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I'm not very well versed in 2 pipe steam and have ran into a problem on this call I received. Here's a little background. Commercial building 850000 btu boiler. (With hole above waterline looking at replacing after heating season) 2 steam mains each with their own return line that drop into a common dry return that runs around the perimeter of basement below the water line of the boiler to boiler feed tank that is vented to atmosphere.. There are f and t traps at the end of each main that are piped into low dry return. The problem the customer seems to be having recently is very little heat on the side of the building fed from one of the steam mains. The main is hot but the return line and most of the rads are cold. The other side fed from the other main seems to be heating fine. As I walked around the basement I found a check valve in common low dry return where the second steam main f and t trap and second return dump into the common lower return. My thought is the reason that one side isn't heating is because the air can't vent out due to the check valve. But I cant figure out why this would just start all of the sudden. Any thoughts?