Knowing how a dry P-trap can cause Sewer Gas Odors

By Thornton Plumbing and Heating

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Blog Highlights:

  • People can easily mistake offensive odors within the home for sewer gas smells
  • Tracking the source of the smell is crucial when it comes to identifying the reason behind it
  • A dried-out or empty P-trap is one of the most common causes of sewer gas odors
  • A P-trap with a proper vent is used to prevent sewer gasses from leaking into the home
  • Proper ventilation is important, otherwise, draining will cause the push and pull of air on the P-trap, allowing sewer gasses to leak
  • Bathrooms, kitchen sinks, and showers that have been unused for months can have a dried-out P-trap

Does your house or building interior smell like a sewer?

A dried-out P-trap or drain with poorly maintained ventilation can allow gas smells to leak inside the home.

A lot of customers look for professional plumbers to help them find where the source of the sewer gas odors come from. And although it can be caused by so many different factors, we try to provide our clients with a few tips over the phone just to see if they can find the source on their own without paying for any cost.

Tracking where the odors come from

The first thing that needs to be answered is whether the homeowner truly smells sewer gas. Many people mistake the smell of a dead animal inside the house for sewer gas. But so you’ll know, a dead animal typically has a pungent and sickening smell that will almost certainly trigger your gag response or make your stomach churn.

Real sewer gas, on the other hand, has the odor of feces. Its stench is typically combined with hydrogen sulfide which has a rotten egg odor, as well as musty, moldy smell.

We also ask owners if they have recently asked for some type of work done in the home or in the business establishment. We ask if the house or building has recently gone through some remodeling, renovation, or pest control work so we can narrow down the culprits.

If the main cause has not been found, we talk to our clients about the possibility of the P-trap drying-out. This is because a dried-out or empty P-trap is one of the most common causes of sewer gas odors.
P-traps and sewer gas odors

All plumbing fixtures and drains have a P-trap installed and it must be installed according to code. This protects us from sewer gas. Unfortunately, as soon as the P-trap becomes dried-up due to various reasons, it will no longer be able to prevent sewer gas from entering into waste pipes and into our houses or buildings.

Each P-trap on each drain must have a vent that can help protect it. These vents are known as plumbing vent stacks and they are typically visible from the outside as they stick out of the roof of houses and commercial buildings.

The importance of proper ventilation for P-traps

When the drain is used, the waste and the water that is coming down the drain displace a lot of air from the pipes. This makes the plumbing system breathe in and out. The water and the waste create an air wave in front of it and it is also dragging some air behind it as it rushes down the pipes.

Without the help of proper ventilation, it will be very hard for us to send water and waste down the drain. This is because the drain pipe has to push and pull air down the drain through the plumbing vent stacks; otherwise, it will pull and push air through the P-trap, allowing sewer gas to leak inside.

In older homes, cabins, commercial buildings, and even in improperly vented additions, one can hear a “glug, glug” noise when you use a drain. This is actually the sound of air and sewer gas being pushed or pulled through a P-trap. Without the proper help, this will eventually cause the water seal to go low, preventing the P-trap from protecting your home or building from sewer gasses.

Knowing why unused or improperly vented P-traps can cause issues

There are different types of P-traps. Those under the bathroom or kitchen sink, for example, are visible. While floor drains, showers, and toilets have hidden P-traps. Each drain must have a P-trap that is vented. And each of these P-traps must have a regular supply of water so as to allow the seal to work properly.

In many instances, talking about the P-trap’s need for a regular supply of water will let clients realize the fact that the odor comes from spare bathrooms, unused showers, or old kitchen sinks. If the smell comes from drains that have not been used for a few weeks, the best thing to do is to perform a quick fix. This done by letting the water run for a few minutes in the shower or sink and by increasing the ventilation in the area in order to let the bad odor out.

In most instances, sewer gas odors come from a P-trap that has lost its water seal. It might have dried-out because it has not been used for a while, it might be leaking, or maybe something has drained the water in the trap. Waste materials like a clump of hair, or fibers, can easily break the water seal. In some areas, it can take three to five months without use for water within the P-trap to dry out and allow the sewer gasses to flow inside.

Tracking down the sewer smell

Sewer gas odors can be caused by different factors, and getting rid of them requires a bit of logical detective work. Whether the source of the odor comes from an unused P-trap, or from an open cleanout that has been covered by a drywall, dealing with the problem will require patience, precision, and experience for both plumbers as well as DIY home

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