Odors
Odor problems are difficult to diagnose because it is an invisible problem. There are two main sources of odors in the plumbing system: sewer gases and odor-producing bacteria.
Sewer gases are kept in the drains and vents by seals and p-traps in the plumbing system. The water in a trap prevents sewer gases from venting back into the home or building. All fixtures connected to the sewer have a p-trap. The trap on a toilet, urinal, and bidet is integral, cast into the shape of the toilet. All sinks, showers, tubs, laundry and floor drains have external p-traps, though tubs and shower traps are under the floor where they cannot easily be seen.
A wax ring on a toilet is the most common example of a seal that prevents sewer gases. It is possible for a wax seal to not leak water, but still allow gasses to escape. This occurs when the outlet of the toilet is directing the water down into the hole, but some of the wax ring is not firmly pressing against the porcelain around the waste outlet.
P-traps only work to prevent sewer gases when they are full of water. The trap may dry out if the water evaporates due to not being used for a long time. This is a common problem with bathrooms that are not often used. One helpful trick to check if the bathtub trap is dry is to flush the toilet and listen near the tub drain for the sound of the flush and water draining. When the p-trap is full, you will not be able to hear the sound. This trick also works with other fixtures, including checking the wax seal if you have another person flush another toilet nearby. Another cause of water evaporating out of the trap quickly is if the trap is full of foreign material. If there is sand or waste in a tub drain, for example, the trap will not hold its normal volume of liquid which will cause it to dry up more quickly.
Another way a p-trap fails is if it leaks water. This would usually be caused by a crack or hole in the trap, or a leaking joint. Cast iron traps often develop holes from rotting over time or become brittle and get holes from being cleaned out with drain cleaning cables. Plastic drains can also get holes or cracks from aggressive cabling with a drain snake. A less common cause of traps leaking water is if there is a hair ball is laying in the trap, and laying down out into the trap arm, allowing water to wick up the hair and out into the building drain.
The final cause of a trap not holding water is due to the water siphoning out of the trap. Vents are what prevent water from siphoning from the traps, so if the vents are clogged or improperly plumbed then the traps may be subject to siphonage.
After dry traps, the next most common cause of sewer odors is cracked or damaged vent pipes. Damaged vents are difficult to find because they are usually hidden in the walls. To find a damaged vent, use one of the troubleshooting methods below.
Smoke testing can be done by pumping smoke into the drain waste and vent system (DWV) and watching for where the smoke comes out into the building. This requires two or more people; one to operate the smoke, and one to observe where the smoke leaks out from.
To perform the smoke test, use a smoke-producing candle, bomb, or fluid. Candles are least expensive but are only good in very small areas. Bombs are also inexpensive but produce much more smoke than a candle. Smoke liquid is more equipment and requires special equipment. Smoke bombs are usually best for most situations. The bombs burn very hot, so a stainless steel shop vacuum is required. Once ready, you light a smoke bomb and place it in the shop vac. After about 10 seconds you can turn the vacuum on to blow the smoke into the DWV system. This can be done in a sewer cleanout or through a roof vent.
There are various strategies for making sure the system fills with smoke properly. You may need to place tape over some of the vents to restrict air flow. You may need to install a test ball in the downstream sewer cleanout so smoke is not escaping out into the sewer. These techniques are only necessary if you do not find the issue with the first smoke bomb, and in general are only needed if the leak is small.
To start, blow in the smoke and watch for when smoke begins to blow out of all of the vents, then shut off the shop vacuum. This will be a sufficient test in most cases when there is a bad enough problem. Look for smoke coming out from under toilets, through traps, or through the wall. Look for smoke coming from around electrical outlets and switches since they are not sealed around, and will be the easiest spot for smoke to escape. If the smoke is leaking out from the system then odors would leak out of the system, too. If smoke comes from the floor this could mean a floor drain has been covered up with flooring.
Sewer Gases
Bacteria
Bacteria can produce odors that are as bad or worse than sewer gases. They may grow anywhere that is a moist environment that is not periodically cleaned. They often occur on synthetic materials, such as certain plastics and rubbers that do not have antimicrobial properties. They also will tend to grow in organic compounds, like natural soaps, lotions and shampoos that build up in areas of drains that are not self-scouring.
Common areas to find odor-producing bacteria:
Lavatory sink overflows and drains
Shower drains
Shower door seals
Clothes washer seals (especially front-load models)
Clothes washer lint traps (lint traps must be cleaned ever 30-90 days)