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Longman Dictionary of Contemporary English
immersion heater
noun
EXAMPLES FROM CORPUS
Immersion heater spanner Electric immersion heaters fit into a large screwed boss on the top or side of a hot water cylinder.
▪ Built-in airing cupboard incorporating lagged hot water tank and immersion heater.
▪ Separate airing cupboard with foam dipped hot water cylinder, immersion heater and timer for boiler.
▪ Simply switch to the Economy 7 tariff and update your immersion heater with an inexpensive Maxistore conversion package.
▪ You can hire the correct immersion heater spanner, but their cheapness probably makes buying one a better answer.
Wiktionary
immersion heater

n. A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water

WordNet
immersion heater

n. a heating element that is immersed in the liquid that is to be heated (as in a hot-water tank)

Usage examples of "immersion heater".

Frau Hufnagel doesn't grasp anything at all, but does the right thing: presuming the intruder in her fragile Christmas room to be a slave laborer from the East, now masterless, she holds out, with bravely trembling hands, the brand-new immersion heater, expecting the brute to blow off steam by demolishing this household utensil.

Now I remembered giving the cigarettes to the plumber who fixed the immersion heater.

While she was there she switched on the electric immersion heater, thinking how, piece by piece, a home was brought back to life.