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I have a tri-level home with a shower on the lowest floor. When I noticed that the bathroom smelled like rotten eggs, I shined my flashlight down the shower drain and saw that the water level was low enough in the trap to let sewer gas in the house. Sometimes after running the shower, I hear gurgling, which must be the water siphoning out of the trap.

The shower drains into the sump hole, and the sump hole vents to the roof. I went on the roof to check the vent, and it didn't seem to be blocked.

Also, I have a radon mitigation system that was installed by the previous owner, and it sounds like water swishing around in the pipes all the time, and I'm not sure if that has something to do with it.

What could be causing the low water level in the trap?

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If your shower drained to your sump, you would not get sewer gasses from the shower, so something doesn't quite add up here. Does your sump go into the sewer? –  The Evil Greebo Aug 15 '12 at 14:18
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Does your sump pump have a pump and drain pipe that goes outside from it? If so, that's not where your sewer pipe is going, I think. Easy test - run the shower, see if the sump well fills up. –  The Evil Greebo Aug 15 '12 at 15:12
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Most likley the air pressure drop from higher water falling down sucks the lower floors trap to get air; so it breaks the water seal. The only way to over come that is install a check valve on various levels of the main drainage or after the shower drainage somewhere. So air gets sucked in when it needs it, without sucking the water out of the showers trap that is affected. –  ppumkin Aug 15 '12 at 15:52
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I'm confused as to why a shower would go to the sump. Don't pee in the shower until it's fixed. –  Chris Cudmore Aug 15 '12 at 16:18
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@woz It sounds like you may want a plumber to come redesign the drains. –  Tester101 Aug 15 '12 at 17:39
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2 Answers

up vote 5 down vote accepted

If your shower (or any drain) is draining to your sump pit, you are overworking your sump pump and shortening it's life span. If you have a radon mitigation system that uses the sump pit, you are also creating a situation where radon can potentially enter your house. The only water that should ever be in the sump pit, is ground water.

Aside from that, here are a few things that may cause your issue.

Evaporation

If the shower is not used very often, the trap can naturally dry out due to evaporation.

Solution

Either use the shower more, or routinely pour water down the drain.

Blockage in dry vent

If the vent becomes obstructed, water flowing down the drain can produce a syphoning action and suck the trap dry.

Solution

Clear the blockage in the vent, by snaking out the vent pipe.

Blockage in wet vent

In situations where a vent is not directly connected to a drain line, a wet vent will be used. A wet vent, is a pipe that serves as both a drain line and a vent.

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If this pipe becomes restricted, syphoning action similar to a blocked dry vent can suck the trap dry.

enter image description here

Air cannot get past the obstruction while water is flowing through the pipe, which creates a pressure difference that must be equalized. There are two things that can happen in this situation. Air can be forced past the obstruction (possibly causing gurgling), or air can be forced through the trap (causing gurgling that you're more likely to hear).

Solution

Snake the drain and clear the obstruction.

If you really are draining to a sump pit used in a radon mitigation system

If both drains and a radon mitigation system are connected to the sum pit, the radon system could be sucking the water from the trap. A radon mitigation system works by sucking radon out from under the house, and venting it outside. If the pressure in the radon vent is lower than the pressure in the house (which is sort of how they work, so it probably is), the water in the trap could be forced out and down the drain by the higher pressure air in the house.

Solution

Don't drain to the sump pit, especially if you have a radon mitigation system.

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I may have to check for blockage with a snake then. Does it change anything, though, that the shower drains to the sump hole? Doesn't that mean there won't be any dry vent? –  woz Aug 15 '12 at 17:25
    
@woz You said the sump hole vents to the roof (which is likely the radon vent), that is the dry vent in your situation. –  Tester101 Aug 15 '12 at 17:36
    
Ok, that makes sense. –  woz Aug 15 '12 at 17:47
    
I hate to criticise your excellent answer, but your use of the term wet vent is wrong. The correct term here is trap arm, the horizontal line between trap and vent takeoff. A wet vent is when a fixture drains into the vent of another fixture. Most commonly seen where a lavatory drains into the vent for a toilet. A +1 for useful answer never the less. To be fair, the IPC definition could easily be misinterpreted to fit your version, but the section on vertical wet venting makes it clear it is as I describe. –  bcworkz Aug 15 '12 at 22:13
    
@bcworkz You are probably right ,I'm not up on my plumbing terminology. However, I'm assuming the vent stack in this situation probably has other fixtures waste lines connected to it (I just didn't include them in the image). –  Tester101 Aug 16 '12 at 11:57
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Gurgling indicates a blocked or improper vent somewhere.

It could be a blockage where the vent goes up immediately after the shower trap.

It could also be an improper vent that is installed too far from the trap, and the sump hole could easily be too far away. For example, if you flush the toilet and it goes down the same drain as the shower, it will be pulling air behind the water. If that air isn't coming from a vent, then it will suck out the water in the shower or sink traps.

One last possibility that comes to mind is a vent that is improperly slopped that has become blocked with moisture (effectively becoming a trap in the vent line).

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The vent being too far away seems likely since there is no vent until it gets to the sump pump, and I think the venting on the sump pump is questionable as it is. It sounds like I'm stuck with the problem unless I do some serious plumbing work, would you say? –  woz Aug 15 '12 at 15:04
    
If your toilet is draining to your sump pit, I think you have bigger problems than the shower trap being sucked dry because of it. –  Tester101 Aug 15 '12 at 15:07
    
No, the toilet is definitely not draining into the sump. That would be terrible! Just the shower and laundry room sink. –  woz Aug 15 '12 at 15:32
    
Sump pumps shouldn't need venting. Sump pumps use a pump to suck water out of the sump well and PUSH it out of the house. Venting is used to allow gravity to PULL water out of the house by allowing air to get behind it in the drain pipes. –  The Evil Greebo Aug 15 '12 at 15:45
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No, they're not. They're saying DRAINS need venting, and they do. Your toilet drain needs a vent. Your shower drain needs a vent. A sump pump does not need a vent because 1) it's using a pump, not gravity, and 2) the sump well is exposed directly to air - it's self vented. (Ok edit- possible exception - a sump well can be sealed to prevent radon escape, in which case that does need an air vent) –  The Evil Greebo Aug 15 '12 at 17:58
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