Why is my boiler making a banging or kettling noise?

What is kettling?

Kettling is named after the noise a kettle makes as it comes to the boil: rumbling, popping or banging from inside the boiler. Water in the heat exchanger is getting so hot in spots that it starts to boil and form steam bubbles, which then collapse. It is a sign that heat is not being carried away properly.

What causes a noisy boiler?

Limescale

In hard water areas, scale can build up in the heat exchanger and, on a combi, the hot water side. It acts like insulation and creates hot spots. A scale reducer on the mains feed helps protect a new combi.

Sludge in the system

Sludge from radiators collects in the boiler and slows the flow of water. Cold-bottomed radiators are a clue that this is the cause.

Pump problems

A pump that is failing, set too low or stuck will not move water through the boiler fast enough.

Low pressure or air

On a sealed system, low pressure or air in the boiler can cause noise and lockouts.

Other noises

• Gurgling: often air in the system, or a frozen or blocked condensate pipe in cold weather.
• Whistling: often restricted flow or scale in the heat exchanger.
• Banging pipes when the heating comes on: often loose pipe clips or pipes expanding against joists, rather than the boiler itself.

Safe checks you can do

These checks are safe for a homeowner. Banging or kettling often means the water in the heat exchanger is overheating because flow is restricted (scale, sludge, air, low pressure or a weak pump). Do not open the boiler casing, dismantle the pump or add chemicals yourself.

1. Check the boiler pressure gauge when the system is cold. If it is below the figure in your user guide, top up only as that guide shows. Low pressure can make the heat exchanger noisy. See boiler losing pressure.
2. Bleed any radiators that are cold at the top, then recheck the pressure. Air slows circulation and can add noise. See how to bleed a radiator.
3. Feel a few radiators. Warm at the top and cold at the bottom on several radiators usually means sludge — bleeding will not clear that, and a powerflush or chemical clean is engineer work. See cold radiator guide.
4. In freezing weather, check the condensate pipe outside for ice. A frozen pipe can make the boiler lock out or run oddly; thaw only as the manufacturer allows. See frozen condensate pipe.
5. Note when the noise happens — heating only, hot water only, or both — and whether it is a bang, whistle, gurgle or kettle-boil sound. That helps the engineer.
6. Stop and book a Gas Safe engineer if the noise continues, pressure keeps falling, or a fault code appears. Do not open the boiler casing, dismantle the pump or dose chemicals yourself. Scale and sludge inside the heat exchanger need proper diagnosis.

Is a kettling boiler dangerous?

Kettling is mainly a sign of strain and lost efficiency rather than an immediate danger, but left alone it can damage the heat exchanger and lead to a costly repair or replacement. If the noise comes with a smell of gas, a fault code, water leaking or the pressure climbing past 3 bar, turn the boiler off and get help.

When to call a Gas Safe engineer

The fixes for kettling, such as descaling or replacing a heat exchanger, a pump or internal parts, are inside the boiler and must be done by a Gas Safe registered engineer. A system clean is often part of the answer; see system flush and filters. Book a boiler repair, and if it shows a code, look it up in our fault code search first.

This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.