Shower drain siphoning and allowing sewer gas through the p trap

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Chad Curtis

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I have an issue on new construction in the shower. The shower was not vented because it's within 6 ft of a vent in the exit drain system. I believe the drain pipe has too much fall and is drawing the water out of the P trap and allowing sewer gas into the shower space. Soap suds tends to amplify the sewer gas into the shower bay. The trap eventually fills back up and the gas is not persistent. I've inserted multiple brands of those flow slowing devices to reduce the flow of water to the P trap, but the siphoning is still happening. Is there some some sort of insert you can install past the P trap in the drain to break the siphon before it draws the water from the P trap? The piping is in a slab and I don't want to break the concrete to add a vent to the shower if possible.
 

hj

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HOW CLOSE a drain is to the "main vent" is irrelevant. The piping between the two points will determine whether it needs a vent or not. Usually, there is no easy cure for an improper installation. Did ANYONE inspect the installation before the concrete was poured?
 

Terry

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If you are getting soap suds from the washer, then it's not being wet vented. A washer can't be part of a wet vent layout because it does siphon traps.
 

Chad Curtis

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https://www.rectorseal.com/sureseal-trap-seal/ might be worth looking into. https://www.ipscorp.com/plumbing/aba/greendrains is another offering.

I have no experience in those.

Was there no warranty on your new construction?
I have one of these in place now and when the siphon starts (not immediate) it makes these fail as well. There was a warranty and the issue didn't show up until after the warranty which is odd. I can't contribute any "change" in the environment to explain this. The builder even sent a different plumber back out after his warranty twice at his cost to investigate with a camera and he could see nothing wrong. The plumber thought the smell was coming from stale water on the shower pan membrane. The shower pan person came out and watched the fitting drain the shower pan so we have eliminated that. Everything that I read tells me that I will need to break up the slab and properly vent the appliance. I think if I could create a "ridge" past the p trap that will hold a small amount of water it would raise the water level in the p trap and break the siphon. I was thinking about some sort of plastic sleeve inserted and glued to create that ridge, but I can't locate any such fitting. I would be willing to deal with the potential drain stoppage if I could stop the siphon.
 

Reach4

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Your theory is that a siphon is being produced that sucks the water from the 2-inch trap. You could test for this.

This test can be done very cheaply but it will be ugly. The procedure would be to run some clear tubing through a shower trap into the trap arm. Blow to clear any water.

Take the near end, and put it into a glass of water. This glass could be elevated, maybe somehow taped into the soap dish. Tape the tubing to support that weight to keep the end of the tubing in place. Normally the level of the water in the tubing should be about the same as the other water in the glass. If there is vacuum in the drain air, the water level in the tubing would drop. An inch, at any time,would indicate a enough vacuum to support your theory, I think. What you would have made is called an open air manometer.

You can buy clear tubing easily and cheaply enough. You can also get it free from somebody who uses an oxygen machine at home and throws away a 30 or 40 ft length every month. Cut off what you need.
 
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