Sewer Gas in Shower

[CanoeCanuck]

Good Morning
I have a 2005 Winnebago Itasca Spirit.
The problem I’m having is when I drain a full sink in the galley there is air backup in the rear shower releasing grey water sewer gas into bathroom. The water drains quickly from galley sink. Drainage from shower works well. I have run a snake from galley sink to grey tank and from shower basin to grey tank and both clear. I went onto the roof and snaked down the vent and it’s clear too.
I’m sure that the grey tank is empty So not overflowing.
I’m sure the trailer is level.
I’ve had same problem at different locations.
These are the plumbing schematics for my motorhome. I’m not sure of the purpose of part VM-1. Is this some sort of vent that may be causing the problem? If so, I’ve found the one under the galley, but where’s the one for the shower? Hopefully not in the wall. Please help. I’m running out of diagnostic ideas.
Thank you for your time.

[Wyatt]

On ours, it’s a one way flow valve that is mounted horizontally under the shower’s drain. If the backflow preventer “flap” in it we’re to get anything, (e.g. hair), stuck in it, it might not close. Could be your issue. Look at the shower plumbing diagrams and see if you have one of those too. If so, take it apart, clean it, and check it out.

It’s similar to what you see in the diagram under your sink, and it works on the same principle.

[backtrack15]

I only see the diagram for the sink. Could you share the one for the shower?

I believe VM-1 is an air admittance valve. The idea being to allow air into the plumbing and prevent loss of the liquid seal in the trap via suction.

Has this always happened? Or is this new? If the vent off the sink drain line is the only vent for the grey tank, I can see how a large inrush of water from the sink might prevent air from escaping the grey tank. The water flowing toward the tank might make it difficult for air to move toward the vent. This could build just enough pressure in the tank to overwhelm the trap on the shower drain. This may just be a design flaw, for which the easiest solution is draining the sink slowly.

[CanoeCanuck]

Thank you for the quick replies. I don’t think this is a design flaw as so far Winnebago seems to have thought of many conveniences with this unit. I just bought it this year so still in the honeymoon phase. 😀. Here’s the specs for shower plumbing.

[backtrack15]

Looks like a simple trap and air admittance valve on the shower drain. Thanks for posting that image. I think you might struggle to solve this without reworking the sink vent. I’d run some water down the shower each time you park (to fill that trap) and consider draining the sink more slowly.

Other options include putting one of those duck-billed back-flow preventers in the shower drain (if you have room) or fitting an actual plug in the shower drain while not using it.

You could also change out that air admittance valve on the shower drain (maybe it isn’t sealing well). The smell might appear to be from the shower drain. I’m guessing you’d smell it more frequently if it was this valve, but maybe it’s worth a try.

[backtrack15]

You could test that air admittance valve by plugging the shower drain completely and then draining your sink. If you get the smell as strong as ever, it’s probably the air admittance valve leaking. If you don’t get the smell, the valve is probably good.

[Bones2003]

CanoeCanuck, from the drawings it looks like you have two air admittance valves. One for the sink and one for the shower. They work great when they work but they stink when they don't work! These are very common failure items and are usually very inexpensive to replace. Your symptoms are the classic complaint for these valves. The hard part might be getting access to replace them. I think they are readily available from big box hardware stores.

[CanoeCanuck]

Thanks for the advice Bones2003. Are the air admittance valves at the top of each line? Ie VM-1 in the diagrams? Sorry resolution isn’t very good. I would assume these would allow air into the line so it flows well, but not release air as then it would release smell as well.
Thx again for your time.
Sean

[Bones2003]

Yes, VM-1 is the air admittance valve. I've also heard them called a mechanical vent.
My understanding of their purpose is that when a big slug of water goes down the drain it creates a suction that would pull the water out of the trap (items P-1 and P-4 in those drawings). Smells from the tanks could then enter the living area of the camper through the empty drain trap. The air admittance valves allows air to enter the pipe and break the suction from the water draining down into the tank. If you have a vent up through the roof on that particular drain line then you don't need an AAV but it is hard for manufacturers to add extra vent pipes up through the roof so they use the valves. They've been used for many years and generally work well.

I think they are very simple and only have some type of rubber flap inside that lets the air in when water is draining but reseals to prevent odors. I'm not sure exactly how they fail - either they don't let air in or they don't reseal - but I expect they don't reseal.

A lot of times people will complain about the smell when they are driving because the motion of the vehicle creates a suction in the camper that pulls tank fumes in through the faulty AAVs.

[imnprsd]

...That, and your gas is penetrating your p-trap.

...Turning on your Fantastic fan will also pull the smell out of your tank... and past the p-trap.

The only alternative is to have 2-traps, but that's not possible.

The water in your p-trap is your main smell-blocker.

And this odor can be very offensive in the gray tank. Maybe even more so than in the black tank. What's worse, I think the gray holding tank smell is harder to get rid of in some ways when it gets real bad. And you can't just dump Rid-X in your gray tank like you can your black tank. ...I don't know why, but I tried that and it just made matters worse.

Further, you can't just bomb your plastic tanks with chlorine...

...Well, you can, but I would not.

Diluted chlorine can is still very "active" and will embed itself in your plastic tank. Plus, the chlorine order then forms with other gases and that smell is really bad.

This is what I do... I use a biodegradable degreaser concentrate. I don't care what type. Just throw it in the tank with 10 gallons of water and then I drive to my next destination; and dump.

==> NEVER USE FABULOSO or some other Pine Sol-like cleaner.

Note: When you first start driving you may get a bloom of stench, but then it will go away as you mix more oxygen in the tank. You might even throw some Boraxo in your tank to help with the "oxygenation" process that hopefully will keep the smell down.

Oxygenating the soup in the tank will help to neutralize the anaerobic bacteria which grow in the absence of oxygen. That's why adding the Boraxo helps. I.e., it helps to add oxygen to the soup, but it will not kill the bad bacteria.

The degreaser will hopefully get rid of the bad bacteria... with each treatement.

So you need to commit to your cleaning process. You can't just go halfway! ...And it may take 2 or 3 cycles to return your tanks to normal, but I got there following these suggestions.

...Fabuloso masks the odors and you will have both a bad smell and the bouquet of whatever lemon or pine scent cleaner you put in the tank. (Not good.)

Anyway, after you add your gallon of biodegradable degreaser with 4-5 gallons of fresh water, and after a 100+ mile drive, when you get to your camp spot you can dump your tanks -- and then re-fill them 3/4 of the way with fresh water. Then use your tank for 1 day. ...Drain. ...The repeat this process 3 more times while you are camping.

The fresh water will help to bond with the bad bacteria and with every RINSE you will be on your way to a fresher smelling tank.

Then you repeat this cycle as often as you can or think necessary. The point is that "fresh water" is your best solvent after you get to the point where your tanks are not lined with "yuck."