Cast Iron baseboard radiators heat v e r y slowly

PEvans

I first raised this issue at the end of a long thread about bringing my steam system back on line:

https://forum.heatinghelp.com/discussion/comment/1661708#Comment_1661708

This is a two-pipe steam system with pumped condensate return and the receiver vented to atmosphere. One steam distribution main serves BASE-RAY radiators on two floors and no conventional radiators.

The baseboards really never heat up, especially on the lower floor. I have confirmed that the supply valves are fully open and replaced those thermostatic steam trap elements but no effect. I recently noticed vents in tappings on the lower floor baseboards. I'm not sure the vents are working. If I remove the vents entirely with the system steaming up I can hear and feel air being pushed out, but the baseboards still heat slowly.

The arrangement for this part of the system is similar to what is shown on p. 5 of the BASE-RAY manual:
https://file.ac/sjX8zTp2UtE/Baseray Radiator I&O Manual.pdf

In particular, the condensate return drops down to the basement floor, then up a foot or so before it drops into the condensate header. It is odd (to me) as the other returns or drips are all dry. Someone went to a lot of trouble to do this this way and I have certainly seen posts suggesting that BASE-RAYs are supposed to have wet returns.

Thoughts?