We moved into an older home 4 yrs ago and shortly after I changed the 2 upstairs toilets for low flush ones. We started getting the occasional smell of septic in the house, which seemed to start upstairs when we showered and then spread, but it always disssapated within a few hrs. I put double wax seals on both toilets and that stopped the smell for almost 2 yrs. Then on Dec 7, 2011 it came back and wouldn't dissapate. After 3 days we had the septic tank emptied and the smell left, but returned on the 17th. We have made sure that daily we put water in all the drains, We have run hot water from the hot water tank down all vent stacks to clear them, we out in 2 cheater vents. And are now trying a Bio product in all the drains overnight. I checked the septic tank and it appears to be working properly. Any ideas would be appreciated.
Thanks
Lost and Confused!

Smells and noises are the hardest to track down if you're not there.
In this case your nose is the best detective. Where's the smell the strongest? Let me know what fixture is the closest to the strongest smell. Back to you, Tom

The smell is the strongest in the basement. There are no fixtures there but the main stack from the two second floor bathrooms passes straight down and through the wall then comes out in the basement where the smell is the strongest. BUT also there is a second stack for the downstairs toilet/sink/shower/washing machine and the kitchen sink - that ties in at that point in the basement. There are no signs of a leak anywhere - in the basement or upstairs walls. The kitchen sink is approx 15 feet from where it joins. There are no bad smells coming directly out of any of the sink etc drains either.
Someone suggested to us that the problem is in our field - being too old the bacterial action has stopped but that it could be reactivated with the bio enzyme product --- any thoughts? To us that doesn't explain how the smell gets in the house. There is also no physical signs of the field not working.

Thanks for your help!