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Old 07-17-2020, 02:21 PM   #1
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Brand: Thor Motor Coach
Model: Axis 25.3
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THOR #19097
Sewage smell from vent pipe area

Hello intelligent Thor owners...
I have a 2017 Axis 25.3. The sewer vent pipe is behind a cabinet and some drawers. When I take a shower, I get out of the bathroom and that area smells horrible. I open those cabinets and drawers and itís about enough to knock me out.
Thinking it could be that the pipe wasnít sealed well but looking for other people with this issue and solution.
Tanks never more than 2 days of use so Iím not letting it al sit in there or anything.
Wondering why taking a shower which drains to gray tank would give the same smell off. Does gray water have a vent that feeds into sewer vent pipe?
Thanks!

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Old 07-17-2020, 02:44 PM   #2
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Originally Posted by Jake-n-Marty View Post
Hello intelligent Thor owners...
I have a 2017 Axis 25.3. The sewer vent pipe is behind a cabinet and some drawers. When I take a shower, I get out of the bathroom and that area smells horrible. I open those cabinets and drawers and it’s about enough to knock me out.
Thinking it could be that the pipe wasn’t sealed well but looking for other people with this issue and solution.
Tanks never more than 2 days of use so I’m not letting it al sit in there or anything.
Wondering why taking a shower which drains to gray tank would give the same smell off. Does gray water have a vent that feeds into sewer vent pipe?
Thanks!
The bathroom sink typically drains into the black tank and thus that vent under the sink vents black tank. On the top of that vent is what is called an "air admittance valve"--it is supposed to be one way and only let air in thus preventing any odor.

Those valves, however, can and do go bad (It may also be loose--try tightening it first). The good news is that the valve that Home Depot sells is a screw in replacement--I just replaced both of ours (there is also one under the kitchen sink) with Home Depot valves.
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Old 07-17-2020, 03:23 PM   #3
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THOR #19097
Thanks so much for the info! I see the valve under the kitchen sink but not in the bathroom. It looks like that one goes through the wall and straight up to the top of the coach. Does that sound right? May be hidden though in the wall.
This all makes sense and I appreciate the heads up on the possible quick fix!
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Old 07-17-2020, 03:40 PM   #4
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Originally Posted by Jake-n-Marty View Post
Thanks so much for the info! I see the valve under the kitchen sink but not in the bathroom. It looks like that one goes through the wall and straight up to the top of the coach. Does that sound right? May be hidden though in the wall.
This all makes sense and I appreciate the heads up on the possible quick fix!
If it goes straight up that may be just the vent pipe through the roof.

May want to get up on top and take a look where your vent pipe is.

Another handy thing to do is grab your VIN and log onto Thor's Owner-Resource page. You'll be able to download the plumbing diagram and see where the pipes go:
https://www.thormotorcoach.com/owner-resource/
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Old 07-19-2020, 08:12 PM   #5
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THOR #19097
Thanks again @Jamiegeek. I called Thor, got the diagrams. The main vent to the roof is located in that exact spot behind the cabinets That has a sewer smell and I went on the roof to inspect and didnít see any blockages. Actually got a nice dose of sewer smell so it seems like itís working up there.
There is no siphon valve on 25.3 for the restroom sink which is very close to the main vent.
Itís boggling my mind and did as well to the Thor tech support I spoke with. He suggested, as you did, to check the valve under kitchen sink.
Turns out that is working fine and no smell emanates from there, or anywhere else for that matter besides the cabinet space containing the pvc main vent pipe and only after taking a shower. Wondering if I have a loose pipe or something. Havenít seen anything out of place yet though.
Scratching head still.
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Old 07-19-2020, 09:00 PM   #6
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Originally Posted by Jake-n-Marty View Post
Thanks again @Jamiegeek. I called Thor, got the diagrams. The main vent to the roof is located in that exact spot behind the cabinets That has a sewer smell and I went on the roof to inspect and didnít see any blockages. Actually got a nice dose of sewer smell so it seems like itís working up there.
There is no siphon valve on 25.3 for the restroom sink which is very close to the main vent.
Itís boggling my mind and did as well to the Thor tech support I spoke with. He suggested, as you did, to check the valve under kitchen sink.
Turns out that is working fine and no smell emanates from there, or anywhere else for that matter besides the cabinet space containing the pvc main vent pipe and only after taking a shower. Wondering if I have a loose pipe or something. Havenít seen anything out of place yet though.
Scratching head still.
The entire purpose of an AAV is to preserve the water seal in the trap. Without one the seal can be siphoned into the tank. Then when the tank gets water from another source guess where the vapors come out that are displaced by the water entering the tank?
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Old 07-19-2020, 09:09 PM   #7
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@16ACE27 that is what Iím trying to figure out- Where the smell is coming from. Again, it is not coming from the kitchen sink siphon valve nor is it coming from the restroom sink or any other drain. Itís coming from somewhere behind my cabinet area near the main vent pipe.
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Old 07-20-2020, 03:25 PM   #8
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THOR #17747
Sounds like a problem with the vent pipe itself - plumbing joint not properly cemented or screw that penetrated the pipe when they installed the cabinet. No way to prove this for sure without getting access to the vent pipe. You might be able to see something with one of those wireless endoscopes they sell on amazon for $35.
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Old 07-20-2020, 03:41 PM   #9
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Originally Posted by Jake-n-Marty View Post
@16ACE27 that is what Iím trying to figure out- Where the smell is coming from. Again, it is not coming from the kitchen sink siphon valve nor is it coming from the restroom sink or any other drain. Itís coming from somewhere behind my cabinet area near the main vent pipe.

16ACE27 is suggesting that if you don't have an AAV under the bathroom sink it is possible that having a shower (or possibly some other water use activity) is siphoning the water out of your bathroom sink trap. Run some water in the bathroom sink to fill the trap; turn off the taps and put the plug into the sink drain. (I assume your bathroom sink does not have an overflow opening). Then have your shower.......any black tank smell this time? If yes then it is some other cause other than an open sink trap.
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Old 07-20-2020, 04:43 PM   #10
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Originally Posted by javelin View Post
16ACE27 is suggesting that if you don't have an AAV under the bathroom sink it is possible that having a shower (or possibly some other water use activity) is siphoning the water out of your bathroom sink trap. Run some water in the bathroom sink to fill the trap; turn off the taps and put the plug into the sink drain. (I assume your bathroom sink does not have an overflow opening). Then have your shower.......any black tank smell this time? If yes then it is some other cause other than an open sink trap.
Except that the bathroom sink drains to the black tank and the shower drains into the grey tank (or is supposed to).
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Old 07-20-2020, 06:09 PM   #11
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True Jamie, "is supposed to" being the key words.
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Old 08-05-2020, 03:06 PM   #12
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THOR #19097
UPDATE- After replacing the air admittance valves and still having the issue I replaced the vent cap on top of the camper with a Siphon 360 as suggested and that immediately solved the issue. There is a small gap in between the PVC pipe and the opening and thatís all it took for the vapors to cascade back down and into the specific area of the camper. Thankfully it was a relatively easy fix.
Thanks again for all the suggestions!
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Old 08-05-2020, 03:21 PM   #13
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