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Old 04-04-2021, 06:32 AM   #1
ChrisM
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Strange problem with Air Admittance Valve

I have a slow draining bathroom sink. I replaced the factory AAV a few years back so I figured it was the issue but to be sure I removed the AAV while the sink was full and draining slowly. As soon as air got into that pipe with the AAV removed the sink emptied immediately. I got a new AAV but the problem exists. I tried the AAV from my half bath ( on a different tank ) and still slow drain. Got a better brand of AAV made by Studor and still the problem. My kitchen sink and my shower are on the same grey tank as my bath sink and neither have issues. My grey tank empties fine so I don't suspect any problem with the roof vent. This one has me stumped.
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Old 04-04-2021, 07:40 AM   #2
NH_Bulldog
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In general, the draining water needs to flow free enough to create a siphon effect. A restriction in the system could cause the issue you describe. I would check 2 things; make sure the sink trap is clean and clear, and also check the roof vent.
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Old 04-04-2021, 09:53 AM   #3
lunge motorsport
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An air admittance valve is used in applications where a VTR (vent through roof) cannot be accomplished due to construction. It is designed to open when there is a pressure change in the drainage piping system allowing a free flow of air and “preventing” a syphoning of the trap. Spring type check valves are similar and have a designated “cracking pressure” that needs to be overcome to allow the valve to operate. They are a different animal however and used typically in pressure piping. I could image that the pressure differential is very low in this application and the the AAV is sticking closed. It’s possible that the new AAV is just defective and not opening.
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Old 04-04-2021, 09:56 AM   #4
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Check under the sink to see if you have a Hepvo valve. These are used in place of a P trap to prevent gases from coming up from the tank. They use a flexible device that opens to allow water to pass and as soon as it’s done draining, the device closes. After a time, these will get clogged and need to be cleaned. With that restriction along with the AAV, it may be enough to cause the slow flow. https://hepvo.com/
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Old 04-04-2021, 10:03 AM   #5
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I don't think I've seen a HEPVO valve on a bathroom sink in an RV, but could be...

My guess is that the roof vent is clogged and there's not enough water force to overcome the resistance in the closed gray water tank...

One way you can check it is to open the dump valve on the gray tank (so it vents to the outside) and then fill the bathroom sink, pull the plug and see if it drains quicker... If it does, then address the rooftop gray tank vent. If it remains slow, then it's something to do with that particular mini-vent.

At Lowe's, there are "cheap vents" and "Studor vents"... Our Cougar doesn't like the $7 vents and only works well with the $28 Studor brand.... Everything that will screw onto a 1.5" threaded fitting is not created equal....
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Old 04-04-2021, 02:55 PM   #6
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I tried dumping the grey tank and leaving it open and the sink drains at the same slow rate. I removed the p-trap and there were no clogs. I checked the tail piece off the sink and removed the horizontal piece which connects the p-trap to the vertical pipe ( with the AAV at the top ) and couldn't see any blockage. The first AAV I bought was https://www.homedepot.com/p/Oatey-1-...oduct-overview and then bought the Studor Redi-Vent https://www.homedepot.com/p/STUDOR-R...oduct-overview. I even tried removing the working AAV from my half-bath ( on another tank ) and nothing has fixed the slow draining. When I think more about the rooftop grey tank vent I'm wondering how that can cause the issue with the sink when the shower and the kitchen sink are all draining without issue. Also, when I remove any of the AAVs I've tried from the bath sink drain the sink then drains very quickly. It's almost as if I've gotten multiple bad AAVs but I've even tried one that appeared to be working on my half bath.
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