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Bryan H · 06-06-2013, 02:48 PM

Ok, a little background first. We recently bought a 2008 Winnebago Destination 39W. Since our purchase, we have always noticed a tank odor when the coach is closed up for a day or two. In an attempt to remedy this problem, we flushed all the tanks thoroughly, and then we added tank powder to the kitchen sink in order to 'sanitize' that grey tank. As we were adding water, the flow from the faucet was more than what the drain could handle, so the sink began filling up. Once that happened, water began gushing out from the piping under the sink. After cleaning up the mess, I noticed there is an anti-siphon valve downstream of the p-trap. The elevation of the valve is right at the bottom of the sink. So basically, as soon as the water reaches a depth of 1" or so in the sink, it starts coming out the valve. So, my question is, is the anti-siphon valve faulty? I have no idea how they are designed, so I'm not sure if they should overflow in this type of situation.

Any thoughts are appreciated.

Bryan

ChasA · 06-06-2013, 03:45 PM

The valve is not your problem right now. You have a blockage between the sink drain and the gray tank. I'd run a small plumber's snake down the drain to see if you can break whatever is. Probably kitchen grease.

Bryan H · 06-06-2013, 03:56 PM

Agreed, there's definitely a blockage of some sort, otherwise the sink would not fill up.

But that raises a question. How does the anti-siphon device keep tank gas from escaping? It's between the trap and tank, so the trap doesn't help. What keeps tank odors from wafting out of it?

UFO Pilot · 06-06-2013, 03:56 PM

My 39W doesn't act that way so I assume you have a blockage. The galley sink drain is very long as it goes thru the slide and under the bed. The hose under the bed may be kinked? You have to remove the mattress and then remove 4 or 5 screws to lift the bed "cover" up. If you do access under the bed, be sure and check the framework that the bed slides over. The screws in mine had all pulled out and the bed was binding when the slide moved in or out.

Gary RVRoamer · 06-06-2013, 04:05 PM

An anti-siphon valve is simply a check vale that passes air rather than the more familiar water check valve. It's formal name in plumbing is actually an "air admittance valve". It allows air to pass inward and prevents air (or liquids) from passing outward. Basically it is just a rubber flapper that moves in only one direction - inward.

Since yours is allowing both fumes and liquid to leak out, if is clearly defective. They are simple to replace - any home center or plumbing supply will have them and RVs use standard 1.5" air admittance valves. They are not glued to the pipe - just a friction fit.

However, as others have said, your main problem is a blockage in the sink drain line.

Bryan H · 06-06-2013, 04:18 PM

Is it possible that this valve, if defective, could be causing the odor AND slow drain problem? If it's not opening properly to allow a large volume of water through, I'm wondering if that could cause the backup. At times, the drain seems to work fine.

mfire1339 · 06-06-2013, 08:02 PM

Bryan,

The valve only deals with air. Not water. It is a result of a drain problem and is easy to change.

Mike

chboone · 06-08-2013, 12:14 AM

If the "air admittance valve" sticks closed it can cause the drain to backup because it is not allowing air into the drain for the water to flow. A friend of ours had the same problem with her 2008 Monaco, replaced the air admittance valves cured the problem. If your is the style that has a light spring in it, they were discontinued because of problems with them sticking closed. Replace your valve(s) and you should not have anymore problems.

Chuck

bobmac · 06-09-2013, 08:10 AM

The fitting is really a "vent" designed for use when a proper vent to the roof to cannot be installed. It allows air in to the drain line as others have stated. It is equipped with a spring loaded shut-off to prevent backflow and the overflow of liquids if the drain is blocked. These are cheap fittings and should be available at an RV supply or Home Depot. Over time, the backflow prevention can clog up and fail as yours appear to have done. Best solution is to replace it once you have cleared the blockage that is restricting the drain. There are likely others installed under other sinks that you might want to check as well.
The odor could very well be the result of the evaporation of liquid in the P-traps - you need to run a bit of water into the traps periodically when the coach is stored. Otherwise, once evaporated, there is a direct path from the tank into your coach for odors to escape.

Gary RVRoamer · 06-09-2013, 10:26 AM

If the air-admittance valve sticks closed, the only effect should be water getting sucked from the nearby sink P-trap with a lot of glug-glug-glug noise. The main roof vent will still provide enough air relief to avoid a back-up.

Selah · 06-09-2013, 05:18 PM

It is the valve, probably installed backwards. Take it off and check the direction of flow through the valve. Flow should be IN only. If it has been installed backwards it would then allow air to flow Out of the gray tank instead of the other way around causing your Oder problem. Check that first.

jeepfreak · 06-09-2013, 06:14 PM

Oatey 1-1/2 in. ABS PTC In-Line Cheater Vent-39012 at The Home Depot

chboone · 06-09-2013, 11:26 PM

On our friends 08 Monaco The only roof vent was directly connected to the gray tank. All of the sinks and shower had the air admittance valves a total of four, when they would stick closed which ever fixture it served would not drain or suck the water from the other P-traps. As soon as the valve was removed the fixture would drain.

Chuck

"007" · 06-10-2013, 05:32 AM

The vent under sink should be higher than the P trap to work correctly.
The power maybe whats causing the blockage and didn't go down into gray tank.
Use bleach to sanitize the gray tank will make a good cleaning.