We have recently had a new septic tank put in our house (about 3 months ago). 1 month after the works were complete, there started to be an overwhelming septic tank smell in the house. There is only one room affected by the smell at the top floor, but the smell is overwhelming.

This shows the ventilation pipe that runs the house, the exit at top of house, and the room impacted from the outside:

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We’ve searched in vain, including even ripping out wall panelling in this room for what might be causing it but haven’t been able to track it down. The smell is only coming into this one room at the very top of the house and I’m wondering if it is because this room is the closest to the septic tank ventilation pipe? Any suggestions? I know the name of our septic tank system, it’s called the “Clear fox” https://clearfox.com/en/domestic-wastewater-treatment/ and the ventilation pipe from the system follows the house roof vertically before it exits into the air at a pipe chimney at the roof apex. This vertical ventilation pipe did not exist with the old ventilation system. I did a smoke test, and the smoke did seem to exit the ventilation pipe at the top without a problem..

The vendor says it’s an internal house problem (however there was no internal septic tank smell before his works) so he is trying to pass the problem to me.

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    Is the vent pipe new with the new system? Does the room have openings (I.e. windows and doors we can't see)?? Not sure if you could use the smoke they use to smoke test sewer lines in this scenario or not. The other possibility is that the vent pipes proximity to the problem is a red herring. It might be that reconfiguring the systems vent changed the overall inside stack venting in a negative way. – Tyson Dec 31 '17 at 13:50
  • Hello, and welcome to Stack Exchange. Where/how did you do the smoke test? What's the vendor's explanation for why the room smells like sewage, without it being a problem in his system? – Daniel Griscom Jan 1 '18 at 13:34
  • @Tyson thanks for the reply. There are two windows on the opposite side of the house that belong to that room. However I've pinpointed where the smell is actually entering from the room and its on the opposite side of the windows. Is there anyway to figure out / diagnose how the inside stack venting might have changed without ripping the house apart? – Java Guy Jan 2 '18 at 22:13
  • @Tyson also one thing I've noticed, the septic sewer gas smell is heavier than air and sinks as opposed to rises; ie if I leave the internal door open on the infected room, the septic gas sinks straight to the basement. Could the septic gas be leaking through the ventilation pipe and then sink through the roof tiles as it's not airtight ie the roof is vented to breathe? Thanks! FYI, these are the smoke pellets I used: amazon.co.uk/Rothenberger-6-7043-Standard-Smoke-Pellets/dp/… – Java Guy Jan 2 '18 at 22:13
  • @Daniel Griscom - thanks for the reply + welcome! For the smoke test, I opened up the septic tank caps, and lit a couple of smoke pellets and then put the septic tank caps back on quickly (the septic tank is new, and so I can easily put my hand down ie the septic tank is not full). The vendor's explanation is that it must be a problem with the house (even though I've explained many times that there was no odor with the old septic tank) – Java Guy Jan 2 '18 at 22:13

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