What the banging noise usually means
Most boiler bangs come from one of three places: inside the heat exchanger, from the pump, or from the pipework around the house. Working out which one you have is mostly about timing. Listen for when the noise starts, where it seems loudest and whether it changes as the system warms up.
| Sound | When it happens | Likely cause | Who deals with it |
|---|---|---|---|
| Rumbling, popping or kettle-like knocking from the boiler | While heating or hot water runs, worse when hot | Kettling from limescale or sludge | Gas Safe engineer |
| A single loud bang or thump from the boiler | As the burner lights | Delayed ignition | Gas Safe engineer, stop using the boiler |
| Grinding, knocking or buzzing near the pump | When the pump starts or runs | Pump seizing, air in the pump or wrong speed | Bleed radiators first, then an engineer |
| Ticking, creaking or knocking in walls and floors | As the heating warms up or cools down | Pipes expanding against joists or clips | Plumber or heating engineer |
| A bang in the pipes when a tap or appliance shuts off | Right after the water stops | Water hammer on the mains | Plumber |
Kettling: limescale and sludge on the heat exchanger
Kettling is the most common reason a combi boiler starts banging. Water passes through the heat exchanger, where the burner heats it. If the waterways inside have narrowed, water moves through too slowly and parts of it overheat. Small steam bubbles form and then collapse, which makes the rumbling, popping and knocking that sounds like a kettle coming to the boil.
Limescale
In hard water areas, including London, much of the South East and East Anglia, calcium deposits build up wherever water is heated. On a combi, scale mostly forms on the domestic hot water side (the plate heat exchanger on many models), so noise that only happens when you run a hot tap points that way. A scale reducer fitted on the cold mains into the boiler slows this down, and some manufacturers expect one in hard water areas as a condition of the warranty.
Sludge and magnetite
Inside radiators and pipes, steel slowly corrodes and forms a black sludge called magnetite. It collects in the boiler heat exchanger and in the bottom of radiators. Signs include radiators that are cold at the bottom, dark water when you bleed a radiator and a heating circuit that has not had inhibitor topped up in years. Noise that happens mainly when the central heating runs points to sludge on the heating side.
Low pressure makes kettling worse, because water boils more easily at lower pressure. If the gauge reads under 1 bar with the system cold, follow our guide to repressurising a boiler with the filling loop before anything else.
Pump problems
The pump pushes water round the heating circuit. When it struggles, flow drops and the boiler can overheat and kettle, or the pump itself can knock and grind. Common pump causes include:
- air trapped in the pump after the system was drained or topped up, which often knocks or gurgles for a short time
- a pump that is starting to seize, often after a summer of little use or because of sludge
- a pump speed set too high for the system, which can make pipes knock and water rush
- a worn pump bearing, which tends to grind or hum rather than bang
Many boilers show a code when the pump fails, such as Worcester Bosch A1 281 for a pump that is stuck or running dry. Freeing, adjusting or replacing a pump inside the boiler casing is a job for a Gas Safe registered engineer.
Pipework expansion and water hammer
Copper pipes grow slightly as they heat and shrink as they cool. Where a pipe is clipped tightly, runs through a tight notch in a joist or touches another pipe, that movement makes ticking, creaking and sometimes a sharp knock. It is usually heard under floors and in walls rather than at the boiler, and it happens as the heating comes on or goes off. It is annoying rather than dangerous. A plumber can ease notches under floorboards, fit pipe clips with rubber inserts or add lagging where pipes touch.
Water hammer is different. It is a loud bang in the pipes when a tap, shower valve, washing machine or dishwasher shuts off quickly, because the moving water stops suddenly. It happens on the mains cold supply, not the boiler circuit. A plumber may secure loose pipes, fit a water hammer arrestor or lower very high mains pressure with a pressure reducing valve.
Safe checks you can do yourself
- Check the pressure gauge with the system cold. It should read 1 to 1.5 bar. Top up with the filling loop if it is low.
- Watch the gauge while the heating runs. A rise to around 2 bar is normal. A climb towards 3 bar is not, and our guide to boiler pressure that is too high explains why.
- Bleed radiators that are cold at the top, then recheck the pressure and top up if it has dropped.
- Check the thermostat and timer settings. If the noise started after the heating temperature on the boiler controls was turned right up, try the setting your manual recommends.
- Note when the noise happens (heating, hot water or both) and any fault code on the display. This saves the engineer time.
If the boiler has locked out, you can press reset as described in the manual, but stop after 2 or 3 attempts.
When banging is urgent
Turn the boiler off at the controls and call a Gas Safe registered engineer if the banging is loud and getting worse, if there is a single loud bang when the burner lights, if the pressure is heading towards 3 bar, or if water is dripping from the pressure relief pipe outside. If you smell gas, do not touch the controls: follow our steps for when you smell gas near your boiler and call the National Gas Emergency Service on 0800 111 999.
A boiler that kettles badly can trip its overheat protection and show a code such as Vaillant F20. That is the boiler protecting itself, but repeated overheating strains the heat exchanger, seals and pressure relief valve. Do not keep resetting it.
Typical UK repair costs
| Repair | Typical UK cost |
|---|---|
| Chemical clean of the system with inhibitor top-up | Typically £150 to £350 |
| Powerflush | Typically £350 to £800 |
| Magnetic system filter fitted | Typically £150 to £300 |
| Scale reducer fitted on the cold main | Typically £80 to £200 |
| Pump replaced | Typically £200 to £450 |
| Plate heat exchanger replaced (combi) | Typically £200 to £450 |
| Main heat exchanger replaced | Typically £500 to £1,000 |
| Noisy pipes re-clipped or notches eased | Typically £80 to £200 |
Jobs come in at the lower end outside London and the South East, for common brands with parts readily available and for boilers that are easy to reach. They rise with London labour rates (Gas Safe engineers commonly charge around £50 to £80 an hour), large systems with many radiators, loft or cupboard installations and brand-specific parts. If the boiler is still under warranty, contact the manufacturer first, as a repair may be covered if it has been serviced every year. On an older boiler, a new main heat exchanger can cost a large share of a new boiler, so ask for a replacement quote too. An annual service, covered in our boiler service cost guide, is the best way to catch sludge before it gets noisy.