hot water heater

noun1

1. •
   An apparatus, usually fuel-burning, electric or solar-powered, in which water is heated to be used directly as domestic hot water or process hot water (but not for supplying space heating, calorifiers etc, for which a boiler or furnace is used).

   • This [warm-air heated] house has a supplemental electric hot water heater and an oil furnace. — 1977