Ball valve: what is it & how does it work?

ball valve or float valve, or less commonly, a ballcock, is a valve which controls the flow of water into a cistern via a plastic or metal float on the surface of the water. When water is drawn out of the cistern, the float falls with the water level. This opens the valve and causes water to enter the cistern. As the cistern fills, the water level pushes the float upwards, closing the valve once the water has reached a predetermined level.

Ball valves are present in cold water storage cisterns and feed and expansion cisterns, where they tend to be made out of brass. Those installed in toilet cisterns are usually made out of plastic. Plastic ball valves are popular choices for reducing the sound of running water as the cistern fills. Ball valves are usually connected to a cistern via two nuts on the valve stem, and are connected to their supply pipe via a tap connector. Plastic cold water storage cisterns and feed and expansion cisterns are generally supplied with a small plastic or metal backing plate. This protects the cistern wall from the stress of the upward force of the float. Service valves should always be fitted on the supply pipe to a ball valve, and as close as possible.

Air gap

An air gap between the water level and the outlet point on the valve must always be present. This is in order to prevent a siphon effect, which would draw water from the cistern back into the mains. For this reason, rigid plastic ‘silencer’ tubes which dip into the body of water in the cistern are prohibited. However, some toilet float valves are equipped with a built-in anti-siphon mechanism, or a collapsible plastic tube attached to the valve.

Faulty ball valves are the most common cause of an overflowing cistern, and are usually simple to repair. Ball valve repair kits are available for plumbers and other individuals who repair a lot of ball valves. They contain low pressure and high pressure valve seatings, split pins, washers for Part 1 valves and diaphragms for Part 2 valves.

Ball valve types

There are several different types of ball valve available. All of them must conform to BS-1212, the British Standard specification code for ball valves. The different ‘parts’ of the code are used to refer to the different types of valve.

Part 1 ball valve – Portsmouth valve

The Part 1 ball valve is easily recognised by its piston-like shape. Inside the valve is a small piston with a washer on the end, and inside the piston there is a slot in which the end of the float arm sits. The arm is secured to the valve via a split pin.

When the water level rises, the float pushes the arm of the valve upwards. This pushes the piston and its washer onto the valve seat, stopping the flow of water. The brass housing of the valve has an unscrewable cap on the end. This is usually made out of brass too, but is plastic in some valves. If the valve has seen many years of use, the cap will most likely be impossible to undo by hand. Part 1 valves usually lack a mechanism to adjust the amount of water in the cistern. The only way to do so is to bend the valve arm. They are typically side entry, but bottom entry valves are available.


Part 1 valves are gradually becoming obsolete in favour of Part 2 and Part 3 valves. This is because a ball valve with an arm that must be distorted in order to adjust the water level is no longer permitted by water byelaws, and that should the water level in the cistern reach the critical level, i.e., level with the centre line of the supply pipe, the risk of siphonage back into the mains is much greater due to the position of the outlet.

The Part 1 valve, or Portsmouth valve as it is sometimes known, is similar to its predecessor, the Croydon valve, whose piston moves up and down instead of backwards and forwards. Some cheaply made ball valves have broadly the same design as the Part 1 valve, except that they lack the plastic or metal cap on the end, exposing the piston. Since water is able to pass the piston when the valve is open (hence the cap), water is likely to spray in all directions inside the cistern. These valves should be avoided.

Part 2 ball valve – Diaphragm valve

Part 2 ball valves operate slightly differently. Instead of pushing a piston, the float arm pushes a small plastic or brass plunger into the valve housing, which in turn pushes a rubber diaphragm against the valve seating, closing the valve. The small plunger protrudes from the valve housing, and is partly visible.

Part 2 ball valves offer a number of advantages over Part 1 valves. Firstly, they have a specific mechanism for adjusting the water level in a cistern. This is done via a small nut which secures the float to the valve arm. The nut can be unscrewed and moved up or down.

Secondly, they offer a much greater air gap between the outlet on the valve and the water level in the cistern. This is because the outlet is located on the top of the valve. A small plastic spout directs water down into the cistern.

Part 3 ball valve – Plastic diaphragm valve

Part 3 ball valves function in exactly the same way as Part 2 ball valves, except they are made out of plastic instead of brass. They are generally used in toilet cisterns, and on the end of the arm, they sometimes have two screws which point out at different angles. This is so that there is room to attach the float for its location between the siphon and the opposite end of the cistern, depending on which side the valve enters.

Part 4 ball valve – Torbeck valve

Part 4 valves refer to compact plastic valves designed for toilet cisterns. These are sometimes known as Torbeck valves (Torbeck is the trademark name of a popular Part 4 valve). The term ‘ball valve’ is inaccurate in reference to these valves, as the float is usually not a ball. The design of these valves means they are compact and don’t require big floats.

Part 4 valves are always equilibrium valves. This means that they use the pressure of the water to help close the valve. Water is allowed to flow behind the diaphragm washer. This ensures that the pressure either side of it is the same – hence the term equilibrium. As a result, significantly less force is required in order to close the valve. The float arm does not have to overcome the pressure of the water entering the cistern.

Equilibrium ball valves

Ball valves which equalise the water pressure on either side of the valve washer are not limited to Part 4 Torbeck models – there are also equilibrium Part 1 valves. In this case, the piston has a channel which allows the water to flow through it in order to occupy the other side of the washer.


A faulty ball valve is the most common cause of cistern overflow, and can easily be repaired or replaced. Typical ball valve faults include:

  • The washer or diaphragm and/or the valve seating has worn and no longer forms a seal.
  • The float has become waterlogged and has sunk.
  • Grit, limescale or other debris is preventing the valve from closing off.
  • The valve arm has jammed down (alternatively the valve arm can become jammed upwards, prevent the cistern from refilling.