Seasonal odor from drains - vent/downdraft?

[Josee]

We bought a house 2 years ago, 1975 build on septic. The septic is pumped and checked every 2 years by the town, last pump was summer before last. We've had no obvious septic issues or smells.

Since moving in we've done a lot of work making the house more energy efficient and even had our house tested. Our house is so tight that we've also installed an ERV type air exchanger.

Last spring I started noticing a musty, dank (not sewer) smell in our upstairs bathroom. No apparent reason, no mold or standing water. The smell was coming from the drains so I opened and thoroughly cleaned years of sludge. That did not fix the problem. Then magically when the weather turned warmer, the smell completely went away.

Now its returned with the cooler weather. I suspect a vent/downdraft issue but could I be missing something? Do I need a specialty plumber? What should a plumber check for such an issue?
Many thanks!
Jo

 

[Dj2]

Could be a small animal in the vent.
Could be some debris stuck up there and collecting moisture.
Could be a faulty P-trap.
Pour some baking soda in the drain, or a table spoon of bleach - is the stink still there?

 

[Sponsor]

 

[hj]

That usually indicates that wherever the A/C condensate drains into has lost its trap seal and the blower is "sucking" the sewer gas into the unit and distributing it around the house. In the summer time the condensate keeps water in the trap, but the dry winter air, and no air conditioning running, lets the water evaporate.

 

[Josee]

I don't know if its pertinent but I can confirm that there's water in the shower drain - it's not dry. And I tried every baking soda, vinegar, bleach and pipe cleaner (not together!) but the smell remains or returns as soon as the products clear.