I've been trying to figure out why I occassionally get a sewer gas smell from my master bathroom shower drain, usually within a few minutes of using the shower. It doesn't happen all the time, and I wonder if it has anything to do with the 3 ft extension to the stack vent above the roofline that most houses in my area of Albuquerque seem to have (this is an apparent retrofit that had to be done to get the discharge from the stack vent above the evap cooler on the roof). I'm wondering if this extra extension has changed the physics of the DWV system enough to sometimes pull the water out of the p trap? Also, the toilet in this bathroom has been retrofitted with a vacuum-assisted flush unit, and this toilet is above the shower in the drain line. I wonder if that, too, is a possible cause by being too strong of a flush that it pulls the water out of the adjacent shower p trap as well? Thanks for any comments or advice.