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Old 06-13-2021, 07:50 AM   #1
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Join Date: Sep 2020
Posts: 7
Anti Siphon Valve

Hello all,
I am having issues with slow shower draining and at this point was thinking either something plugging my vent stack or a bad Anti Siphon valve. My stack has a cap (factory installed) so I don't think anything got in there but before I get on the roof I was thinking I would check the valve.

If I "burp" the grey tank the shower drains right away, like it is a pressure issue.

I looked under all the sinks and can't find one and it does not appear to be in an obvious place, if there even is one?

I have a 2018 Tuscany and was wondering if any one had an idea...and yes the holding tanks are empty.

And yes I called Thor and asked for a diagram but "crickets".

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Old 06-13-2021, 09:16 AM   #2
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Join Date: Jan 2018
Location: Capistrano Beach, California
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First question, what do you mean by "burp" the grey tank?

Here is a generic drain plumbing diagram that may help you visualize where the problem may be.

The grey tank is not only vented through the roof, but each drain line from a sink has an air admittance valve (AAV) that allows air into the drain line when negative pressure from running water exists. These valves are on top the the grey water vent by the sink in the diagram.

Again, your coach is probably not plumbed exactly like the diagram but you can see the shower is vented through the main vent through the roof. You may also not have a P-trap but a Hepvo waterless waste valve instead. If only the shower is draining slowly, or not at all, and the main vent is located between the shower and the grey tank, it is possible there is a blockage to the main vent which is not affecting the other drains due to the presence of AAVs in their drain lines.

Your coach may have an AAV in the shower line as well, but it would/should be is some accessible space between the shower drain and the grey tank. Most coach builders don't provide the access nor put AAVs in the shower line.

Again, sorry I don't know what "burp" the grey tank means but whatever you do seems to open the shower line for drainage. It may be possible the main vent pipe was installed too far into the grey tank, or has slipped down into the tank. However, since the draining issue happens even when the tank is empty, that possibility seems remote.

Whatever you do, don't push anything like a snake down the shower line unless you know for certain you do not have a Hepvo waste valve. Water from a hose, flowing down the main vent pipe from the roof, should go straight into the grey tank with no problem.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.
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Old 06-14-2021, 05:59 AM   #3
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Join Date: Feb 2016
Posts: 15
I had the exact same problem

I had the exact same problem on my 19 M37 Venetian. The interim solution was to take a toilet plunger into the shower to get it to drain. You can ONLY push down on the plunger not up as it will fowl the flapper valve that follows the shower pig tail.
I tried everything , took it to a couple of shops to get it fixed, that Thor paid for. Finally I got an appointment and took it to Thor. I have got to tell you they are fantastic, fixed the problem and haven't had an issue since.
The problem is a design issue with my model. I have a mid bath with a washer/dryer in the rear curbside corner. The drain line for the washer is the same line as the vent for the grey tank which is fine if the line drains perfectly every time, but it doesn't. You can look up your pluming schematics on the Thor website.
What Thor did to resolve the problem was to go into the shower drain line after the check/flapper valve and vent it to the black tank vent. The lines are pretty close in that area, not a big problem if you know what problem you are fixing.
Hope this helps.
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Old 06-14-2021, 07:34 AM   #4
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Join Date: Sep 2020
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Thanks all

Thanks for the comments...what I mean by "burp" is that I open the grey tank discharge valve for a few seconds (not a full drain) and the shower drains. This is why I suspect some kind of air pressure issue.

Also thanks Longshot, I did try an look up my plumbing schematics on the Thor site and they are not there...they said they would send them to me but have not received them yet. I am thinking you are right with regards to a venting issue.

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Old 06-14-2021, 08:08 AM   #5
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Just into the owners resource page and found the coach schematics! Yippee...

Still not sure how to fix the issue but at least I know where everything is and where it goes.

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