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I am trying to figure out how to properly vent a new washing machine drain line that will tie into an existing drain; however, the existing drain is 10.5' away from the washing machine drain.

Sources seem to indicate this is either fine, or needs to have a vent within 5' or 8' of the washing machine p-trap. The first figure shows the setup.

Is this setup ok as is, b/c the existing vent is above the water line of the washing machine p-trap? (See https://www.jlconline.com/how-to/plumbing/maximum-length-for-fixture-drains_o).

Or do I need to add in a vent for the washing machine? If so, can I connect it to the existing vent to keep from having to run a second vent line. See the second figure.

Thanks in advance for any help!

Proposed drain Proposed drain w/possible vent connection

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Something like your second drawing should work, though it's not technically correct (nor is the first) by the code I'm under (IPC, with some local amendments,) which requires that all vents run at least 45° upward (which they define as "vertical") until they are 6" above the flood rim of the highest fixture on the floor served.

So a correct version would be with both vents running upward until that level, and then they can turn horizontal (actually 2% slope to drain), and they can be joined to the single vertical vent (assuming it's of adequate size, but that's likely unless it's tiny, as a dry vent.)

Alternatively, I think you could use 3" pipe for the washer drain (but I haven't double-checked that.) However, the original vent would still be wrong, if it's as drawn.

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  • Thank you! Would I be better off maintaining the new vent line at a 45° until it meets the existing vertical vent? I cannot modify the existing horizontal vent b/c of a window. For second part - does a 3" drain pipe mean I don't need a vent?
    – r_alanb
    Jun 19, 2023 at 13:13
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    Yes, that would be better. You always need a vent. The 3" line might, depending on your applicable code, allow you to use the existing non-compliant vent. I'd consider 2" line and the 45 to the existing pipe (or straight up and then over at the correct level to it - whichever is easier) preferable.
    – Ecnerwal
    Jun 19, 2023 at 13:51
  • Thank you very much!
    – r_alanb
    Jun 19, 2023 at 20:06

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