We have a horizontal shower drain installed that looks similar to this: The contractor who installed it failed to place a syphon in the drainage pipes which are accessible in our basement. We soon had sewer odors coming though our shower drain. Recently, we had another plumber install a syphon which corrected the odor problem. However, the drain in our shower is now very slow. In fact, we have to turn if off after 2 min to allow the drain to catch up. I've noticed that there seems to be air bubbles coming trough the water when we first start the shower. When the syphon was installed, the pipe that feeds into the outside sewer pipe was sealed and made air-tight. Could the air pressure in the pipes prevent water from draining fast enough?