As I write this the wind is blowing 30 to 35 mph with gusts to 57 mph. This is not uncommon here in the Cascades. Every time we get strong wind it lowers the water level in all of the toilet bowls.
The other day I smelled sewer gas and went exploring, to find that the toilet bowl in the basement had lost its seal. This toilet is out of the way and doesn't get used much. So there may be 2 things going on, the lowering of the level and then evaporation of the small amount of water remaining.
The house is new construction, great experienced plumber did the waste system. Three vent stacks, all on the lee side of the roof from the prevailing winds. The house has a conventional septic system.
My biggest fear is being away for a week or two and having sewer gas fill the house.
So what can I do to reduce the effect?
My guess is there is a venturi effect on the vent stacks which creates a negative pressure and pull down the water level. I guess there could also be a positive pressure wave that creates a rocking effect in the bowls. It also might be both effects working together.
I assume its happening with all the traps, but the floor drains have trap primers, so if there is activity they refill. Maybe it only is happening with toilet bowls because of their design
Will putting a trap upside down on the top help?
How about drilling holes in the stack above the roof.
Would adding a double wye to the top of the stack help?
How about extending the stack above the highest point of the roof?