Why is my boiler gurgling?

Written by ian kelly
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Why is my boiler gurgling?
Boiler kettling is caused by limescale build-up inside the heat exchanger. (Comstock/Comstock/Getty Images)

Unlike industrial steam boilers, domestic combi boilers and regular hot water boilers are designed to heat up water via a heat exchanger to below its boiling point. However, limescale deposits and water-born sludge eventually clog heat exchanger outlets. This tends to restrict flow, causing trapped water to reach boiling point and generate pressurised steam in a process known as ‘kettling’. This phenomenon not only emits the characteristic gurgling sound of boiling water, but it also greatly reduces boiler efficiency.

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Apart from replacing the heat exchanger, a partial solution is to appoint a qualified heating engineer to either install a mechanism know as a ‘filler loop’ into the water feed line, or to install a specialized radiator in the heat exchanger plumbing. This feeds an acidic chemical or ‘system restorer’ to the boiler feed water. Although both methods retard limescale build-up, they do not remove heavily encrusted deposits from the system. The major disadvantage is that acidic chemicals circulating throughout the system can eventually cause pinhole leaks unless the entire heating circuit is periodically flushed with clean water.

Scale inhibitor

The second solution is to have your heating engineer install a dual technology magnetic scale inhibitor to your system. This consists of a magnetic bar that induces single polarity to the water. This single polarity causes magnesium and calcium particles to repel each other, thereby preventing limescale build-up on the interior heat exchanger walls. A mechanism containing parallel zinc plates encased in a copper tube is also installed downstream of the heat exchanger to induce inter-granular corrosion. This causes the zinc plates to release minute particles designed to inhibit corrosion by preventing air bubbles forming.


If you live in one of the UK's hard water areas, the best solution is to have the system powerflushed once a year. To carry out this procedure, the heating engineer first drains the entire household system and connects a high-velocity, low-pressure powerflush machine to the hot water circuit. Neutralising chemicals are then introduced to loosen encrusted sludge and limescale. Continuous high-pressure flow then causes the solution to flush out all traces of debris, leaving the inside walls of your heat exchanger, radiators, and pipes, clean and smooth.


Annual powerflushing is a long-term cost-effective solution to kettling. It restores boiler and central heating efficiency, reduces circulation pressure, improves heat transfer efficiency, and accelerates radiator heat-up time. The end result is not only a rapidly heated home during those harsh winters, but it also provides a substantial reduction in the amount of gas burned. Finally, regular powerflushing extends boiler life.

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