Top valve - Automatic air/bleed valve ( they leak just for fun)
Valve below - Service valve - not designed for Central Heating rather for hot and cold water and shouldn't be used in that situation
The top one should have a small threaded stopper in the top that is missing, so that can't shut that off,
the lower valve one can be isolated so the top one can be replaced but the bottom one can only be replaced. When manifulating the lower one though, if it's already leaking, may leak more.
As above auto air vent dreadful things I'd replace with a manual one.
Look for 15mm compression manual air vent that and a 15mm compression straight job done.
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