The short answer
A boiler that bangs, whistles or sounds like a kettle coming to the boil is usually suffering from kettling: water in the heat exchanger is overheating in spots because limescale or sludge is blocking its narrow waterways. Other causes are trapped air, low pressure, a failing pump and a faulty thermostat. Check the pressure, bleed the radiators, and book an engineer if the noise continues. A chemical or power flush often fixes it, though a badly scaled heat exchanger may need replacing. London repair costs run from £90 to £750.
What the law requires
What is kettling?
Kettling is the name engineers give to the rumbling, banging or whistling sound a boiler makes when steam bubbles form and collapse inside the heat exchanger. Water passes through very narrow channels. When limescale or magnetite sludge narrows them, water slows down, the metal gets hotter than the water around it, and small pockets boil. The bubbles collapse with a noise that sounds like a kettle on the boil, or like knocking.
It is more common in London than elsewhere because the water is hard, and because many older systems have had no inhibitor and no filter. It also tends to get worse over a few months rather than appearing overnight.
Which noise do you have?
Banging or knocking when the burner fires. Often kettling, or sometimes delayed ignition.
A high whistle. Low flow through a restricted heat exchanger, a partly closed valve, or a pump running too fast. It may also be a radiator valve passing water at speed.
Gurgling or bubbling. Air in the system, or low pressure, or a partially blocked condensate pipe.
Clicking or ticking as it heats and cools. Pipework expanding against brackets. Usually harmless.
A loud whooshing or roaring at start-up. A different problem: a fan, ignition or flue issue. Book an engineer.
A humming or grinding that is constant. The pump, which may be failing.
What you can safely check yourself
None of these involves opening the boiler case.
- Check the pressure gauge. It should be between 1 and 1.5 bar cold. Low pressure causes noise. Our guide on a boiler losing pressure shows how to top up.
- Bleed the radiators, starting with the one furthest from the boiler, and check the pressure again afterwards.
- Turn the flow temperature down a little if your boiler has a dial. A lower flow temperature reduces localised boiling and can quieten kettling while you wait for a repair.
- Check the radiator valves are open and the thermostatic valves are not stuck.
- Listen for when it happens. Only at start-up, only on hot water, only when the heating has been on for an hour. Tell the engineer. The pattern is a diagnostic clue.
If the noise continues, book a visit. Ignoring kettling lets scale build, and a scaled heat exchanger is the most expensive repair on a boiler.
Rumbling boiler in a hard-water postcode? Get an honest assessment before anyone sells you a full flush.
What an engineer does
First the pressure and the pump are checked. Then flow and return temperatures are measured with the boiler running. A gap that is too wide means water is not moving fast enough. Next the system water is tested and the filter or drain-off inspected for black sludge. If scale is the suspect, the heat exchanger is inspected or tested.
The solution depends on the diagnosis.
- Air or low pressure: bleed and top up. Under £130.
- Stuck pump or wrong pump speed: adjust or replace. £220 to £420.
- Sludge in the system: a chemical flush or a power flush, with a magnetic filter and fresh inhibitor. See our central heating power flush page.
- Limescale in the heat exchanger: descale chemically, which works where scale is moderate and the heat exchanger is accessible, or replace the heat exchanger.
Will a flush fix it?
Often, yes. A power flush clears sludge from radiators and pipework, which restores flow. A chemical descale of the heat exchanger can bring back much of the lost water flow. But there is an honest caveat. Where scale has built for years and the waterways have narrowed badly, a flush may reduce the noise without silencing it, and a replacement heat exchanger or boiler is the real fix. A good engineer will tell you that before charging you.
How much does it cost in London?
Fixed price quoted after diagnosis.
- Bleed, top-up and flow adjustment: £90 to £130.
- Chemical flush and inhibitor: £150 to £260.
- Power flush: from £350, depending on the number of radiators.
- Magnetic system filter fitted: £130 to £220.
- Pump replacement: £220 to £420.
- Heat exchanger descale: £180 to £320.
- Heat exchanger replacement: £450 to £750.
On a boiler over twelve years old, a heat exchanger replacement is the point at which a new boiler becomes a fair comparison. See the repair or replace guide.
Is a noisy boiler dangerous?
Usually not. Kettling is a sign of wear and reduced efficiency, not an immediate hazard. Two exceptions: a loud bang followed by a lockout, and any noise accompanied by a smell of gas, fumes or burning. Switch the boiler off and call an engineer, or the National Gas Emergency line on 0800 111 999 if you smell gas.
How to stop it coming back
Fit a magnetic filter. It catches the black sludge that causes much of the noise.
Add inhibitor at every service. The annual service should test the inhibitor level, not just assume it.
If you live in a hard water area, ask whether a scale reducer on the mains feed makes sense for your property.
Noisy boiler, quick answers
Why does my boiler sound like a kettle?
Limescale or sludge in the heat exchanger is causing localised boiling. It is called kettling.
Is a banging boiler dangerous?
Rarely, but it is a sign of wear and should be checked.
Why is my boiler whistling?
Restricted water flow, a pump issue, or a partly closed valve.
Can I fix kettling myself?
You can check pressure and bleed radiators. A flush or descale is a job for an engineer.
Does a new boiler stop the noise?
If the system is dirty, no. A new boiler on a sludged system will start kettling again. Clean the system first.