Part 3 ball valve: what is it & how does it work?

A Part 3 ball valve or diaphragm ball valve is a ball valve made to British Standard specification 1212, Part 3. It consists of a plunger and a diaphragm washer inside a plastic housing. The plunger is partly visible and protrudes from the face of the valve. Water enters the cistern via small plastic spout on the top of the valve which directs the water down into the cistern. The float arm is secured to the valve via a plastic fitting. On the end of the arm is a plastic float which rises and falls with the water level.

When the water level in the cistern rises, the float arm pushes the plunger into the valve. The plunger then pushes the diaphragm washer onto the valve seating. This closes the valve.


Part 3 ball valve overview

Part 3 ball valves look similar to Part 2 ball valves and operate exactly the same way. The only difference is that they’re made out of plastic instead of brass. While Part 2 ball valves are intended for loft cisterns such as the cold water storage cistern and the feed and expansion cistern, Part 3 ball valves are typically fitted in toilet cisterns.

Interchangeable valve seatings are available for Part 3 ball valves. This means they can be supplied by the mains or by the cold water storage cistern. Depending on the valve and the manufacturer, there may be two different colour valve seatings in the box. White seatings are to be used for mains-fed valves; red seatings for those supplied by a water tank.

On some Part 3 ball valves, you can clip the unused seating onto the arm. Some Part 3 ball valves also have two screw fittings for the float, which come off from the arm at different angles. This is so that, in a toilet cistern, the float can be screwed on at an angle which allows it to float in the right position between the siphon and the cistern wall.

Part 3 ball valve advantages

Like Part 2 ball valves, Part 3 ball valves have a key advantage over Part 1 ball valves, or Portsmouth valves as they are also known. The position of the outlet spout at the very top of the valve means that even if the valve fails and the water level reaches the centreline of the valve, water can’t be siphoned back into the mains.

Another advantage over Part 1 ball valves is that they have a specially designed mechanism for adjusting the cistern water level. This is typically a plastic nut on the arm of the valve which comes into contact with the plunger.

Since part 3 ball valves are commonly used for toilet cisterns, it’s also possible to buy them in bottom entry form.


A faulty ball valve is the most common cause of cistern overflow. The float can become perforated and sink, or the arm can become jammed down by limescale or debris. The arm can also become jammed up, preventing the cistern from refilling.

The most common reason for a Part 3 ball valve to fail is wear and tear on the washer and/or on the valve seating, preventing the valve from shutting off.

Repairing a part 3 ball valve

Repairing a Part 3 ball valve is relatively straightforward, and can be achieved without undoing the tap connector attached to the valve stem and removing the entire assembly from the cistern. While holding the valve body with a pair of grips, use an adjustable spanner to loosen the nut against the cistern wall. Hold the valve body in place with your hand and unscrew the nut. You should then be able to remove the valve. Take care not to drop the washer into the cistern while you remove it.

Once you’ve removed the valve, you can disassemble it, and fit a new washer or valve seating as necessary.

While it is possible to repair a faulty ball valve with a new diaphragm or a new valve seating from a ball valve repair kit, replacing a faulty valve with a new one may be more convenient.

Where to buy

Part 3 ball valves are available from B&Q and Wickes.