Help identifying a pipe please

[Hadlowj]

Hi everyone I’m looking for a little help to identify a pipe so far I think the below is correct (see pic)

A - hot from boiler

B - expansion to expansion tankin loft

C - control for hot water tank

D - control for central heating

E - return from hot water tank coil, return from central heating T’s into it from the floor I believe

F - goes upto expansion tank

Now the one I’m not sure about.
What is pipe G, I was thinking some kind of expansion again maybe for central heating but it has a valve that seems unwise for expansion.

Any ideas?
Whole system both central heating and hot tank are vented.

Many thanks
James

 

[king of pipes]

Combined Cold feed and vent for the heating system . cheers kop

 

[Best]

Not sure why the valve has been fitted on the feed pipe there. It shouldn’t be on any feed pipe