Part 3 ball valve

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 which are both situated 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 in turn pushes the diaphragm washer onto the valve seating, closing the valve.



Part 3 ball valves look similar to Part 2 ball valves and operate exactly the same way – the only difference is that they are made out of plastic instead of brass. Whereas Part 2 ball valves are primarily 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, which means the valve can be supplied by the mains or by the cold water storage cistern. Depending on the valve and the manufacturer, you may find two different valve seatings in the box: a red one if the valve is to be supplied from a cold water storage cistern, and a white one if the valve is to be supplied directly from the mains. On some Part 3 ball valves, the valve seating which isn’t fitted can be clipped onto the arm for storage. On the end of the float 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.


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 it is impossible for water to be siphoned back into the supply, should the water level in the cistern reach as high as the centreline of the valve.

Another key reason why Part 3 ball valves are preferable to Part 1 ball valves is because they are equipped with 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.

In addition to side entry configuration, Part 3 ball valves are readily available in bottom entry form due to their common use as a valve for toilet cisterns.


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 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. Undo the nut while holding the valve body in place with your hand, and the valve can be removed. Take care not to drop the washer into the cistern as you remove the valve.

Once you have you 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.



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