E350 vs E450 chassis pros and cons for a short 24 ft class C

Alternately, you can just realize that a hot water heater is the heater for the hot water system, just as the engine block heater is the heater for the engine block and a space heater is a heater for the space enclosed in a room.

One may equally well ask why residential hot water heating systems (not potable water heaters, but hydronic room heating) have what is generally called a "boiler" even though the water never ever reaches a boiling temperature....