It's all to do with the air gap between outlet and water level when fitted
to the cistern. The airgap is required for backflow prevention. The part 1
valve has the outlet on the under side of the valve. This is not suitable
for fitting to WC cisterns where the inlet hole is at the same level as the
overflow hole. In the event of the valve failing, the water level rises to
the overflow level and the outlet of the valve ends up under water, thus the
required air gap for backflow prevention is lost. The part 2 valve has the
water outlet via an upstand. If this valve is fitted in a cistern where the
inlet and outlet holes are at the same level, then in the event of an
overflow situation, the airgap is maintained.

In general if the inlet hole and overflow holes are at the same level then
Part 2 valve is required.

If the inlet hole is above the overflow hole and the airgap is maintained in
the overflow situation then part 1 can be fitted.

A part 1 valve can be replaced with a part 2 valve with no problem.
But a part 2 valve can't be replaced with part 1 unless you first check that
the air gap will be maintained in overflow situation.

Part 1 valves usually found on potable water cisterns, cold water tank in
loft.

Part 2 valves usually found on WC cisterns and CH feed/expansion tanks.

BillP.


Thanks all.

I need to replace a fault valve in my parents CH header tank. The
existing one wont shut off properly and is slowly filling up and
overflowing. The one they have is a part 1, but when the tank fills,
it is under the water level, so I guess I will replace it with a part
2.

Cheers

Alan