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We are having a problem with water backing up thru a return to a Hoffman 4a vent located on a dry return. The boiler is a Weil-McLain EG series Steam boiler. We have removed, cleaned and inspected all return piping. Boiler is piped with equalizer as per manufacturers drawing. We have replaced pressuretrol and cleaned steam siphon connection. Pressuretrol is set at 1psi differential, .5 psi cut-in. Gauge on boiler functions and indicates corresponding pressures pressuretrol is set at. House is two story home. The system was originally a one pipe system with CI Radiators at some point (30+ yrs ago) The radiators on main floor were replaced with baseboard with piping added to dry returns. Hoffman 17c traps were installed on each lenghth of baseboard. CI radiators remain on second floor. Have replaced 2 traps on baseboard because steam was present in return. Have also skimmed boiler twice and added 1 can of Squick boiler cleaner. Boiler has been in around 5 years and seemed to run fine until about a month ago with a call that water was leaking out above mentioned vent and no heat in one room on main floor. My first concern is trying to stop water from backing up return any suggestions?