Hot water cylinder making a banging or popping noise

Banging or popping from a hot water cylinder is most commonly caused by one of two things: scale build-up on the immersion heater element (a similar effect to a kettle kettling) or expansion noises as the copper cylinder heats up.

Scale on the immersion heater element: in hard water areas, calcium deposits coat the element surface, causing localised boiling and the popping noise. Descaling the element is possible but often a full element replacement (£15–£40 for the element, 1–2 hours labour) is more practical.

Thermal expansion clicking: as copper heats up it expands, and if the cylinder is tightly clipped or in contact with a pipe or wall bracket, it can click loudly. Check that the cylinder is sitting correctly in its cradle and that hot water pipes have some flexibility at the connections. A pipe that is too rigidly fixed against a timber joist will click and bang as it expands.

If the cylinder is an open-vented type (hot water vent pipe runs up and over into the cold water tank), check the vent pipe is clear and not blocked — a blocked vent can cause pressure build-up and banging.

Check the thermostat setting — an immersion thermostat set above 70°C can cause excessive boiling noise. The optimal setting is 60°C (hot enough to kill Legionella bacteria, cool enough to prevent excessive scale).

If the cylinder is old (15+ years) and repeatedly noisy, scaling and corrosion may be severe. A replacement cylinder (vented copper cylinders start at £150–£250; unvented cylinders £400–£900) may be the most cost-effective long-term solution.