An elevated storage tank (or gravity tank) of adequate capacity and elevation is an acceptable source of water for a sprinkler system. The capacity required depends on the intended use of the tank and the number of gallons available from the tank. Standard sizes of elevated steel gravity tanks range from 30,000 to 500,000 gallons.
It’s very important that water stored in the tank doesn’t freeze. During cold weather, water in the elevated tank may become unavailable because of an ice plug in the riser. Such a plug could burst the pipe.
The type of heating method depends on the tank’s height, construction, size, and shape — and on the lowest temperature of exposure.