What "kettling" actually is
The name comes from the sound — a rumbling, popping noise similar to a kettle coming to the boil. It happens when limescale or sludge builds up on the heat exchanger, restricting water flow. Water trapped in those restricted areas boils and turns to steam before it should, and that's the noise you're hearing.
Is it dangerous right now?
On its own, the noise isn't an immediate safety emergency the way a gas smell is. But it's a genuine warning sign, not just an annoyance — the underlying build-up that causes kettling puts real, ongoing stress on the heat exchanger, and left unaddressed it can lead to cracks or more serious failure over time. It's worth treating as "get this looked at soon," not "get this looked at eventually."
Other noises, briefly
- Banging or clunking when the heating fires up is often trapped air — see our no heating page for safe bleeding steps.
- A buzzing or humming can point to a pump starting to wear.
- A whistling sound can indicate restricted flow, similar territory to kettling.
What we'd do about it
Diagnosis starts the same way as any fault — £99, including the first hour — to confirm what's actually causing it. Depending on severity, that might mean a power flush to clear the build-up (from £550), or in some cases a magnetic filter fitted to help prevent it recurring (£320). See our cost guide for the fuller picture, or our problem finder if something else is also going on.