to a faulty thermostat. If you have a faulty thermostat, take a look at our other guide on why your thermostat may not be working to help find the solution.

Overheating can also be caused from blocked/partially blocked pipework that can happen over time depending on the age of the system. If you have a blockage then you potentially would need the system looking at and a powerflush may be required to remove and clear the blockage.

The kettling noise occurs when a boiler is overheating because the inside of the heating system is getting too hot. The hot water in the boiler system turns into steam, creating the strange kettling noise you can hear coming from your boiler.

Cause 2: Limescale buildup

Another reason for boiler kettling could be limescale buildup. No boiler is immune from limescale buildup. But even though it is a natural occurrence, it does need to be dealt with.

If the limescale begins to cause a blockage, the heat and water will only have so much space to move around, which can cause your boiler to make a kettling noise.

Limescale buildup can also cause blockages in the system to the extent that some radiators or appliances aren't working as well as they once did.

Thankfully this isn't a safety issue, but it will need flushing to release any buildup.

If you live in a hard water area, you're much more likely to have limescale buildup in your heating system, as hard water contains more minerals.

Cause 3: A leak

A boiler leak can also cause kettling, as the system will try to balance the water flow and pressure. If you think this could be the case for your boiler, you'll need to check for leaks. The leak could be close to the boiler unit or anywhere throughout the pipework.

Your central heating system could be leaking for several reasons, from poorly installed pipework to general corrosion.

How to fix a kettling boiler

The right solution for the boiler kettling will depend on what is causing the noise to occur in the first place. If you've read the list of causes above and think you've figured out what might be causing your boiler to make strange kettling sounds, read on to find out what can be done to stop your boiler from kettling.

Solution 1: How to fix an overheating boiler

If you think the cause of your boiler kettling might be overheating, check the settings on your thermostat. If you're confident enough, adjust or reset the thermostat to bring your boiler back to a suitable temperature.

The safest option would be to call a heating engineer to find a solution. If your thermostat is broken, you will need a boiler repair or replacement.

Solution 2: How to remove limescale buildup in a boiler

If the boiler kettling noise is a result of limescale buildup, you'll need to get in touch with a heating engineer. They can perform a system flush to clear your system of debris or limescale.

A chemical flush uses high-strength chemicals to corrode the debris, whereas a power flush uses pressurised water to do the same.

If the boiler kettling noise doesn't stop, there will be a different cause elsewhere in the system.

Solution 3: How to fix a boiler leak

If you suspect a boiler leak, you'll need a Gas Safe registered engineer to solve the problem. If you attempt to solve the problem yourself, not only could you be putting yourself in danger, but you could also invalidate your boiler warranty or guarantee.

The heating engineer will be able to inspect the entire central heating system and pipework for any potential leaks causing the boiler kettling.

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Is boiler kettling dangerous?

Boiler kettling doesn't usually indicate immediate danger, but finding a solution as quickly as possible is wise. The high pressure in the boiler could eventually burst internal pipework. However, modern boilers have built-in safeguard features to shut down the boiler before it reaches this point.

Waiting to fix a problem with your boiler could lead to a much bigger issue later down the line, which could be much more costly. If you notice that your boiler is making a kettling noise or not working as well as it once did, it's best to get in touch with a Gas Safe registered engineer as soon as possible.