hot water heater
noun
°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).
"1977, Kaiman Lee, Encyclopedia of Energy-Efficient Building Design: 391 Practical Case Studies - This [warm-air heated] house has a supplemental electric hot water heater and an oil furnace."