No water flow through zone of water boiler, all other zones working. How to get water flowing?

Question

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.

Answer 1

In my personal experience, this is most often caused by a section of the pipe loop in question being frozen. Tends to be aggravated if there's a setback (automatic or manual) at night causing a long "off" time when the weather is particularly cold, or a woodstove causing a long off time for the boiler. Having a "time override" on the circulator/zone can be helpful in that case (to make it run a few minutes every hour, even if there's no call for heat from the thermostat) since the heating pipes are often located in the coldest edges and corners of the house.

Could also happen due to an air blockage causing no flow in that loop while it's particularly cold out.

I managed to thaw one by making sure the boiler inlet was open to refill it from the auto fill valve and wet-vacuuming (a few gallons of) water from the bleeder until I had hot water to the bleeder, which then warmed enough of the pipe to let the ice clog melt and flow resume. After which, more bleeding, since new water always comes with some dissolved air that needs to be bled out.

A different (long-term) solution to loops freezing is to convert the system to polypropylene glycol and water antifreeze mixture. That complicates the fill procedure: you need a pump (not your circulator, which is optimized for a different sort of pumping) rather than just letting water pressure do the trick and you need a reservoir for the antifreeze (and no leaks, but you want no leaks anyway.)