Bib Tap - small screw

[Andrew May]

I have just replaced a bib tap outside the house. I notice that both the
one that I took off and the new one have a small screw on the underside
of the tap near to where it connects to the supply.

Removing this screw reveals a small O ring around the screw and hole to
the water inlet.

But what is it for?

 

[Dave Liquorice]

Perhaps to drain the tap/pipe for cold spells.

You could just open the tap. But some Bib taps have a built in double
check valve this could be to drain the bit between the two valves.

 

[Andrew May]

You could just open the tap. But some Bib taps have a built in double
check valve this could be to drain the bit between the two valves.

That would make sense although I have a feeling that one did have a
non-return valve and the other did not.

It was the failure of the non-return bit that prompted its replacement.
A little O ring that seals it closed was coming loose and I was bu**ered
if I could find a way of getting it all apart to replace it.

 

[Andrew Gabriel]

This is what I believe it to be: to drain the non return valve.

It's also to check that both sides of the double check valve
are working (like you do;-). Without access between the two
valves, you can't test they both still work.

I think it is preferable to use a tap without a valve and install a
check valve inside the house to minimise the chances of it freezing?

That is the requirement nowadays - the double check valve must
be indoors, as they are otherwise silently destroyed by the first
frost.

At least, that's what I have done. The taps without an integral check
valve (ime, ymmv, etc) do not have this drain nut.

Fitted one for my brother a couple of weeks ago, and used a
separate double check valve on the indoor pipework. The
brass casting for the tap had a bulge where the access nut
would be, but it wasn't machined out.

The ones on double check valves usually have fibre washers
IME. Not seen one with an O-ring like the OP's.

 

[Andrew Gabriel]

How would you test this? Remove the tap and apply water through the
"wrong" end with a hose pipe and if it comes out the drain hole, the
first valve is broken; if it comes out the "back" of the tap, both are
broken?

I've never heard of testing them before. Is it done on installation or
is it supposed to be done annually?

I don't seriously believe anyone does, hence the sarcy comment.