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Math time: condensate drain on common exhaust
I have 2 Navien NFB-200s that are exhausting via a common vent. Per Navien's instructions, we put a tee facing downward just downstream from the last unit to catch condensate (otherwise, condensate can collect on top of the little dampers in Navien's special kit, preventing them from opening properly). Venting is perfect. Drain line is 1" PVC.
Based on Navien's instructions to put a loop in some flexible plastic tubing drain line, my plumber built an impromptu -- and short -- PVC p-trap (he and I both like that better than plastic tubing). Purpose of this loop / trap is to ensure you aren't venting exhaust into the house via your drain line. But, since neither he nor condensate had filled the trap yet, we ended up ..... blowing exhaust out the drain pipe into the house.
This prompted some debate and I'm not finding the right sites to tell me the math on this: how many inches of water sitting in 1" PVC will weigh enough / provide sufficient head to prevent exhaust from blowing the water out of the trap? Please point me to the math so that I can over-engineer for safety. Thanks!