Heating Help

WATERLOGGED 2 PIPE RETURN

WE SERVICE AN OLD COAL TO GAS CONVERSION (CONVERTED 1948?) TWO PIPE SYSTEM HANDLING A 3 STORY MUNICIPAL BUILDING WITH ATTACHED 1880s 3 STORY HOME.  IN THE HOME WE HAVE A PROBLEM WITH A SERIES OF PARALLEL RADIATORS THAT BACK UP WITH WATER AND COOL OFF.
ALL HAVE 17C TRAPS ON THE RADIATOR DISCHARGE, AND F & T BUCKETS ON THE END OF THE SUPPLYS & ONE ON ONE RADIATOR RETURN
THE SET OF RADIATORS CAUSING PROBLEMS DRAINS INTO A 3/4 THEN 1, THE 1.5 THEN 2" IRON LINE, 7 FOOT OFF THE FLOOR.   THE LINE SLOWLY DROPS THEN DIVES TO THE FLOOR RUNS FOR ABOUT 20' HORIZONTAL AND RISES INTO A MANIFOLD 2 FOOT OFF THE  GROUND, AND INTO AN UNTRAPPED LINE ENTERING THE CONDENSATE PUMP.
IT APPEARS WE HAVE 2 OR 3 OF THE 7 RADIATORS WITH BAD T-STAT ELEMENTS (BAD IN THE CLOSED POSITION)
I AM THINKING THE MAIN PROBLEM IS A VACUUM BUILD-UP IN THE RETURN BECAUSE OF THE CLOSED / BROKEN T-STAT TRAPS NOT VENTING AIR / BREAKING A VACUUM,   AND BACKING UP THIS ALREADY SERIOUSLY TRAPPED SYSTEM.
I AM CONSIDERING ADDING AN AIR VENT ( ANTLER SYSTEM) TO THE TOP OF THE BEGINING OF THE RETURN SYSTEM (GORTON #2s) 
ANY OPINIONS OR ADVICE?  

If you were air,

could you get out? That's a perfect P-trap.

One does have to wonder

how air is supposed to get out of that line...

If the traps are bad -- stuck closed -- those radiators will never heat properly; that's a simple fix, though.

The problem isn't a vacuum buildup, though -- it's apparently a total lack of venting.  Your idea of a couple of Gorton #2s at the top of the beginning of the return is a very good place to start.

One does have to wonder

how air is supposed to get out of that line...

If the traps are bad -- stuck closed -- those radiators will never heat properly; that's a simple fix, though.

The problem isn't a vacuum buildup, though -- it's apparently a total lack of venting.  Your idea of a couple of Gorton #2s at the top of the beginning of the return is a very good place to start.

You might try

just opening up a capped Tee or something in the return (if there are any), and run the system. This will tell you if this is the problem or not, and if any traps are failed open and steam is blowing through. It would be very unlikely to have multiple traps failed closed - they generally fail open.

Before I put vents on that system, i'd replace the dip on the return line. What you probably have there is a system that used to be a gravity return with a wet return, that has been converted to a condensate pump system, and wants to have a dry return. If you can replace the 20' run that drops below the tank inlet and put it up higher so that it continuously runs downhill to the tank, then you don't need vents, the tank's open pipe vent will vent the returns for you. (be sure there's an open pipe sticking up out of the tank, it must be vented either way).

Also, PLEASE TURN OFF THE CAPS LOCK!!! IT MAKES IT A REAL PAIN TO READ YOUR POSTS!!!