Boiler making a banging or kettling noise
A boiler that bangs, rumbles or sounds like a kettle boiling is telling you that water is not moving through it properly. It is worth acting on rather than living with.
What it usually means
The noise engineers call kettling is exactly what it sounds like: water boiling inside the heat exchanger. It happens when water moves through the exchanger too slowly to carry the heat away, so instead of warming steadily it flashes to steam against the hot metal and collapses again. That is what produces the rumbling and banging.
Two things cause it. Scale, if you are in a hard water area, coats the inside of the exchanger and insulates the metal from the water. Sludge, which happens everywhere, restricts the waterways so less water passes through in the first place. Both have the same effect and both get worse.
It is worth acting on. A boiler that kettles is repeatedly heating a component past its design temperature, and the heat exchanger is the most expensive part in the appliance. Living with the noise for two winters is how a £400 system clean becomes an £1,100 exchanger replacement.
How to narrow it down
Work through these in order.
Are your radiators cold at the bottom and hot at the top?
That is sludge. Magnetite settles in the bottom of radiators and restricts flow through the entire system, including the heat exchanger. The fix is a proper chemical clean or powerflush, a magnetic filter, and fresh inhibitor.
Do you live in a hard water area and is your hot water weaker than it used to be?
That is scale. It builds up in the plate heat exchanger and the main exchanger, degrading hot water first and eventually causing kettling. Descaling plus a scale reducer on the incoming main is the answer.
Does the noise happen only when the heating is on, or on hot water too?
Heating only points at the heating circuit and system sludge. On both circuits points at the main heat exchanger itself, which is the more serious of the two.
Is it more of a gurgle than a bang?
Gurgling is usually air rather than boiling. Bleeding the radiators and checking the system pressure often clears it, and it is a much less serious symptom.
Codes commonly shown with this
- F.20VaillantThe overheat thermostat has tripped because the boiler got too hot. It has shut down to protect itself and it should not be restarted repeatedly.Stop — call an engineer
- E9 219Worcester BoschThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Stop — call an engineer
- E02AlphaThe overheat thermostat has tripped. Almost always this means water is not moving through the appliance properly rather than that it is running too hot.Get it checked
- 101AristonThe heating circuit has overheated and the appliance has shut down. The usual reason is that water has stopped moving, not that it is firing too hard.Get it checked
- E110PottertonSafety thermostat operated — the primary water overheated — on every Potterton code display. One reset is permitted (5 s knob or 1–3 s button); a second E110 is a circulation fault for an engineer.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£400–£900
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.