[Closed] Help, Plumbers/boiler central heating air or pressure relief valve - information

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 ho****er 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