Why did a new drain pipe cause my septic tank to smell outside?

Question

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?

Answer 1

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.