Hot water loop from steam boiler

FredS

We have a 2 pipe vapor heat system with a Burnham  V904A boiler. All the radiators have steam traps. When the old coal boiler was removed, the vapor/condensate tank was removed and the return line was vented into the chimney to expel air as the system filled. There is no main air vent. The boiler operates at 10 oz. A hot water coil in an air handler was connected below the water line with a circulator (B&G 100). This system worked OK.



Now we added another hot water loop to convert part of the house to radiant and ducted heat. A second circulator(Grundfos UP26-96BF)  feeds a 100 MBH heat exchanger for this load.



With one circulator running, water level drops 1/2" and no feed is called for. With both circulators running, the drop is enough to call for feed. This causes overfilling after several cycles. This will occur whether the boiler is firing or not. The pumps draw from a 1" tapping several inches below the low water line, and the piping is 1-1/4". The returns are into the rear mud leg tappings.



Any suggestions on how to prevent the overfilling are welcome.

TomM

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i have no clue, and did not stay at a holiday in last night, but are your check valves ok?