Part 1 and Part 2 ball valves: what’s the difference?

What is the difference between a Part 1 and a Part 2 ball valve? These brass fittings are some of the most common types of ball valves found in British homes. They are typically found in loft cisterns, such as the cold water storage cistern and the feed and expansion cistern. They both work in roughly the same way: as the cistern fills and the water level rises, the float pushes the float arm up. This in turn moves a rubber washer against the valve seat, closing the valve.

However, Part 1 and Part 2 valves differ in two key ways. The first is in relation to the point on the valve where water is discharged.

On a Part 1 or ‘Portsmouth’ valve, the outlet is on the underside of the valve. On a Part 2 or ‘diaphragm’ valve, the outlet is on the top of the valve. Water is directed down into the cistern via a small plastic spout which screws onto the valve body. Should this spout become detached from the valve, then water will spray up into the lid of the cistern. This water will probably leak out from the top of the cistern. If the cistern doesn’t have a lid, it will deliver a fountain of water up into the loft space!

If a Part 1 ball valve fails and the water level in the cistern reaches the pipe centre line, there will be no air gap between the valve and the water in the cistern. This means that the outlet on the valve will be completely submerged. More to the point, it means that the valve would fail a back-siphonage test. Should there be a negative change in water pressure in the supply pipe, water in the cistern will be siphoned back into the mains. This is prohibited by water regulations and should never be allowed to occur.

As for the other reason why Part 2 ball valves are preferred? That’s because they are equipped with a specifically-designed mechanism for adjusting the water level. The position of the float on the arm can be raised or lowered via a wing nut. This feature is not found on Part 1 ball valves – the only way to adjust the water level is to manually bend the float arm.

So, the difference between a Part 1 and a Part 2 ball valve – or differences, to be more precise – are pretty straight-forward. They also explain why Part 1 ball valves are poorly suited, and why it is a good idea to replace a Part 1 valve with a Part 2 valve, especially if the opportunity presents itself.

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