out of solutions

Two thoughts

First, as Charles said, you are way undervented on the mains.  In fact, the main and return giving trouble may quite possibly not be vented at all.  Have you checked that?  Does it use a crossover trap, perhaps, to a dry return instead of a vent, and is that trap failed closed?  It happens...



Second, and more serious, you are running too high a pressure.  That system should run very happily on less than two pounds.  The traps will be happier... the system will be happier... you will save fuel.

OK then...

If you have a crossover, and the only way you can get heat into the problem radiators is to close the crossover... you have steam pressure in the return.  From somewhere.  When you close the crossover, you eliminate that back pressure and, voila, heat.  So I would start looking for some situation which is allowing steam pressure to get into that return.



Possibilities.  A failed open trap somewhere else on that return.  Much more subtle -- but at least as likely -- a place where you have a steam line drip to a wet return, and the dry return also going down to the wet return -- only the wet return isn't wet any more.  How long has this been going on?  I have seen this when a new boiler was installed with a lower water line than the old one, and I can assure you it can be a devil to find.

Could be, could be...

sometimes traps get added to systems which don't need them, or in places they shouldn't be.  Usually as part of "maintenance" to correct some other problem -- which they don't correct.



What it seems to me is needed at this point is to step back a pace or two, and go through that problem section (and might as well look at the other sections as well) to see if you can figure out how it was supposed to work originally.



For instance, on a system with radiator traps, crossover traps, dry returns, and wet returns, very often the idea was that all venting would take place on the dry returns, often at the boiler, and condensate would drip from both the dry returns and the steam mains into the wet returns.  Now sometimes a trap would fail open somewhere, and steam would get into the dry return.  Instead of fixing the trap, an extra trap would be put in to prevent the steam from getting to, say, part of the dry return system (there are countless variations on this -- as numerous as the guys who played with the system!).  Which did that -- but also prevented that section of the system from venting at all.  And so on.



Very often systems had no return traps at all -- just nice gravity flow (great stuff, gravity -- always works).  Double trapping is not always a problem -- but is rarely a solution!



So my suggestion for the moment is to spend a little time figuring out what the original system was, and why it worked.  Remember that you have to get air out somewhere, so steam can get in.  That water has to flow unimpeded back to the boiler.  And the steam goes from high pressure to low, always!