On the other side of my mechanical room's wall, we are putting in a small powder room. The sink is going to be on the wall below the boiler valves, which is represented by my awesome graphic. I was hoping to just pop the drain through that wall and down to the current ABS drain line in the photo. The drain just services the boiler and the hot water tanks relief valve, so hopefully never used besides testing the valve. The slope is adequate from there going to my stack.

  1. Would this be appropriate to do?
  2. Should I have a little trap on the bottom of the vertical pipes coming down from my tank and boiler? I was once told I should, but these would be dry since those relief valves hopefully aren't releasing water often. So, would that do anything?

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  • 2
    Without a trap, it's a sewer gas vent, assuming it runs to the sewer. Beware that assumption, it's both typical and (IIRC) code that they should not connect to the sewer.
    – Ecnerwal
    Oct 15, 2020 at 16:49
  • 1
    I think that what @Ecnerwal is saying is that the line you're planning on using for your sink drain may not run directly to the sewer. (Nice graphic, BTW!)
    – FreeMan
    Oct 15, 2020 at 17:15
  • Looking down the run, the waste line ties into the waste lines from the upstairs bathrooms, before dropping into the slab next to the stack. So it sounds like gray water is not getting dropped into the stack, which would be getting dropped into the sewer. Is this correct? Assuming this is all up to code, would I be good to tie the sink drain into that line? I can add photos of the waste line if that helps anything. Thanks!
    – junta
    Oct 15, 2020 at 18:21

1 Answer 1


Are you smelling bad sewer gasses down there? I would assume you don't so it makes me think the black drain line from the water heater runs to a sump drain unless there's another trap somewhere else along the black drain line before it goes to the house's main drain run.

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