Why does my kettle switch off before boiling?

Usually limescale on the element. Scale insulates the element, so it overheats locally and trips the thermostat early even though the water has not boiled. The other common cause is a partly blocked steam channel misdirecting steam onto the bimetallic strip too soon.

Why it happens

A kettle has no temperature sensor in the water. It works by routing steam from the spout down a channel onto a bimetallic strip — two bonded metals that expand at different rates. The strip bends as it heats and trips the switch. It is purely mechanical, which is why kettles work with no electronics at all.

Scale breaks this in two ways. It insulates the element, so the metal runs far hotter than the water and can trip a secondary thermal cut-out. It also changes how and where steam forms, which alters when the strip sees it.

If descaling does not fix it, suspect the steam channel or the strip itself. Both sit inside the housing, and most kettles are assembled with clips that are not designed to be reopened.

What to do about it

1. 1Descale first — white vinegar or citric acid, left to work, then rinsed thoroughly. This resolves the majority of cases.
2. 2Check the lid closes fully and seals. An open or badly seated lid vents steam away from the channel and produces the opposite fault, where the kettle never switches off.
3. 3Descale on a schedule rather than waiting for symptoms. Scale makes the element run hotter, which shortens its life as well as causing this fault.