A float detects changes of elevation in the liquid free surface and opens or closes a valve accordingly. Large numbers of small float valves are used in domestic water systems. A float valve maintains the level in the cold water header tank which regulates the pressure of the hot water system.

Float valve or a ball valve automatically regulates the flow and maintains constant level in a service reservoir or elevated tank or stand pipe (pipe that connects main supply to tap directly). It consists of a valve connected to a hollow sealed float (ball) by means of a rigid bar, which is mounted on the top of the tank. The valve is connected to the incoming water supply.
It works on principal of maintaining the level of free water surface. A float valve allows water to flow until a predetermined level is reached. Once water reaches predetermined level, float will rise with water due to buoyancy and stop the flow of water. As water level drops the valve will open to refill the tank.