One of my zones does not work. I think the controlling unit, first picture above is bad but I can trouble shoot this, it may be air as well.
My question is, how do I get rid of air in the system? I can drain the system by opening the valves in the second/third picture and then the system refills. However, if you look at the second and third picture, the valves are kinda double, the knob and just below there is a slit for a screw driver that is a quarter turn. When I drain the system, should this second valve be open or closed?
Any direction on how to do this would be great. There are no bleeder valves by the individual radiators.