No Heat in Basement Radiator #1 (two-pipe)

Neil Spiegel

I just recently had basement apartments put into two homes. I have GC skills, but not heating skills. I had the household radiator system done by a local heating contractor. Both of the houses have some problems, I think they may be related issues, and the contractor has been avoiding me for over a month. Ripping down the drywall and starting again is not an option. The only thing I can think of for sure is removing the rads and putting in a baseboard electrical rad. I would prefer to avoid that.

Situation 1.
3 Story Home
2 Pipe hydronic system
5 used cast-iron rads introduced into the basement
4 of them heat up (not sure if they heat up equivalently to the ones upstairs but they definitely get some circulation and some heat)

1 of them does NOT heat up at all
(only difference I can think of is that it is shorter than the others, and a different style of rad)
I tried bleeding it for like an hour, warm water came out but once I stopped bleeding it it cooled right down again
The rad is right on the floor.

I have heard something about cold water sinking causing a problem for basement rads.
1) Does this sound like the likely problem.
2) What are the solutions?
3) Is there some electrical resistant heating device you can wrap an old cast iron rad in to keep the rad a little hotter than the rest of the water in the system to keep circulation going? Or a mini circulator you can put in line?
4) Or is there some other solution.