Septic Smell from Washing Machine

[Gene86]

Hello, everyone! We added a washing machine and dryer to our 2018 Montana 3921FB. The washing machine was originally rigged to empty into black 2, but that quickly became a nuisance because the tank would fill up too quickly. So, we had a local shop re-route the washing machine plumbing to drain directly into the common pipe where gray 2 and black 2 meet near the outside dump hookup. Meaning, now the washing machine drains directly overboard (we have to be hooked up to septic to use it, otherwise it drains directly onto the ground even with gray 2/black 2 closed). Anyway... sounds better, right? Well, the issue now is that we get a terrible septic smell in the coach through the washing machine anytime an exhaust fan is pulling air into the unit. This only happens when we're hooked up to septic. When we cap the dump pipe there's no smell at all, so we know the smell is coming from the septic hookup and not from our black or gray tank. I'm thinking installing a one-way valve in the washing machine drain pipe will fix this. Does anyone have recommendations? Thanks!

[DQDick]

You could do what many of us have done and put a knife valve on the end of the dump pipe. Just open it to do laundry or dump. It also keeps you from having that little surprise when you arrive and take the cap off.

[BB_TX]

The washer drain should have a P trap to block that odor. Or do the newer ones have those new tangled traps that sometimes don’t close properly?

[Carl n Susan]

If the drain doesn't have a P trap after the retrofit, then ensure you create one using the sewer hose. You want to avoid a straight down slope from the coach outlet to the sewer receptacle.

[Gene86]

Thanks to everyone for your replies. Very helpful. And wouldn't ya know it? I went on a hunt around the washing machine and adjacent cabinet and located a waterless trap! Apparently the shop removed it and forgot to put it back on.

It's the Hepvo variety. I wonder why they favor this over a conventional p-trap. Space savings? Or maybe concern that a p-trap would splash dry while driving down the road so there'd be an open path for sewer gas as soon as you hooked back up (until you ran the washer).