Vent under sink cause of smelly trailer?

If you have a black tank flush you could have a bad back flow preventer, my last 5th Wheel had this device mount under the bathroom sink.

Yes i do have that back flow dealio!! Saw a guy on this forum talk about how they can leak if water flow gets low. So i shine a flashlight every time a dump and rinse.

I feel like the smell is more gray tank than black tank. But please point me in any direction to fix the issue. Only seems to happen come summer heat.

Those vents go bad, I would replace it.

I have read where the air vent pipe from the top of the tank to the roof can lose its seal at the top of the tank. Even where the pipe falls down into the tank and can't vent anymore.

I would dump a bottle of Pine Sol down the kitchen sink and then run water till the grey tank fills so high that water comes up into the kitchen sink. Look under the trailer and see if water is coming down from the top of the tank. Anyway leave it soak for a while and then drain the tank.

If it is just a "galley grey" then add some laundry soap while the tank is still empty. If it is a combined grey tank with the shower, then it will get soapy water anyway. If combined , then the overfill might come up in the shower instead of the higher sink. Whatever works.

The AAV (Air Admittance Valve) does fail and is easily replaced. When it fails, it typically sticks open so that there is a direct opening to the gray tank and can really smell. Best bet is just to replace it... it's cheap and easy to installed and can be purchased at Home Depot, Lowes or even on line at Amazon.

lanerd wrote:
The AAV (Air Admittance Valve) does fail and is easily replaced. When it fails, it typically sticks open so that there is a direct opening to the gray tank and can really smell. Best bet is just to replace it... it's cheap and easy to installed and can be purchased at Home Depot, Lowes or even on line at Amazon.

Mine got neutered...

Grey water tanks do get "sour" and can smell as bad or worse than the black water tank in very short order in this summer heat.

For that reason, I installed a "flusher" in my grey water tank with a selector valve to flush either the black tank or the grey tank - works great!

I also started using Happy Camper Holding Tank Treatment that's done wonders by eliminating those unpleasant smells. It was recommended to me by several other Trek owners and after using it for awhile, I'm now a Happy Camper booster. The link here isn't the only source (try searching for it on the internet) and a few of the better RV supply houses have it in stock.

The air admittance valve is a simple check valve usually with a thin rubber disc shaped flap. It is sucked open by the rush of water passing by to allow air into the drain pipe to keep the p-trap from getting sucked dry when the sink drains.

Eventually they can get hard and mis-shapened keeping them from making a proper seal allowing odor from the grey tank to get into the trailer. If the bathroom sink goes to the black tank it will have one that can let black tank odor into the trailer.

I never heard of an AAV until this thread started, so no help with that. I Googled to see what they are.

My 2003 5er didn't have any, and my stick house doesn't have any. Sort of reminds me of the dishwasher drain, where you have to leave the sink drain open as a vent for the dishwasher, but not exactly I guess. Learned something new again on this forum!

EDIT- seems they had to amend the plumbing codes a few years ago to allow these AAVs. As alternatives to roof venting, they could be handy for RVs, not just for island sinks.

https://garynsmith.net/venting-101-air-admittance-valve-pros-cons/

BFL13 wrote:
I never heard of an AAV until this thread started, so no help with that. I Googled to see what they are.

My 2003 5er didn't have any, and my stick house doesn't have any. Sort of reminds me of the dishwasher drain, where you have to leave the sink drain open as a vent for the dishwasher, but not exactly I guess. Learned something new again on this forum!

EDIT- seems they had to amend the plumbing codes a few years ago to allow these AAVs. As alternatives to roof venting, they could be handy for RVs, not just for island sinks.

https://garynsmith.net/venting-101-air-admittance-valve-pros-cons/

In 40 years and still working as a RV tech, I have never really seen a RV Gray Tank drain system from the sinks NOT HAVE a AAV. They are standard. The usual fix besides replacing is to install a ziplock bag over the AAV and secure with a rubber band. That usually fixes the smell problem. Doug