Reply To: main stack vent

Sounds like venting problem. If you see condensation coming from the main vent stack, it probably is open. I’d be more suspicious of your bathroom sink(lav) vent. The lav drain should have a “P” trap installed to prevent sewer gas from entering the building. The trap holds a water seal blocking the passage of gasses. If the vent serving the lav drain is not functioning properly (or if there isn’t one) the water draining from the sink will siphon the water from the trap, leaving an open airway from the sewer pipes and drain lines.