Central heating making gurgling or bubbling noises

Gurgling and bubbling noises in a central heating system are almost always caused by trapped air in the pipework or radiators. Air enters the system through the feed-and-expansion tank (open-vented systems) or through micro-leaks in sealed systems.

Start by bleeding all radiators — begin with the ground floor radiators first, then upper floors. Use a radiator bleed key on the air vent at the top corner of each radiator. Open it slowly (anti-clockwise) until air hisses out, then close it as soon as water appears. Work through every radiator in the house.

If gurgling persists after bleeding, the noise may be coming from the pipework under floors or in walls — air pockets trapped in low points of horizontal pipe runs. These can be difficult to purge without professional power flushing equipment.

Check the feed-and-expansion cistern in the loft (open-vented systems) — it should have 150–200mm of water in it. If it is very low or empty, the system is losing water and drawing in air as replacement. Top it up and investigate where the water is going.

On a sealed (pressurised) system, if bleeding does not help and the gurgling is close to the boiler, the heat exchanger may be partially scaled. A system flush with a descaler chemical (Fernox DS40, for example) added to the inhibitor may resolve it.

If gurgling is accompanied by the boiler firing and cutting out repeatedly, or overheating noises, this is more serious — call a Gas Safe engineer to assess the system.