Plumbing fixtures whose traps have gone dry or have lost enough water that the seal within the trap has broken.
· Cracks in either plumbing drain lines or vent pipes are the other sources of sewer gas leaks. If the crack is in a drain line, there is often an associated visible water leak. But vent-pipe cracks are far more elusive and can go unnoticed.
· Sewer gas problems can also be caused by clogged pipes. This can happen in old homes where a cast-iron vent pipe gets clogged by years of rust scale that falls off the inside of the pipe.
· Leaves and all sorts of other debris can clog plumbing vent pipes. When that happens, the replacement air needed by the system will get sucked into the pipes through a fixture inside the house. Once this happens, sewer gas immediately enters the room through the dry fixture trap.