hot water in zone when T-Stat not calling
I have a 4 zone hydronic heating system. 4 taco pumps off a supply manifold. 3 zones are copper piped fin tube baseboard. The basement was added and has flexible rubber pipes and runs buderus panel radiators.
The second floor zone just started running at about 72-74 regardless of what the T-stat is calling for (66 for example). First I swapped out the T-stat, which didn't make any difference. I turned the Tstat to "off" and overnight it dropped to 68 and the fin tube is just a tiny bit warm.
In a possibly related problem, the basement zone has one radiator which stopped working a 6 weeks or so ago. We found we could get it to work a little bit by bleeding it. The radiators on that zone are below the high point in the supply and return lines, and I worry that there's air in that part of the system. After the initial bleed, the radiator stopped working again, until I bled several quarts of water from it last week. The water was coming out without bubbles FWIW.
My suspicion is that there's a bubble in the high point of the rubber tubing.
1) could the problems be related? (resistance on the basement supply leading to backflow to the 2nd floor zone?) It's not happening to the 1st or attic zones).
2)Does the bubble in the basement pipe sound plausible, and if so, how to remove it? Drain the zone? There is an air removal device mounted on the system over the expansion tank.
3)If they're not related, could there be a problem with the 2nd floor zone circulator? How would I diagnose that? (other than just replacing the cartridge)
Thanks so much for any advice