small language problem

My aunt has a smallish water container attached to her bathroom wall, which heats the water (so if you use too much hot water at once, it runs out).
It was always referred to as a "boileri" which is basically the word boiler, only modified a bit into Finnish. BUT, would a thing like that actually be called a boiler in English?
Ah, a water heater, apparently. *goes to correct* :)