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Sewer gas smell

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TMC

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I have solid plumbing and building knowledge, but a problem I can’t grasp. Hoping for direction from the experts.
Builder and plumber have been Zero
Help.

Pre-engineered home 2018 build
Single story
Flat roof
Crawl space
Single bathroom
Laundry

I began experiencing sewer gas smell from the bathroom sink. My nose directly to the sink confirmed it’s coming from there. No smell from shower , tub, toilet , kitchen sink or laundry.

Investigating myself a found an “s” trap installed In the crawl space connected to the bathroom sink. I assumed siphoning was occurring causing a dry trap and the smell. Told the plumber what I found. He changed the s to the correct p trap. It was bone dry.
Smell went away for a while.

smell came back. I started to track the factors I think could be related. . Cold weather. Dryer running. After I shower Or do a load of washing laundry. running water into the sink stops the smell but only until I stop running the water.

Sorry for the long post. I wanted as much detail included.

I’m reading about super air tight homes and dryers drawing fresh air. Not sure if I’m on the right path.

HELP !!

thanks for your time and expertise.
 

Geofd

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the p trap may be getting siphoned also, the developed length can't exceed 24inches from fixture outlet to trap weir....depending on your code restrictions a drum trap may work it hols more water and i think its less likely to siphon
 

TMC

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Thanks for the reply.
What is meant by developed length ?
 

Geofd

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from the bottom of the sink to were it exits the trap if your look up trap weir there are examples
 

Diehard

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When, "reading about super air tight homes and dryers drawing fresh air", this info may be of some use to you.
But I would think that anything in these homes that can create a positive or negative pressure greater than 1" WC would have already been taken into consideration relative to the typical trap seal designs.

"1002.0 Traps Protected by Vent Pipes.
1002.1 Vent Pipes. Each plumbing fixture trap, except as otherwise provided in this code, shall be protected against siphonage, backpressure, and air circulation shall be assured throughout the drainage system by means of a vent pipe installed in accordance with the requirements of this code.


The section (see also Section 901.2) requires the trap to be protected from siphonage and backpressure by a vent (see Figure 1002.1), and that air circulation is assured throughout the system. Most of the measures to accomplish this are discussed in Chapter 9 and should be reviewed along with this chapter.


The protection of the waterseal trap is assured upon a simple pneumatic principle, namely, equal atmospheric pressure must be maintained on both sides of the trap. When a trap is filled, the atmospheric pressure on the fixture side of the trap water seal is equal to the atmospheric pressure on the outlet side of the trap water seal. The vent connection to the trap arm maintains that equal pressure on the outlet side of the trap. As is discussed in Chapter 9, the vent system is designed to maintain pressure variation within the system of plus or minus one-inch water column (.25 kPa). Pressures above or below that design pressure will cause a trap to lose its water seal. A positive pressure on the downstream side of the trap will cause air to blow through the trap seal, pushing its contents into the fixture. A negative pressure will siphon the trap seal into the trap arm and the drain.


This allowable pressure variation of ± one-inch water column is a considerable factor of safety for a trap seal of 2 inches or greater. For example, if a trap seal has a depth of 2 inches (see Figure 1005.1) and is subjected to a negative pressure of one-inch water column, then the remaining trap seal depth would be 1 1/2 inches. If the trap seal is 4 inches, then the remaining seal depth would be 3 1/2 inches. Hence, it would take a negative pressure of a four-inch water column to siphon the water from a trap with a 2-inch seal below the dip of the trap, and eight-inches of water column to siphon a 4-inch seal below the dip of the trap. Designing a venting system according to Chapter 9 ensures considerable safety margins by maintaining the pressure variations within ± one-inch water column."
 

Jeff Handy

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Be sure to wipe your history off of her computer, if you go anywhere sketchy, haha.

A good free cleanup program is called CCleaner.
 

TMC

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What a great forum. Quick replies.

here are photos of the components and layout. The sink does not have an overflow - I have a thing for funky sinks.

when it was installed he said something about needing a specialty stub piece because there want an overflow.

Here is the layout.
Below grade the trap on the left is the one in question as it is from the sink.

re: bleach use. The odour is not a cleanliness or residue thing. 100% it’s sewer gas coming through bathroom sink.

thanks for ongoing help everyone.
 

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Jeff Handy

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Is there any vent on that drain line where it leaves the sink cabinet?

Your pics don’t show, but is there a Tee just behind the back of the cabinet, with a vent riser getting air from the roof?

If not, that looong drop is likely sucking the trap dry.

If so, maybe eliminate the trap below, put a P trap in the sink cabinet, add an air admittance valve right before it leaves the cabinet.

That would still be close to an S trap situation, but the AAV might prevent trap getting sucked dry.

Pros on here will probably have better info.
 

voletl

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Your trap cant be below the floor like that it needs to be in the cabinet

Dosent matter is there was a vent in the wall a vent before a trap doesnt do anything
 

Diehard

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