Back pressure on condensate line

DBSJr

I'm looking at a 14 story pre-war building in New York.  The building has a 2 pipe steam system.  At two radiators on the top floor of the building we are not getting heat.  The remainder of the radiators on that floor seem OK.  The floor is served off of multiple steam and condensate risers.  The plumber checked the traps on the two radiators in question to see if they were blocked and found them to be clean.  When he manually vented the radiators they got steam and everything worked fine.  However when he removed the steam trap and opened the condensate line he was getting positive pressure.  Any ideas what could be causing this kind of issue?

Paul48

Traps

There are traps stuck open elsewhere in the system. Need to survey all the traps.

Pumpguy

pressurized return line

A pressurized return line can be caused by improper venting at the condensate pump's receiver and/or bad steam traps in another part of the system.  I have seen condensate receiver vents capped "because there was lots of steam shooting out of it!" If that's the case, gotta fix the traps.



What is the condensate temperature?  If it's hot, like 180 or higher,  this is another indication of bad steam traps.



Is or was this a vacuum return system? If it was but is not now, the vacuum should be restored.  Vacuum systems typically will have smaller return pipes than a gravity return system.  If it was a vacuum system but is not now, there is a possibility of lift fittings out in the system.  Without a vacuum on the returns, the lifts will act as water seals and a positive pressure will be needed to push the condensate up the lift pipes.



Hope this helps.  Others may have other ideas to offer

Pumpguy

Another cause...

could be if some knucklehead put a "master trap" at the inlet of the condensate pump's receiver.  When it's open, everything's fine.  When steam hits it and it closes, the return line goes to the same pressure as the steam side.  This could show up as other problems because, now with no pressure differential, we have no steam flow and this could cause heating problems throughout the system.