I've fitted these Hep2O double check valves on the pipework to my outside taps
[img] http://www.plumbingplace.co.uk/image.php?type=P&id=559%3E [/img]
I can't find any instructions or guidance for them - what's the little screw down piece on it for?
Isolating valve? Does turning it shut the water off?
Definitely not. Not connected to a pressurised water supply at the moment so hard to test.
Kind of looks like it might open a little hole - I'm wondering if you use it to drain off the system beyond the valve by letting some air in through it but that's complete guess.
I think it's a test point for testing downstream valve like here under product features.
pressure gauge can be screwed in.
I've never seen one before, but I'm guessing it's a white hole.