Which method is not used for heating a gravity tank?

The method that is not typically used for heating a gravity tank is heating with electric coils. Gravity tanks, which are often used in water supply systems, primarily rely on methods that involve direct interaction with the heating medium, like steam or hot water, to effectively raise the temperature of the stored fluid.

Using steam coils placed inside the tank is a common way to heat the water, as the coils can efficiently transfer heat from the steam to the water surrounding them. Similarly, discharging steam directly into the tank can rapidly increase the water temperature by introducing a significant amount of heat energy directly into the tank's contents. Circulating hot water is another effective method as it maintains temperature by continuously moving heated water through the tank, ensuring even distribution of heat.

In contrast, heating with electric coils presents several challenges for gravity tanks. This method is generally more appropriate for smaller, localized applications rather than larger systems like gravity tanks, where indirect heating methods are favored due to their efficiency and effectiveness in handling larger volumes of water. Thus, the use of electric coils is not a conventional approach for heating gravity tanks.