What’s That Smell? Common Causes and Solutions of Sewer Odor.

By: Patrick Hooper

The smell of sewer odor in a house is a very disconcerting problem for a home owner. We all recognize it when we smell it and we definitely don’t want it in our homes. In addition to being horrifyingly embarrassing and nauseating, it is also very dangerous.

Sewer gas is created by the decomposition of waste materials found in both public and private sewer systems. The characteristic odor possesses toxic methane gas that, with excessive exposure, can lead to physical ailments such as hydrogen sulfide poisoning, or even asphyxiation. It can also be explosive. Should enough methane gas build up in a home, the potential for ignition is a definite possibility. For the safety of your family, this is not a problem to be ignored or put off. It needs to be addressed immediately.

Diagnosing the source of an invisible sewer odor can definitely be a challenge. Knowing where to look is the first step to identifying the origin of the smell as quickly as possible.

Worn Wax Ring

A worn wax ring underneath a toilet may be the culprit behind the stinky smell that comes from the sewer drain pipe. You may or may not be able to notice a water leak coming from beneath the toilet at the same time. If the wax ring is not the culprit, another part of the toilet may be to blame.

To identify whether a wax ring is the cause behind the smell of sewer gas, look for other symptoms that may possibly accompany the smell. After you flush, look around the base of the toilet for escaping water. If there is caulk installed completely around the base, you may not be able to spot a possible leak. If the toilet moves or rocks, instead of being secured to the floor, the wax ring has likely failed and could be the reason for the smell.

Blocked Or Lack Of A Sewer Vent Pipe

soil vent pipe_draindomain.com

A sewer vent pipe not only allows foul air to vent from the drainage system, it also allows air into the system. When an upstairs toilet is flushed, a block of water drops down the vent stack. If there is not an open vent at the top of the pipe a vacuum can momentarily occur behind this falling plug of water. If your bathroom sink, shower or bath waste lines discharge directly into the sewer vent pipe this vacuum can draw water out of their traps. Once the water in the traps drop to a certain level the barrier fails and you have, in affect, a direct vent from the drainage system into your bathroom. Until the next time water passes through the trap(s) and the water level is restored, sewer odor may be allowed to enter the home.

The above scenario can also affect kitchen sinks if they are plumbed directly into a sewer vent pipe. This can also be a problem with basement bathrooms that have no vent system at all. Additionally, if a hand wash basin is plumbed into the pan connector at the back of toilet, this same siphon action may occur.

There are 2 simple tests you can perform if you think the above applies to your situation. First, using a garden hose–simply fill the vent stack with water from the roof. If the pipe fills and overflows–there is likely an obstruction that is restricting air flow. Secondly, just leave the plug(s) in the sink, bath or shower drains and fill with about an inch of water, this should stop the venting. Then, just run water through the trap(s) after each flush of the toilet to see if this cures the problem.

If you have confirmed that a siphon is indeed drawing water from the traps and established that there is a vent on the system that is blocked–it likely can be cleared by auguring or hydro jetting.. Blockages at or near the top of sewer vent pipes can often be the result of things such as wasps nests, birds nests, dead animals, leaves, debris, etc. On more modern properties the vents finish within the roof space and have an air admittance device fitted that can be prone to failing due to the build up of dust and loft insulation fibers. If you can reach the valve they are easy enough to dismantle and clean out.

While siphoning is caused by low pressure in the vent stack, blockages can lead to high pressure as well–resulting from the build up of sewer gas that is then forced out through the  traps and toilets. So, this situation is the converse of the siphoning effect. Instead of water from the trap being sucked into the vent stack by a low pressure vacuum, sewer odor is forced out through the trap from the high-pressure build up of gas that cannot escape through the blocked vent. You’ll probably hear gurgling and bubbling in the fixtures and may also smell a vile bacteria odor mixed with sewer gas.

Dried Drain Traps–Water Seal Evaporation

As perhaps the most common cause of sewer odor in a house, it is also one of the simplest to remedy.

As with a clogged or blocked air vent, the problem is an emptied or dried drain trap. Unlike a clogged air vent, instead of the water seal being siphoned out of the trap, it simply evaporates. Infrequently used drains can dry out due to evaporation. With no water seal in the trap, sewer gas odors can easily infiltrate the home. Regularly flushing unused drains with about a gallon of water will maintain proper water levels in the trap and keep sewer smells where they belong. Additionally, you can also try pouring olive oil or mineral oil directly into the drain. The oil will separate from the water and float to the top– creating a barrier above the water seal–preventing or at least minimizing evaporation.

Other Potential Causes

Cracks in either plumbing drain lines or vent pipes are other potential sources of sewer gas leaks. If the crack is in a drain line, you often see an associated water leak. But vent pipe cracks are far more elusive. They can leak vast amounts of sewer gas and be difficult to locate. Some plumbers can do a pressure checks of vent stacks by closing off both ends of the pipe and then slightly pressurize the line while injecting an artificial smoke that can identify exactly where the leak or leaks are.  Sometimes a furnace or air conditioner can add to the problem if the vent pipe is close to a return air duct. The leaking sewer gas can be sucked into the return air system and then broadcast throughout the entire house by the air handler in the furnace.

I once had a customer who lived in an older home call me because of sewer odor that seemed to be in the area of his front porch. Many older houses in the Cincinnati area have their gutter down spouts tied directly into the main sewer line. After completing the inspection I was able to diagnose the problem as a leaking, dried trap right where the down spout connected to the sanitary . As a result, with no viable water seal, sewer gas was backing up through the line and coming out of the gutter outside the home, right next to the porch.

“Ask the Builder” is hosted by Tim Carter and offers all kinds of great advice on home maintenance.  I’ve included a link below to one of his articles about Sewer Gas Smell. Check it out.

Sewer Gas Smell







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