What is this Fitting?

[John Carroll]

As above, installed between the mains and a newish electric shower, shower flow stops when weather gets warm or during the day as it heats up, its a dedicated supply to the shower.

 

[Alan Ambrose]

Looks a bit special. Dare to take off the caps (maybe noting position) to see what's under there?

 

Says 'wetsco' - what's the other writing say on there? Can't see anything here, but I didn't spend much time looking:

 

https://shop.westco.co.uk/

 

Best guess Google came up with was a baby manifold, but that doesn't make any sense as the dimensions of the two take-offs are different.

 

[crispy_wafer]

would it be some kind of flow regulator?

Edited July 30 by crispy_wafer

[joe90]

2 hours ago, John Carroll said:

shower flow stops when weather gets warm or during the day as it heats up,

That’s odd. Whatever it is it sounds broke! Try removing it and plumbing straight through 

 

@Nickfromwales any idea mate?

[John Carroll]

Not mine but definitely seems to be some form of flow regulator based on the incoming water temperature but why it is installed on the supply to a electric shower is very strange so will suggest just removing it.

[SteamyTea]

If it is a thermostatic flow controller, was it part of an old heating system and never got removed.

[saveasteading]

I recognise it, ie have seen one before.

I think it may be a backflow prevention valve, fitted just before an outside tap.

It is so that a hose immersed in a bucket cannot provide a flow back into the mains system in the case of reverse pressure.

[Alan Ambrose]

Could ask Westco I guess - in principle they should know 

[garrymartin]

Closest match seems to be a "pressure test valve" https://shop.westco.co.uk/15mm-cxc-pressure-test-valve.html

 

But that doesn't explain what you're observing so not really sure.