Sewer gas can be a difficult problem to solve, but it is absolutely not impossible to solve. Sewer gas is created by the decomposition of waste that is found in public and private sewer systems and septic systems. The odor caused by the gases can be overpowering and it is toxic. The gas is also explosive as it often contains methane.
Plumbing drainage systems are designed to keep sewer gas inside the pipes and any gas that does exit to the atmosphere goes outside the home through the vent pipes that stick up through the roof of a house. These vent pipes on the roof are intake vents, not exhaust vents as some believe. When a large volume of water enters a plumbing drain it pushes the air in front of it towards the sewer or septic tank. The air must be replaced and is sucked into the plumbing system through the roof vents.
The source of sewer gas odors can be plumbing fixtures with traps that have gone dry or have lost enough water that the water seal within the trap has broken. Water can rapidly evaporate from toilets and the traps below tubs, floor drains and just about any fixture within a few months. Cracks in either plumbing drain lines or vents pipes are another source of sewer gas leaks. If the crack is in a drain line, you will often see a water leak associated with it. But vent pipe cracks are much more difficult to locate. They can leak a great deal of sewer gas and you may not be able to locate the source easily without a special machine.
Sewer gas leaks can be quickly discovered by a plumber that owns a machine that generates artificial smoke. This smoke is simply visual and does not create a lasting odor and it won’t stain surfaces in a house. The plumber will connect the smoke generation machine to the plumbing drain system and then will block off the drain pipe leading to the sewer and cap off all roof vent pipes.
When the smoke machine starts, it will slightly pressurize the plumbing system. If there is a cracked pipe or a loose fitting joint, the smoke readily exits at that point before going up through a fixture trap filled with water. Usually the source of the sewer gas leak can be discovered in less than an hour.
Sewer gas problems can also be caused by clogged plumbing vent pipes. This can happen in old homes with cast iron vent pipes that get clogged by years of rust scale that falls off the inside of the pipe and forms a clog at a 90 degree bend in the pipe.
Leaves and all sorts of other debris can clog plumbing vent pipes. When a vent pipe gets clogged, the replacement air needed by the system will get sucked into the pipes through a fixture inside the house instead of the vents. When a large amount of water is placed into the drain pipes by a toilet or a powerful washing machine pump, it can suck the water out of a nearby bath tub trap or kitchen sink trap. Once this happens, sewer gases immediately enter the room through the dry fixture trap.
Sewer gas odors can occur in manufactured and mobile homes when valves, designed to open when water is draining to allow for air to get pushed out of the drain pipe and close when water isn’t draining, get stuck open allowing sewer gas to escape into the home. These valves are located underneath sinks and are easily replaced. Simply locate the valve on top of the vent pipe under the sink and unscrew it. Get a new one from the local hardware store and screw it on.
To slow down evaporation from fixture traps that are not used frequently, fill them with water then slowly pour four ounces of mineral oil into the fixture or floor drain. The mineral oil will evaporate far more slowly than water and the trap will remain wet for many months.