unfired hot water storage tank
(12) (A) The term “storage water heater” means a water heater that heats and stores water within the appliance at a thermostatically controlled temperature for delivery on demand. Such term does not include units with an input rating of 4000 Btu per hour or more per gallon of stored water. (B) The term “instantaneous water heater” means a water heater that has an input rating of at least 4000 Btu per hour per gallon of stored water. (C) The term “unfired hot water storage tank” means a tank used to store water that is heated externally.