GGRR

Came home to find ceiling below boiler a lot yellower than it used to be. Off the pump comes a pipe (upwards) which
A – to the right feeds into middle of hotwater cyclinder (and out at bottom of cylinder – so I assume this is the 'screw' that transfers heat to hot water)
B – straight up, ending in what I believe is a either a pressure relief valve or an air release valve. This was weeping and had been for a while (scale etc deposits) …
It is brass 1/2 bottom fitting, about 1" in dia and with a red plastic 'valve' cap screw in bit on top. It is just a brass cylinder with a cap – there are no outlets (except at top via red cap))

Anyway
1 – anybody know what it really is called and what it does.

2 – we stopped ch water header tank filling, drained a rad – this stopped the weep (as this valve is the second highest point in system below ch header), any other suggestions

Thanks