Boiler Making a Banging Noise

Which bang is your boiler making?

Stand near the boiler for one full heating cycle, from the moment the thermostat calls for heat to a few minutes after it stops. Most people can place the bang in one of four groups, and each has a different cause and a different level of urgency.

• One sharp bang or thud each time the burner lights, sometimes with a puff of air from the flue: delayed ignition. Treat as urgent.
• Repeated bangs, pops or a rumbling like a kettle while the heating or hot water is running: kettling. Not dangerous today, but it gets worse.
• Bangs or cracks from under the floor or behind walls as the system heats up or cools down: pipes expanding against joists and clips.
• A single thump in the pipes when a tap or the heating valve shuts: water hammer.

A bang when the boiler lights: delayed ignition

Normally the spark and the gas arrive together and the burner lights with a soft whoosh. If the spark is late or weak, or the gas and air mix is wrong, a small amount of gas builds up before it catches, and it lights all at once with a bang. You may see the casing jump slightly or hear a puff from the flue outside.

The usual causes are a worn or badly gapped ignition electrode, a dirty burner, a gas valve that is opening incorrectly, or a partial blockage in the flue or air intake. None of them is something you can check yourself, because all of them are inside the sealed combustion part of the boiler.

This is the one banging noise not to live with. Each ignition bang puts stress on the combustion chamber seals, and a damaged seal is how combustion gases can escape into the room. Turn the boiler off at the switch next to it, keep a working carbon monoxide alarm in the room, and get an engineer to look at it. If you smell gas, open the windows, turn off the gas at the meter if it is safe to, avoid anything that could spark, and call the National Gas Emergency Service on 0800 111 999 from outside.

Gas work is carried out by Gas Safe registered engineers. Never take the boiler casing off to look at the burner or electrode.

Repeated banging while it heats: kettling

Kettling is by far the most common reason a boiler bangs, and the same fault is behind most whistling boilers. Limescale from hard water, or black sludge from corroding radiators, coats the inside of the heat exchanger. The water trapped against that hot, insulated surface boils into steam bubbles that collapse with a bang as they meet cooler water. It sounds alarming, but the overheat thermostat and relief valve are there to shut the boiler down safely, so an explosion is very unlikely.

Kettling points to one of three underlying problems, and it is worth telling the engineer which fits:

• Scale: common in London’s hard water, especially if the system has been drained and refilled often or has a slow leak that means it keeps being topped up with fresh mains water through the filling loop.
• Sludge: likely if radiators are cold at the bottom, bleed water comes out black, or the system has never been flushed.
• Poor flow: a pump that is failing or set too low, or most radiator valves closed, so water sits in the heat exchanger too long.

What you can safely do tonight

If the bangs are the kettling kind and there is no smell of gas, leak or lockout, these checks can quieten the boiler until someone looks at it. All of them are on the outside of the boiler.

• Check the pressure gauge. With the system cold it should read between 1 and 1.5 bar on most boilers. Low pressure makes kettling worse, so top up if it is below 1 bar.
• Open the thermostatic valves on the radiators, at least in the rooms you are using, so water has more places to go.
• Turn the boiler’s heating flow temperature down a notch. A slightly cooler heat exchanger kettles less.
• Bleed any radiators that are cold at the top, then check the pressure again. See our guide on whether to bleed radiators with the heating on or off.
• If the bangs are in the pipes under the floor rather than at the boiler, turn the flow temperature down too. Slower heating means less sudden expansion.

Do not try descaler products in the heating system yourself. Adding chemicals without draining and flushing properly can loosen sludge that then blocks the heat exchanger or pump.

How an engineer fixes a banging boiler

For delayed ignition, the engineer strips and inspects the burner and electrode, checks the electrode gap against the manufacturer’s figure, measures the gas working pressure and runs a flue gas analysis to confirm the boiler is burning cleanly. The fix is usually a new electrode or a burner clean, and occasionally a gas valve.

For kettling, they first confirm it is the heat exchanger and not the pump, then choose between a chemical clean and a power flush depending on how bad the sludge is. A magnetic filter on the return pipe and fresh inhibitor stop it coming back. On a combi with hard water, fitting a scale reducer on the cold feed protects the hot water side.

Typical UK market prices, parts and fitting included, are often £80 to £200 for an ignition electrode, £250 to £500 for a chemical clean with a new magnetic filter, and £400 to £800 for a full power flush. These are typical market ranges, not our prices. Book a boiler repair with us and you pay the £60 call-out, and the repair price is agreed with you on site before any work starts.

Banging pipes that are not the boiler at all

If the boiler itself is quiet and the bangs come from the floor or walls, the most likely cause is copper pipe expanding as it heats and catching on a notch in a joist, then slipping with a crack. It is harmless but annoying, and it is the same ticking and creaking you hear from radiators as the system warms up. A plumber can lift a board and pack the pipe with a felt or foam sleeve where it rubs.

A single heavy thud when a tap closes is water hammer, the shock of fast-moving water stopping suddenly. It is often worse on high mains pressure or with quarter-turn taps and appliance valves. If it is loud and frequent, it can loosen joints over time, so treat it as a plumbing job rather than a boiler one. For the wider range of sounds, see our guide to what each boiler noise means.