![]() I'm not sure how plumbers account for this in normal layout of the vent stack. Meanwhile on the face of the house you have slightly elevated pressure that would add to the problem as you surmised. The roof or any obstruction acts like an airfoil, reducing the local pressure on the back side. Caps might fix it if it is just venturi (an effect of the opening itself), but I suspect the issue might be because the vents are on the lee side. How about extending the stack above the highest point of the roof? Would adding a double wye to the top of the stack help? How about drilling holes in the stack above the roof. ![]() Will putting a trap upside down on the top help? Maybe it only is happening with toilet bowls because of their design I assume its happening with all the traps, but the floor drains have trap primers, so if there is activity they refill. It also might be both effects working together. I guess there could also be a positive pressure wave that creates a rocking effect in the bowls. My guess is there is a venturi effect on the vent stacks which creates a negative pressure and pull down the water level. My biggest fear is being away for a week or two and having sewer gas fill the house. The house has a conventional septic system. Three vent stacks, all on the lee side of the roof from the prevailing winds. The house is new construction, great experienced plumber did the waste system. ![]() So there may be 2 things going on, the lowering of the level and then evaporation of the small amount of water remaining. This toilet is out of the way and doesn't get used much. The other day I smelled sewer gas and went exploring, to find that the toilet bowl in the basement had lost its seal. Every time we get strong wind it lowers the water level in all of the toilet bowls. This is not uncommon here in the Cascades. As I write this the wind is blowing 30 to 35 mph with gusts to 57 mph. ![]()
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