Greenstar 30i drips when filling.

[The Donut]

Hi all , went to a small top up on 30i yesterday at daughters house. Turned key , opened white valve gently , then water started to drip from right of filling key. Appeared to be coming from a small black knob to right of key.
This seem a little loose so gave a gentle tighten. Any ideas what this is?
Pressure relief? Need to loosen again?

Cheers for looking

D

 

[ShaunCorbs]

Just a backflow device eg check valve should be hand tight

 

[The Donut]

Just a backflow device eg check valve should be hand tight

Ok thanks Shaun