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

Question

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?

Answer 1

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.

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

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.

Answer 2

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).