Why Is My Boiler Making a Kettling Noise?

What does kettling sound like?

Kettling sounds like a kettle coming to the boil - a rumbling, banging, or whistling noise coming from inside the boiler. It is most noticeable when the heating or hot water kicks in. It is not dangerous in the short term, but it means something is wrong and needs attention.

What causes it?

The most common cause is limescale building up on the heat exchanger, especially in hard water areas like Surrey. The limescale restricts water flow through the heat exchanger, so pockets of water overheat and turn to steam - that is the noise you hear. Sludge and debris in the system can cause the same problem.

Is it dangerous?

Modern boilers have safety devices that will shut the boiler down if it overheats, so there is no risk of explosion. But kettling means the boiler is working harder than it should, which wastes energy and puts extra strain on the heat exchanger - the most expensive part to replace.

How do we fix it?

For sludge-related kettling, a power flush will clear the system and usually resolves the noise. For limescale buildup, the heat exchanger may need descaling or, in severe cases, replacing. We will diagnose the cause and recommend the most cost-effective fix.

How to prevent it

A magnetic system filter fitted to your boiler catches debris before it reaches the heat exchanger. In hard water areas, a scale reducer on the mains inlet helps too. Combined with annual servicing, these measures will keep your boiler running quietly and efficiently for years.