I have an old farmhouse with a bathroom vanity, laundry room, and kitchen sink/dishwasher draining into a single septic run. There aren’t any vents installed in the system with only a crawl space under the two rooms. Is there a way to add a vent in the crawl space or without running it up through the walls or ceiling?

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    Vent stacks were very often run outside the home in the old days. Attached to siding and ending above the eaves. – Kris Jan 17 at 14:16
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    Related question with good info diy.stackexchange.com/questions/58657/… – Kris Jan 17 at 14:32
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    Not a duplicate they are asking if they can vent to a specific location that is not covered in that answer. The vent needs to be above the level of the drain at a minimum or any backups the vent becomes the drain. – Ed Beal Jan 17 at 16:54
  • Wouldn't moisture be an issue? I saw where someone had vented into an attic. It was dripping wet up there, even with gable vents. – Keith McClary Jan 18 at 3:44

If allowed in your location an air admittance valve can be added. It needs to be close to the level of the drain. I put them in the sink cabinet as close to the top as I can. I agree with @kris my 1930 farm house has the vent outside several Victorians I remodeled also had them outside. But an AAV if allowed is the easiest but not in the crawl space.

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