Part 1 ball valve: what is it & how does it work?

A Part 1 ball valve or Portsmouth ball valve is a ball valve made to British Standard specification 1212, Part 1. It consists of a piston inside a brass housing, with a float arm secured to the valve via a split pin. Inside the piston is a slot which accommodates the end of the float arm, and on one end of the piston is a washer. When the water level in the cistern rises, the arm pushes the piston and its washer onto the valve seating, closing the valve.

Part 1 ball valves are gradually being phased out in favour of Part 2 and Part 3 ball valves. Unlike these valves, Part 1 ball valves do not offer an effective air gap in the event that the water level reaches the pipe centre line, due to the position of the water outlet at the lowest point of the valve housing. Also, they are not equipped with a mechanism to adjust the position of the float and thus the level of water in the cistern: the only way to do so is to bend the float arm. Some Part 1 ball valves on pre-existing installations may have a rigid plastic or metal tube which dips into the body of water in order to reduce the sound of running water as the cistern fills – these ‘silencer’ tubes are no longer permitted by water byelaws.

For these reasons, Part 1 ball valves are no longer permitted on cold water storage cisterns due to non-compliance with water byelaws. They may be found on older installations, and should ideally be replaced with a Part 2 ball valve.


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 1 ball valve to fail is wear and tear on the washer and/or on the valve seating, which prevents the valve from shutting off.


Repairing a Part 1 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. First, isolate the water supply. Then, 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 then be removed. Take care to make sure that the washer doesn’t fall into the cistern as you remove the valve.

Once you have you removed the valve, you can disassemble it, and fit a new washer , piston, or valve seating as necessary. If you are replacing a Part 1 ball 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.