I had a washer draining into a laundry tub for 20 years in the 2nd floor laundry room. Worked fine.

I got a new washer and now every 4th or 5th wash, I smell methane gas in the laundry room. I finally figured that it was the new washer draining into the laundry tub was somehow causing all of the water to be siphoned out of the trap.

I run a little water in the tub and the smell eventually goes away but looking for a more permanent solution.

Any thoughts on whats going on and how to fix?

 

After some online reading, I now know more about self-siphonage than I ever thought I would, that's when the waste water fills the entire pipe causing a vacuum that sucks the water out of the trap behind it.

So it seems that either (a) the trap or the drain pipe has accumulated so much stuff (hair, clothing threads, etc.) causing the effective size of the pipe to be smaller and therefore more susceptible to self-siphonage, or (b) the drain was never designed properly and needs an additional vent connection. Presumably the new washer has a greater flow rate than the old, causing all of this to be a problem now, or the clogged trap only recently became clogged.

 

I would venture to say the drain on the tub is at a maximum 1 1/2", and a washer needs 2", so yeah, you probably got a lot of friction in the pipes. There should be a vent. If not you may want to entertain installing an air admittance valve (Studor).

 

Yep, it's 1 1/2".

So I saw how to install one of these Air Admittance Valves here:

Air Admittance Valves Install

I'll have to open the wall either way so the choice is to install one of these or if close by, tap directly into the vent stack 3' above (on 2nd floor, no fixtures above) - but I'm guessing installing one of these will be a whole lot easier. Agreed?

Typically are these installed in the wall or do I try to rework the drain to put it under the tub - that might be even easier.

 

AAV's are installed under the sink. They should not be hidden nor closed up. The top of the AAv should be above the drain level. If you have a vent stack, you don't need an AAV. Clean out your vent stack.

 

I'm the OP on this a few months ago.

I had smelled methane gas in the laundry room every other time that we wash clothes. (The washer drains into the laundry tub and the water is sucked out of the tub's trap.)

I never got around to installing an AAV because I'd have to re-work some of the drain pipes in the wall.

Yesterday another thought came to mind and I wanted to get an opinion.

The 3" drain/stack that the laundry room tub drains into goes to the basement, across the basement horizontally (with pitch) and meets a 4" main pipe.

Can I make a trap and put in that horizontal 3" section using 4 - 3" 90 degree elbows to solve this problem?

Thanks.

 

No. Either vent the laundry tub properly when you have the walls open, or install the AAV.....