In alt.home.repair, on Sun, 22 Dec 2019 14:12:13 -0800 (PST),
wrote:

I have hydronic heat system at home. One zone. Boiler in basement with 1-1/4" diameter main loop around the basement ceiling perimeter, with a set of venturis and Tees going to each baseboard upstairs and even to the 2nd floor. Each branch from main loop is 1/2" diameter. Every unit works well, except one baseboard unit on 2nd floor doesn't heat up. When i bleed the one bleed valve on it, water immediately comes out. So, no apparent air lock in the pipes. I'm at a loss as to what else it could be. The system has performed perfectly for years. Suggestions appreciated.
Thanks
Theodore

There's nothing wrong with the system. There is something wrong with
that baseboard unit.

If you keep bleeding for a long time, does the water ever come out hot?
If not, where is the cold water coming from?

(Put a hose over the bleeder and run it out the window.)