This one has me puzzled. When we take a shower in our first floor walk-in tiled shower we get a septic smell from the drain. The house is only 4 years old, the problem started about 1 year ago, is intermitten and most noticeable in the winter. I inspected the shower drain trap in the basement and it appears to have been installed correctly. The bottom of the drain pipe from the trap is 4 inches higher than the bottom of the trap. Is it possible that something could cause the water in the trap to be siphoned off? There are no problems with any other drain in the house including a second floor shower, everything drains ok and our septic system appears to be operating OK. I'm about to call a plumber but I'm not sure they could solve the problem.