On Sunday, December 22, 2019 at 5:12:17 PM UTC-5, 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

It would seem to indicate that there is a blockage in that one that's
preventing water flowing out of it. I assume you bled it for a long
time, hot water comes out?