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

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.)

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?

Thank you Micky. Water comes out hot eventually, of course. But it's obviously not circulating through this unit as it does all others. I wonder if it's a clog inside the baseboard, or the Tee, or the Venturi? And what's the best way to diagnose?

On Sunday, December 22, 2019 at 6:34:00 PM UTC-5, wrote:
Thank you Micky. Water comes out hot eventually, of course. But it's obviously not circulating through this unit as it does all others. I wonder if it's a clog inside the baseboard, or the Tee, or the Venturi? And what's the best way to diagnose?

I guess that depends on how it's put together. If there is a union,
you can disconnect it see if water will flow through it with a garden
hose, snake the pipes, etc. If no unions, then cut we must. I don't see
how it can be on the goesinto side, since you get flow through the bleed.
Must be on the goesouta side or the radiator. Have an air compressor?
How about taking out the bleed fitting or other plug, rigging up
some compressed air with a place in the system open so it can blow
it out?