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We have (what I think) is a pretty standard setup: various drains converge in the basement and exit through a single pipe out to the septic tank. I have lived in this house for 13 months and never noticed a smell from the septic tank outside the house.

A family friend "helped" install a new 3" drain from the upstairs into the basement. The drain had nothing "on top" yet, just an open pipe. The drain line runs down through an HVAC duct, exits into the basement and ties into an existing drain line. The design is bad and flawed but that's for another discussion. Of course, we have a smell in the house due to the open drain.

However, the day that this work was done, I started to notice an intermittent raw sewage smell outside the house from the septic tank. Septic people told me it was normal to occasionally have a smell, but this was too much of a coincidence to me. As a hedge I had the tank pumped, the intermittent smell persisted.

A contractor has since removed the new drain line and put things back as they were. The smell outside straight away is gone.

On to my question: why can reconfiguring the drains in this fashion cause a septic smell outside the house?

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    Apparently you have the answer in your question ...which is YES!
    – RMDman
    Aug 31, 2023 at 14:41
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    Haha, yes I see that it can be the cause but I was mainly curious as to the why :)
    – TheNextman
    Aug 31, 2023 at 14:51
  • Were there open windows where the eau de toilet was wafting out?
    – FreeMan
    Aug 31, 2023 at 14:52
  • Because largely it makes no sense to me (as a total layman) that reconfiguring the drainage inside the house can cause the tank to start smelling... But there obviously is a reason, and the septic people I spoke with just said "oh it's normal for the tank to smell sometimes".
    – TheNextman
    Aug 31, 2023 at 14:52
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    The only reason I can think of as a non plumber, is that the pipe was adding extra air into the tank. I do not think this is a good reason, since the pipe should have let the gases out in the house.
    – crip659
    Aug 31, 2023 at 15:22

1 Answer 1

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If your HVAC system has a fresh air intake -- for example a small duct to the outside to add 10% fresh air whenever the blower runs -- then you could have positive pressure in the house. If you feel air in your face when you open the front door slightly from outside when the HVAC is running, you have positive pressure.

You had a faulty drain setup, and if there was no P-trap between the open drain pipe in the house and the septic tank, you would have been forcing air into the septic tank through the new drain whenever the HVAC runs.

That air would have to go somewhere. It would seep out of the septic tank cover, which is not air-tight, and assuming it is buried by a couple inches of soil, the air and consequently odors would emanate from the area of the septic tank.

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  • Marking as answered in the face of any other suggestions. Thank you!
    – TheNextman
    Aug 31, 2023 at 15:30
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    This sounds quite logical, @TheNextman, however it only applies if you have a fresh air intake. Also, you might want to wait a bit before clicking that check mark, as it may discourage others from answering and you might miss out on some other good info.
    – FreeMan
    Aug 31, 2023 at 15:39
  • The HVAC indeed has a fresh air intake
    – TheNextman
    Aug 31, 2023 at 15:55
  • @TheNextman FreeMan beat me to it! I was going to mention that it's customary here (for future reference) to wait 24 hours before accepting an answer because we have regular contributors all over the world. It's nice to know that you do indeed have a fresh air intake on your HVAC so the answer is plausible.
    – MTA
    Aug 31, 2023 at 16:57

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