Hot water going in header tank

[Steve barber]

The water in some rads is a bit black, all the system is going and it’s very warm very vey warm hot water and rads. My pump is on the slowest but it still has water going in the header tank. It’s faster with just the hot water on and just a bit more than a dribble when the rads are on.
As I put on a early post I noticed little tiny green looking plastic bits coming out the hot tape,,,,

 

[Madrab]

Can I presume that the HW cylinder is an open vented system, it's not unvented is it?

 

[Steve barber]

Vented system. It’s 11 years old and it’s oil