New build basin inlet plumbing confusion

[mayfel]

Hi,

I'm in a new build and there's an arrangement behind the hand basins I've not seen before with a valve that actually connects between the hot and cold feeds. I can't work out what this valve does. See pics. I'm assuming it's a more recent regulation as I've never had it in any older houses.

Basically, the Grohe mixer tap suddenly has very little pressure on the cold side. All other cold feeds to other sinks are fine including the bath in the same room. No leaks. I'm wondering whether this valve has failed and causes the issue. The hot water pressure is perfect so it's not a general cold water pressure problem. Presumably it's either the tap or this valve?

 

[CBW]

TMV - thermostatic mixing valve - only required on baths on new builds/building change.

 

[cross thread]

Cold goes straight to the tap the TMV should have no impact on the cold,
If the cold fails it should shut down the hot to stop scalding.

 

[mayfel]

Ah okay, that's good to know. Any ideas why the cold pressure would suddenly fail on a non thermostatic mixer tap?