A boiler should be more or less silent. If yours is not, the specific sound tells you a lot about what is wrong.
Below is what each noise usually means, what you can safely check yourself, and the point at which it needs a Gas Safe engineer.
Banging or clunking, also called kettling
Sounds like a kettle coming to the boil, or someone hitting a pipe with a hammer. The noise comes from the boiler itself.
Caused by limescale build-up, which is very common across West Sussex and Surrey because it is a hard water area, and magnetite sludge restricting flow through the heat exchanger.
When the flow is restricted, the water trapped against the hottest surfaces boils and turns to steam instead of circulating. The banging is those steam bubbles forming and collapsing.
Why it matters: kettling wastes gas, and the thermal stress can eventually crack the heat exchanger, which turns a cleanable problem into a new boiler. A scaled heat exchanger passes less heat into the water, so the boiler runs longer to do the same job and your bills go up while it gets worse.
Fix: a professional powerflush to remove the scale and sludge. In severe cases the boiler has to be stripped down.
Have it diagnosed before you pay for a flush. Restricted flow can also come from a sticking pump or a closed valve, and cleaning the system will not cure either of those.
Whistling or whining
Sounds like a high-pitched whistle, often continuous while running.
Usually trapped air creating turbulence, worn pump bearings, a partially closed valve forcing water through a small gap, or a faulty thermostatic radiator valve.
What each one needs:
- Air in the system. Bleed the radiators and check for leaks.
- A worn pump. The whine gets louder as the bearings wear. The pump will usually need replacing.
- A partly closed valve. Check the radiator valves are open, particularly any you have turned down recently.
- A faulty thermostatic radiator valve. If the whistle is coming from one radiator rather than the boiler, the valve on that radiator is the likely cause and can be replaced.
Try first: bleed the radiators. If that does not resolve it, it needs an engineer.
Gurgling
Sounds like water draining down a plughole. It can come from the boiler or from the radiators themselves.
Usually air in the system, low water pressure, a frozen condensate pipe in winter, or the pump running too fast.
Try first: bleed all radiators starting downstairs, then check and top up the pressure to 1 to 1.5 bar. If it persists, air is getting in continuously and there is likely a leak.
Our guide on how to bleed a radiator covers the bleeding step by step.
If the gurgling started during freezing weather and the boiler has stopped working, check the condensate pipe. It is the white plastic pipe that runs from the boiler to a drain outside. Pour warm, not boiling, water over the frozen section, then reset the boiler.
When the air is coming in through a leak or a faulty component, leak detection can find the source without pulling up floors on guesswork.
Humming or vibrating
Sounds like a continuous low hum, sometimes felt through walls or floors.
Usually loose brackets, pipes or panels, a pump set too high or wearing out, water pressure too high, a failed expansion vessel, or a blocked magnetic filter.
Usually minor on its own. If it comes with low pressure, poor heating or error codes, it is part of something bigger.
One quick check is worth doing. With the heating off and the system cold, the pressure gauge should read between 1 and 1.5 bar. Above 2 bar is too high and puts the whole system under strain. If the pressure climbs sharply every time the heating runs, the expansion vessel is the likely cause.
You can check pipe brackets and clips you can see by eye. Do not remove the boiler casing to look for loose parts inside.
A loud bang when the boiler fires up
This one is different. Do not ignore it.
Sounds like a single loud bang when the heating comes on, almost like a small explosion inside the boiler.
Cause: delayed ignition. Gas builds up and then ignites all at once, usually because burner jets are dirty or blocked, ignition electrodes are worn, or gas pressure is wrong. On older boilers with a pilot light, a problem with the pilot can cause the same thing.
What to do: turn the boiler off, do not attempt to fix it, and call a Gas Safe engineer. This is a safety issue rather than a comfort one.
Do not keep resetting the boiler to see whether it happens again. Leave it off until it has been checked.
If you smell gas at any point, do not operate any switches, including lights. Leave the property and call the National Gas Emergency Service on 0800 111 999.
Tapping or clicking
Sounds like regular tapping or clicking, often with a steady rhythm.
Usually harmless: pipes expanding and contracting with heat, loose pipe clips, or pipes touching joists.
Some clicking is simply the boiler working normally, such as a click as it fires up or as a valve opens and closes.
Occasionally a faulty fan, worn pump bearings, or a diverter valve problem.
Repeated clicking with no ignition, where the boiler keeps trying to light and fails, is an ignition problem. That needs an engineer.
When to call
- The noise is getting louder or more frequent
- You notice any gas smell
- The boiler is also losing pressure
- Bills are increasing
- Heating is less effective than it was
- Error codes are appearing
- The boiler is over eight years old and has not been looked at recently
- Basic checks have not helped
Before you call, note what the noise sounds like, when it happens, and where it seems to come from. As the boiler fires up, while it is running, or as it cools down all point to different faults. Add any error code on the display and the diagnosis gets much quicker.
Preventing it
Annual servicing catches most noise-causing problems before you hear them. A magnetic filter prevents sludge accumulating, and a powerflush every five to six years is sensible in a hard water area like this one.
Annual servicing
During a service the engineer cleans the components, checks for limescale, and adjusts the settings. Worn parts get spotted before they start to whine or rattle. A boiler service is from £105.
Protecting the system water
A magnetic filter catches sludge before it reaches the boiler. Corrosion inhibitor in the system water slows the corrosion that creates the sludge in the first place. Between them they keep the heat exchanger clear, which is what stops kettling coming back after a flush.
Dealing with noises early
A noise that starts small rarely stays small. Getting it looked at when you first notice it is almost always cheaper than waiting until it becomes a breakdown.
Frequently asked
Is it safe to use my boiler if it is making noises?
It depends on the noise. Minor gurgling or clicking is usually harmless. A bang on ignition, loud kettling, or any noise alongside a gas smell needs investigating immediately. If you are unsure, switch it off and call a Gas Safe engineer.
Will a boiler service stop the noises?
Often, yes. Many noise problems are caught and resolved during an annual service, since the engineer cleans components, checks for scale and adjusts settings. It is the cheapest way to deal with most of this.
What does kettling actually mean?
It means limescale and sludge have coated the heat exchanger, restricting water flow. Trapped water boils locally and flashes to steam, and the expanding steam bubbles make the banging noise. It is the same physics as a kettle.
How much does it cost to fix a noisy boiler?
It depends entirely on the cause. Bleeding the radiators costs nothing. A powerflush to cure kettling is from £400, typically £400 to £780 for an average three bedroom home. A worn component such as a pump or a valve is quoted once the fault has been diagnosed, and you get the price before any work starts.