The term Ball Valve is often applied to the device more properly known as a float valve, or ballcock.
Ball or float valves are used to regulate the flow of water into a tank, such as a toilet cistern. When the toilet is flushed, the valve opens to allow more water into the cistern until it reaches a certain level, at which point the valve is closed.
The valve is operated by means of a lever attached to the valve; at the other end of the lever a hollow ball or cylinder is attached, which floats on the water. When the water level falls the floating ball allows the lever to drop, causing the valve to open and water to be admitted. As more water flows into the tank the float is pushed upwards until the valve shuts once the water reaches a suitable level.
Should the valve fail to shut off the water properly, an overflow pipe will carry water outside the house to prevent flooding. If water is seen to be leaking from such an overflow pipe the valve should be adjusted or renewed as necessary.
The term ‘ball valve’ also describes a type of valve in which a sphere with a hole or port through its centre allows water (or other fluids) to flow when the port is in line with the inlet and outlet of the valve, but will shut off the flow when the ball is turned within the body of the valve. Such valves are suitable where the water supply only needs to be shut off occasionally, such as to allow a washing machine or dishwater to be disconnected from the main water supply.
Ball valve repairs are generally straightforward, and a property surveyor will advise if more fundamental issues with the cistern or general plumbing are suspected.