Tuesday, December 6, 2022


Ask Dave: Gray water tank gurgles. What’s wrong?


Answers to questions about RV repair and maintenance from RV expert Dave Solberg, managing editor of the RV Repair Club. This column appears Monday through Saturday in the RV Travel and RV Daily Tips newsletters. (Sign up for an email reminder for each new issue if you do not already receive one.) Today he talks about gurgling drain pipes.

Dear Dave,
I have a 2010 Damon Avanti which has been a great rig. I am having a problem with the plumbing. When I have the gray water valve shut I get gurgling noises from the drain pipes, especially when running a sinkful of water through either sink. It all works fine when the valve is open.

I have done the baking soda/vinegar trick in both sinks and the shower to unclog any obstructions. I have replaced the air vents under both the kitchen and bathroom sinks. And I just replaced both roof vents with 360 Siphon Vents. Replacing the roof vents helped, but did not solve the problem totally. Any ideas on what is going on or what else I may try?  —Laurie

Dear Laurie,
All tanks in an RV need to be vented such as the fresh water, black water, and gray water tanks. They start out full of air and as the water or waste enters and starts filling the tank the air needs to go somewhere and that is designed through some type of vent. If not properly vented, the air pushes back up the drain pipe in the case of black and gray water tanks, or full tube of a fresh water tank. This will cause a gurgle or bubble back up the pipe as it is being filled or will stop filling/draining completely and cause backflow.

The best solution to vent a tank

The best solution for venting a tank is to run a 2” PVC pipe from the top of the tank to the roof and put a cap over it.  This allows maximum air flow that can match the volume of water going into the tank.

You indicated that both roof vent covers or caps were replaced with the 360 Siphon and this helped. This would indicate that the old vent cap might have been plugged, somewhat restricting the venting. It could also mean the vent design of your rig was not sufficient for the amount of water volume going into your gray water tank. Has this happened from the start or just started? If it is from the start, it could be a design issue. If it just started, it would be an issue with a plugged vent somewhere.

“Cheater vents” under the sinks

You also indicated you changed the air vents under the kitchen and bathroom sinks. These are commonly referred to as “cheater vents” because the vent comes up from the tank. Since there is an open countertop it cannot go all the way up to the roof. So manufacturers bring the vent tube up a couple of feet and put on a cap that allows air to come in, but restricts the odors from emitting out. These are a constant issue as they not only allow odors to come into the coach but also create too much resistance for venting.

I would suggest removing the cap from under the sink or bathroom and draining the sink to see if that eliminates the gurgling – which I believe it will. If you bought the replacement “cheater vent” cap from a home improvement store, it would be designed for residential applications and might have a higher pressure. You would need to get a cap for an RV application.

If the problem is still present, then I believe it’s a design issue with a vent pipe that has a sharp 90-degree bend that restricts the air flow.

Read more from Dave here.

Dave Solberg worked at Winnebago for 15 years developing the dealer training program, as marketing manager, and conducting shows. As the owner of Passport Media Creations, Dave has developed several RV dealer training programs, the RV Safety Training program for The Recreation Vehicle Safety and Education Foundation, and the accredited RV Driving Safety program being conducted at rallies and shows around the country. Dave is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.


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360 Siphon® RV Vents on Amazon


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1 year ago

I have that with my bathroom and kitchen sink. Usually means it is getting clogged. Either use the plunger on it (which is quicker) then pour vinegar down it and leave overnight.

I would never mix vinegar and baking soda to pour down my drains. I have heard stories of people who do that but the baking soda is not dissolved totally. Because in many RVs the kitchen pipe especially does not have a steep angle the baking soda will dry in the pipes.

Ron Lane
1 year ago

There are different AAV’s for home or rv use? This is the first I’ve ever heard of this. Are the AAV’s marked to tell the difference with pressure limit readings or just a simple “rv”/”home” labels??? I replaced my AAV in my motorhome with one that I purchased from Amazon and it looked identical….but there were no indications that it’s intended use was for either an rv or home.

Royce Hershberger
1 year ago

I had a problem similar to this in an rv I had. I ended up going on the roof and taking the cap off the vent and running a hand operated augers down it to unplug it. Worked like a dream after that.

1 year ago

Just guessing here, but if I encountered that on our RV, I would firmly tie a heavy fishing weight or bolt to some line and try to drop it into the tanks. If it doesn’t come back wet and smelly, it’s probably clogged with a bug’s nest or something.

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