What to do with this drain situation? The kitchen sink drain shares the galvanized pipe there. Problems have been that the water from the kitchen sink backs up into the bathroom drain and there is always water in the bath sink drain up to the level of the sink. I don't know if this is a vent problem, a drain problem, or both. It looks quite a mess to me, but I'm not a pro. I am willing to jump in and do what it takes but not thrilled with the idea of messing with that cast iron stack.

There is no venting of these drains. Rather than run vent pipes and have to tap into the cast iron vent pipe on the adjacent wall I was hoping to install Air Admittance Valves under both the bathroom and kitchen sinks, but the drain pipe on the bath side is so high there isn't enough room for the AAV under the sink.

Just to clarify, the kitchen sink is on the wall behind and above about where the heat register is. The drain setup under the kitchen sink is pretty average with just a few inches of drain from the p-trap to the wall.

How should this drain set up be configured?

Bathroom Drain question

  • 2
    Wait, is that sink expected to drain up, over the level of the toilet? How is that ever going to work?
    – JPhi1618
    Apr 5, 2018 at 2:33
  • As long as the sink bowl is higher than the drain it'll theoretically work. (Gravity doesn't care.) It makes for a nonstandard trap height, though, which can be problematic.
    – isherwood
    Apr 5, 2018 at 16:16

1 Answer 1


You're going to need to replace the existing cast iron tee with an elbow (or just plug the top), and install a sanitary tee lower in the galvanized pipe to connect the bathroom drain. I probably just cut off the galvanized down near the floor and use a no-hub coupling to run new PVC to the sinks.

The next question is what to do with that HVAC register.

  • Thanks for the response! Yes, what to do with that register....next puzzle to solve.
    – JLL
    Apr 5, 2018 at 16:27
  • You're welcome. Please make sure to take the tour to understand how this site works. "Thank you" comments are typically discouraged.
    – isherwood
    Apr 5, 2018 at 16:31
  • I would install the kitchen sink drain lower , the bathroom drain I would then elbow it and drop down near the floor and go across to the drain pipe - just below the register. However that set of plumbing in there looks quite the mess could probably stand a complete rework to neaten it all up ..Glad I am not dealing with it.
    – Ken
    Apr 6, 2018 at 7:47

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