Solution to P-trap gas from self siphoning?

[tobinator]

Just finished making some major changes to my drain and septic systems and I've created a problem with sewer gas. Can I get some advice on fixing?

I rerouted my main 3" drain pipe to exit from a different point in my house, closer to my new septic tank. Used to have a pretty gentle slope and exit my house through the floor slab in my finished bsmt. I rerouted the run to exit though the stem wall in my laundry room so I didn't have to build out a chase to cover the drain pipes. After the pipe exits the bsmt., it drops pretty sharply to the level of the septic tank inlet. Drop is about 3' in 15' run. Final drain pipe is 3" ABS.

I'm getting gas through the vent/drain of my main floor shower, the last drain connection before the aformentioned changes. I'm pretty sure that the increased drop is siphoning the water from my tub trap.

Switching back to the old drain exit point is not an option. Short of that, would a deeper or bigger trap help (currently trap is 1 1/2' ABS that's only about 1" deeper than the depth of the 'P'). Could I "couple in" some depth to the trap to add a couple inches of water? Or could I switch to a 2" trap? What about an AAV? If they come in 3" ABS that might make it easy, otherwise I might be able to make a 1 1/2" or 2" AAV work?

Ideas are appreciated! Drawing attached.View attachment plumbing.bmp

Thanks,

Matt

 

[Geniescience]

Deeper than 1" is my idea. For the p trap.

Matt, can you explain whether there is something exceptionally shallow about your p trap? You explain things well.

Is venting is sufficient and adequate?

david

 

[Sponsor]

 

[Jimbo]

If the trap is a standard trap, and not something homemade, it should be fine. If you make a trap too deep, it will drain slowly. Even with the slope you have, that trap should not siphon, unless the vent is blocked, or is not as shown.

 

[hj]

trap

Unless there is something you are not showing, that drawing does not indicate any reason for the trap to be siphoning, regardless of how deep or shallow it is. If the drawing is accurate, you have a different problem and until it is found any attempt to correct it will result in spending time and money, but still having the original problem.

 

[tobinator]

REVISED: Solution to P-trap gas from self siphoning?

Thanks for the responses.

House venting should be adequate. I have at least 2 ea 2" vents through the roof. All home plumbing is new (as of two years ago). But the problem didn't start until sometime after I changed the drain exit locations.

I could snake the vents, to see if I've got a vent clog... probably not a bad idea.

My plumber was a guy I hired "on the side". My bad. He was recommended by a friend and I was already HUGELY over budget on the house. He was a "commercial" guy and did more refrigeration than residential. I think that he underestimated how difficult a remodel could be. He made a ton of mistakes. Forgot to sweat fittings, cut through floor joists, ran pipe outside the wall studs, etc. When all was said and done, I spent more paying him "under the table" than I would have paid a full fledged contractor. Lesson learned.

I didn't detail it in the crude picture, but the vent line does NOT connect directly to the overflow. Should it? It connects downstream from the trap, but before the waste pipe increases to 2". My guess was that the waste water fell from ceiling height, and hits a 45 and a 90 and "whirlpools" creating suction that pulls water from a shallow trap. The trap is a standard one, but it seems very shallow to me as it will only hold about 1.5" of water.

Any way to figure out exactly what is happening short of opening up the walls? I know it is sewer gas, and I know that it is coming in through the tub overflow vent. I also know that the vent stack is not directly connected to the overflow, but the drain line just beyond the trap is connected to the vent stack.

Thanks for your help!

Matt