I am getting a strong septic smell after flushing the commode or running water in the sink on the 2nd floor. I added an addition to my house 3 years ago and tied a new bathroom into the septic system. All was well for a while but now we get a strong septic smell every time we flush any commode in the upstairs of the house. Our sink in the new bathroom (2nd floor) also releases a smell when running water. I have also been having an unsual amount of clogged toilets. It almost seems like they are vapor locked. I have a vent pipe from the new bathroom sink going into the attic and ending at the peak vent inside. I also added another vent at the other end of the new plumbing for a whirlpool tub (tub never installed). It seems like sometype of venting issue but I am not sure. Any ideas?

Is there a floor drain? If there is try pouring some drain cleaner in it and flushing it with water. If there is not then pour a small amount of drain cleaner in the toilet, the sink, and the tub drains. Run all three simulatanously for a minute of so. Most sinks also have an overflow section. NOT at the same time as the drain cleaner, you can pour some bleach carefully into the top of the overflow to kill the fungi and bacteria that build up in there and cause a nasty odor. Good luck.