Which is NOT a method for heating water in a gravity tank?

The method that does not typically apply to heating water in a gravity tank is using electric heating pads. Gravity tanks are generally designed to distribute water through gravity flow, relying on methods that can heat larger volumes of water in a system.

Hot water recirculation, direct steam discharge, and passive solar heating are all practical approaches for warming the water stored in a gravity tank. Hot water recirculation efficiently uses existing hot water and keeps it at a usable temperature, while direct steam discharge involves injecting steam to rapidly increase water temperature. Passive solar heating utilizes solar energy to warm the water naturally, making it an environmentally friendly option for such systems.

Electric heating pads, however, are typically used for localized heating applications rather than for heating large quantities of water in a tank. Their scale and application limits make them unsuitable for effectively heating the volume generally stored in a gravity tank configuration. Therefore, this method stands out as the least relevant for the context of water heating in this setting.