Part 2 ball valve: what is it & how does it work?

A Part 2 ball valve or diaphragm ball valve is a ball valve made to British Standard specification 1212, Part 2. It consists of a plunger and a diaphragm washer which are both situated inside a brass 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 brass split pin.

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, closing the valve.


Part 2 ball valve overview

Part 2 ball valves are intended for mains-fed cold water storage cisterns and feed and expansion cisterns, and are by far the preferred choice over Part 1 ball valves. This is because they offer a much greater air gap between the water exit point on the valve and the water level in the cistern if it reaches “critical level”, i.e. the same level as the pipe centre line. Part 2 ball valves also have a specifically designed mechanism for adjusting the water level in the cistern. In contrast, the only way to do this on a Part 1 valve is to bend the float arm.

As a result, current water regulations no longer permit the use of a Part 1 ball valves on storage cisterns. While a faulty Part 1 ball valve on a pre-existing installation can be repaired, it should ideally be replaced with a Part 2 ball valve. This is especially the case if the old valve has a rigid tube which dips down into the water in the cistern. These ‘silencer’ tubes are prohibited by water byelaws.


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 2 ball valve to fail is wear and tear on the washer and/or on the valve seating. This prevents a watertight seal.

Repairing a Part 2 ball valve

Repairing such a valve is relatively simple. It can be achieved without undoing the tap connector attached to the valve stem and removing the entire assembly from the cistern. After isolating the valve, hold the valve body with a pair of grips. Next, use an adjustable spanner to loosen the nut against the cistern wall. Then, undo the nut while holding the valve body in place with your hand, and you can remove the valve. Take care not to drop the washer into the cistern as you remove the valve.

Once you’ve you removed the valve, you can disassemble it, and fit a new washer or valve seating as necessary. If you’re replacing a Part 1 ball valve with a Part 2 ball valve, you can simply undo the valve nut on the new valve, and screw it into the pre-existing fitting on the cistern.