I recently had my basement sewer lines replaced. The original line had an s-trap in the basement and the cap for the trap had a bell on it that covered the trap. The new line has what they called a p-trap and has only a grate covering the trap. I've noticed every week or two an odor from the trap. There seems to be sewage in the trap. If I flush it out with a hose, the odor goes away for awhile but then returns. It seems that sewage is getting into the trap and not being flushed out. I didn't have this particular problem with the old trap.
Any ideas before I call the plumber who did the work?
Are you sure there is actually sewage in the trap? It sounds like the trap is losing its trap seal. (The water level in the trap is dropping below the trap wier and allowing sewer gas into the home.)
There are a few things that could cause this:
- Evaporation - The water may be evaporating out of the trap. Try putting a little vegetable oil in (1/2 cup should be plenty) after you flush it next time. The oil will sit on top of the water and slow the evaporation down.
- Siphonage - The trap arm may be too short, or too small, and could be getting siphoned out by sewage moving past it in the main line. To properly fix this means having the plumber come back, and making sure to leave enough pipe between the trap and the drain line it is dumping into. In Canada that is a minimum of 18" and if it is anything smaller than a 3" floor drain, it must be vented separate from just venting through the main stack.
- Wicking - The trap may have something in it like hair, or a mop string that is drawing the water through it, and slowly causing the trap seal to be lost. You may or may not be able to clear it by hand or with a shop vac into the trap.
These are the common things I have encountered, but there are probably other causes as well.
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