Shower p-trap going dry quickly

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JeffeVerde

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I have a shower drain in a new house that's consistently admitting sewer gas two days after its last use. We have 50% humidity, no heating or a/c running, and none of the other drains are doing this, including two guest baths that go weeks without having water run through them. The shower is on the ground floor, and there are stacks to upstairs bathrooms both up and downstream of this fixture.

Before taking this to the builder (new home warranty), is there anything I can do to help define the problem so I know they're correctly addressing it? The house is slab on grade, so I can't visually examine the shower drain. My test so far has been to run the shower, then check the bathroom periodically until I smell sewer gas.
 

Terry

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You should be able to shine a light down the drain and see if there is water there.
You may even be able to put something down there and see how deep the water is on the seal.
 

Nukeman

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I had the same issue. In my case, it was the basement bathroom shower and the cause was lack of venting. There are two bathrooms at the front of the house (one on 1st floor, one on 2nd). This stacked dropped vertically under the basement slab and went horizontal towards the back of the house (where the basement bathroom tied in). After a day or two, the trap would be dry as the slugs of water moving past that connection would pull water out of the trap. I took up the concrete, rerouted the plumbing, and ran a vent all of the way to the roof. That fixed the problem.

I would take sort of a dipstick approach. Get a small wooden dowel that will fit through the grate on the drain (or remove the grate). Pour some water in the trap to make sure it is full and fully insert the dowel. When you pull it out, you will see how deep the trap is when full. Wait a couple days and try again (without adding water). I'm sure you will see it is now lower (if you write down the depth each time you will also know how much it dropped.

The process won't tell you how you are losing water. It may be a venting issue or there could be a crack/leak in the trap (especially is they didn't use a solvent weld trap. If you know how the other plumbing is run, you could try not using the upstream bathroom for a few days and see what happens. That could let you know if it is a vent problem or a leaking trap.
 

Jadnashua

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Either no venting, or improperly installed venting...the vent could be too far from the trap, or they just didn't vent it at all.
 
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