The most common cause is kettling — a build-up of limescale or sludge on the heat exchanger that restricts water flow and causes it to overheat and produce steam, creating that distinctive banging sound.
A second common cause is a failing circulation pump. When the pump begins to deteriorate, water flow through the system becomes inconsistent or restricted. Without adequate circulation, heat cannot be distributed away from the heat exchanger effectively — causing localised overheating and the resulting banging or thumping noise. A pump running at reduced capacity will often make the boiler work harder and run noisily before failing altogether.
Other causes include low system pressure, loose pipes expanding against joists, or air trapped in the system.
Look at the gauge on your boiler. It should read between 1 and 1.5 bar when cold. If it's below 1, you need to repressurise it.
Trapped air causes banging. Turn off the heating, let it cool, then use a radiator key to bleed each radiator until water (not air) comes out.
If kettling is the cause, add a central heating descaler to the system. Run for 1-2 hours then flush.
Pipes expanding when heated can bang against joists. Feel along accessible pipes when the heating is on to identify movement.
If sludge is the issue, a power flush clears the entire system. This is best done by a professional.
⚠️ Important: If you smell gas or suspect a gas leak, do not attempt any fixes. Leave the property immediately and call the National Gas Emergency line: 0800 111 999.
💡 Pro tip: Adding a magnetic filter (like a Magnaclean) to your system catches sludge before it builds up and can prevent kettling from recurring.
Describe the noise and symptoms to Reslvr and get an instant AI diagnosis and step-by-step fix guide.
Get instant diagnosis — free →