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?