Identify Component in Heating Cylinder Installation Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[AspanCT]

There is water leaking out of what looks like some sort of valve from the pipe running to my hot water cylinder in my airing cupboard.
Can someone help me identify the part so I can replace it (see image attached).

[ericmark]

I think it is a by-pass valve, but not sure. Idea is as the TRV close this valve opens and returns hot water to the boiler so turning down boiler output.