Boiler kettling

What it usually means

Kettling is the one boiler noise that is named after exactly what it is. A whistling, rumbling or bubbling sound that rises as the boiler heats up, coming from the appliance itself rather than from the pipes, is water boiling in small pockets and then collapsing back into liquid. The sound is the collapse. Being able to identify it matters, because the causes and the urgency are quite different from the other noises a heating system makes.

The mechanism starts with how little water is in contact with the burner at any moment. A modern heat exchanger passes water through narrow passages across a very hot surface, and it relies on that water moving briskly enough to carry heat away as fast as the burner delivers it. Slow the water down, or put a layer of something between the water and the metal, and the water touching the hottest part reaches boiling point locally while the rest of the system is nowhere near it. It flashes to steam, the bubble is swept into cooler water further along, and it implodes. Thousands of times a minute, that is a kettle.

So there are only two families of cause, and they are worth telling apart because the remedies differ. The first is restricted flow: magnetite sludge in the exchanger or lying in the bottom of radiators, a circulating pump set to too low a speed or failing, thermostatic valves all closed at once with no bypass open, a blocked filter, or air. The second is insulation on the metal, which in practice means limescale. Scale comes out of solution wherever water is hottest, so it forms precisely on the surface you need to keep clear, and it arrives with fresh water — which is why a system that has been topped up repeatedly in a hard water area kettles sooner than one that has not.

That connection is worth following up. If you have been adding water to the system every few weeks, you have been adding scale-forming minerals and dissolved oxygen with it, and the oxygen makes the sludge that causes the other family of cause. Kettling on a system that keeps losing pressure is usually two symptoms of one problem, and fixing the leak is part of fixing the noise.

Two other noises get called kettling and are not. A single bang or a whoomph as the boiler lights is delayed ignition, which is a gas fault and belongs in a different and more urgent category — that one is a stop and call. Ticking or creaking from under floors as the heating comes on is pipework expanding against a joist or a clip, which is annoying and harmless. Kettling proper is continuous, follows the burner, and gets worse the hotter you ask the boiler to run.

Which points at the one adjustment worth making while you wait for someone. Turning the flow temperature down — the dial or setting that decides how hot the water leaving the boiler is, not the room thermostat — reduces the local boiling directly, and on a condensing boiler it usually improves efficiency at the same time. It treats the symptom rather than the cause, but it buys time without doing harm.