tl;dr - How can I get water flowing through one of my zones of a baseboard boiler heat system?
I have baseboard boiler heat with 10 zones (5 per boiler). One of the zones (unfortunately, it's our bedroom) is cold. I have vented the air using the 2nd floor vent valve. There was some air initially, but water now comes out.
I disassembled the zone value thinking that was the issue, but when I open it up with the valve fully removed (see picture), water flows initially and then stops. When I reconnect everything and open inlet and outlet valves water seems to flow in the reverse direction (i.e., the outlet pipe is warm and inlet pipe is cool (though not cold).
I've been troubleshooting for a few hours, but I'm stumped. Any suggestions?
Edit: Adding more pictures
Here's the system as I have it now:
Here's a picture of the (removed) valve system. It's a Honeywell 40004850 valve.
Edit #2: the saga continues
All valves for the problematic zone have been checked. I took apart the inlet valve (which was dirty but not clogged). My best guess now is that there is a lot of air in the lines preventing flow. This goes from basement to 2nd floor bedroom. They system's pressure if a bit high (>30 psi according to the pressure gauge), but I'm struggling to get the air out.
I've isolated that zone by closing the other 4 for that boiler hoping that I can get some movement of the air.