Boiler not firing - which valve?

[haplessdiyer]

Firebird boiler isn't firing so I assume I need to bleed it, but I'm not sure where the valve is. I keep seeing different things online that say I should loosen the valve where the Allen key is, or the lower hexagonal nut. Which is it?

 

[Terrywookfit]

Try both of them.
I love to see a well maintained boiler.