Harold Kestenholz
One pound steam pressure will push water down 14 inches and up the other side of a loop 14 inches, so keeping the pressure in the system low enough (Systems are designed at 1 psig) to prevent water rising higher than the distance from the boiler water level to the main is important.
The proper way to pipe the returns is to drop them both below the water line and up to the Hartford loop inlet tee at the equalizer pipes to the boiler. This gives a water seal for boiler supply main steam to be separated from the return. Vents eighteen inches from the drop elbow to the seal trap can then expel air from the one-pipe main in the right direction toward the Hartford inlet. You might have the pressure too high, no water trap drop, and no vent near the end of the longer run; check to see the cause in the first piping diagram at: http://www.oldhouseweb.com/stories/Detailed/10464.shtml
