Could be a munging of "water heater" with "hot water tank", since the most common type of water heater seems to be the storage water heater, which consists mainly of a large tank.
At the end of the day, it's just a name. It doesn't have to "make sense".
"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons