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Old 08-18-2022, 06:21 PM   #1
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Vacuum Breaker Check Valve for Black Tank Flush

We've been enjoying our nes Dynamax DX3 37ts since we picked it up this past February. However, after our last trip, I could not flush the black tank. As soon as I turned the water to the hose on, there is a single clunking noise followed by the water pressure backing up in the hose. Nothing is getting through to the internal spray head inside the black tank. After Googling the issue, I'm fairly certain that I have a bad Vacuum Breaker Check Valve. These valves are available on Amazon for around $25 ($10 if you go with the no-name Chinese brand). The problem is in getting to the check valve to replace it. on the DX3 37ts, the check valve appears to rest inside a false wall between the washer/dryer closet and the bedroom closet. Can anyone confirm that location? If that is the correct location, then how do I get to it to replace the check valve? I've seen a couple of online posts where people recommend doing away with the check valve altogether and building a simple bypass out of plumbing fittings. Thanks in advance for any help in resolving this issue.
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Old 08-19-2022, 02:17 PM   #2
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I've no idea how to help you. You need an administrator to move you to the proper forum for your DX 3 where other owners of your rig can help.
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Old 08-19-2022, 03:16 PM   #3
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Nothing is getting through to the internal spray head inside the black tank. .
A clogged spray head nozzle will have the same affect. have you verified this at the spray head?
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Old 08-19-2022, 03:19 PM   #4
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I know that this will be controversial, but here goes. If you can access the area that the flush line goes into the wall and comes back out you can bypass the back flow preventer by tying them together. I know why the valve is there, (I am licensed plumber) it is to prevent black water from the tank from going back into the potable water system. In reality, as long as you do not leave the flush hose connected to the black tank flush without the water being on, it can never happen. the water will come out of the vent on the roof before it will have enough pressure backing up from the tank to override the fresh water system.
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Old 08-19-2022, 03:40 PM   #5
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I know that this will be controversial, but here goes. If you can access the area that the flush line goes into the wall and comes back out you can bypass the back flow preventer by tying them together. I know why the valve is there, (I am licensed plumber) it is to prevent black water from the tank from going back into the potable water system. In reality, as long as you do not leave the flush hose connected to the black tank flush without the water being on, it can never happen. the water will come out of the vent on the roof before it will have enough pressure backing up from the tank to override the fresh water system.
Yes... and no.

In reality, it is there in case there is a sudden LOSS of city water pressure (think water line broken downstream by a backhoe) while using the tank flush, which would siphon the contents of a full black tank back into the municipal water system.
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Old 08-19-2022, 03:42 PM   #6
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Originally Posted by wvcampers View Post
I know that this will be controversial, but here goes. If you can access the area that the flush line goes into the wall and comes back out you can bypass the back flow preventer by tying them together. I know why the valve is there, (I am licensed plumber) it is to prevent black water from the tank from going back into the potable water system. In reality, as long as you do not leave the flush hose connected to the black tank flush without the water being on, it can never happen. the water will come out of the vent on the roof before it will have enough pressure backing up from the tank to override the fresh water system.
Agree, and yeah, it's overkill, but the OP needs to find his if just to bypass it at its location because the bypass needs to be done there where it's elevated above the floor 3 or more feet. Now, wait for the naysayers.
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Old 08-19-2022, 03:54 PM   #7
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Yes... and no.

In reality, it is there in case there is a sudden LOSS of city water pressure (think water line broken downstream by a backhoe) which would siphon the contents of a full black tank back into the municipal water system.
The reality is, it's only connected when wanting to clean out the tank and the dump valve is open, so the nozzle isn't even in the black water.
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Old 08-19-2022, 04:09 PM   #8
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A clogged spray head nozzle will have the same affect. have you verified this at the spray head?
Short of removing the spray head from the tank, there is no way to tell if it is clogged. The inlet has a mesh screen and its worked fine before. I pulled the mesh screen and inspected the line and it looks clean. The sprayer spins with pretty good force and I would think that it could easily break through soggy TP. Normally, when you remove the hose from the black tank flush fitting, a little bit of water runs out as the line clears itself. In this case, there is almost no water coming out. When I turn the water on, there is a quick clunk and then the hose expands from the built up water pressure.
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Old 08-19-2022, 04:16 PM   #9
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Originally Posted by wvcampers View Post
I know that this will be controversial, but here goes. If you can access the area that the flush line goes into the wall and comes back out you can bypass the back flow preventer by tying them together. I know why the valve is there, (I am licensed plumber) it is to prevent black water from the tank from going back into the potable water system. In reality, as long as you do not leave the flush hose connected to the black tank flush without the water being on, it can never happen. the water will come out of the vent on the roof before it will have enough pressure backing up from the tank to override the fresh water system.
Thanks for the suggestion. I've actually seen other people online recommending the same thing. My preference would be to replace the black tank flush check valve, but if it is nearly impossible to get to, then making a bypass might end up being the best option. Can anyone tell me exactly where the check valve is located in the DX3 37ts and what is the easiest way to access it?
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Old 08-20-2022, 05:07 AM   #10
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The check valve is usually behind the fuse panel, but may have moved since my previous 2018 TS. I have not had to locate it on my 2021. My experience, however, after having to do clean it out twice, is it’s a clogged sprayer nozzle. I would suggest trying that first. Likely the check valve is just fine…mine was.
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Old 08-20-2022, 05:51 AM   #11
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↑X2

Here's a link that discusses it.

https://www.forestriverforums.com/fo...ve-247115.html
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Old 08-20-2022, 06:14 AM   #12
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Click image for larger version

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ID:	278041Nasty project, but relatively easy.
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Old 08-21-2022, 09:09 AM   #13
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I spotted the vacuum breaker behind the bedroom drawers in our 2021. If the check valve is part of that unit, I don't know.
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Old 08-21-2022, 09:19 AM   #14
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I have heard of stink bugs going into the valve and causing it to do what your saying. but you still need to get to the valve to remove the stink bug
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Old 08-21-2022, 09:34 AM   #15
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I spotted the vacuum breaker behind the bedroom drawers in our 2021. If the check valve is part of that unit, I don't know.
There is a check valve built into the vacuum breaker. There is a threaded fitting on each side of it for easy removal. Here's a factory pic of a Triple Slide standing outside of the bath lav looking toward the back of the bedroom dresser. If you remove the fuse panel, the vac breaker is right there. It is on top of the vertical blue pex tube.
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Old 08-21-2022, 09:36 AM   #16
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I have heard of stink bugs going into the valve and causing it to do what your saying. but you still need to get to the valve to remove the stink bug
This is a true story.

I did have stink bugs in the pex between the spray head and vacuum breaker. I'm sure they crawl into the pex at the factory to stay out of the cold.
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Old 08-22-2022, 09:44 PM   #17
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Attachment 278041Nasty project, but relatively easy.
You were right on both counts. Since the spray head was easy to get to, I decided to check that first. Not to get into the gross details, but the spray head was clogged with a thick gel. After cleaning it out and re-installing it, everything worked perfectly.

Lesson Learned: Don't drive home with the black tank registering at 2/3rds full. You can always add some water for proper flushing when you get home. On the DX3 37ts, the bathroom sink drains into the black tank, so that's an easy way to add water to the tank.

Thanks to everyone for your suggestions.
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Old 08-23-2022, 11:51 AM   #18
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Actually if you drive around with something in the black tank it helps to break up the solids. An old trick is to dump the tank, add about 10 gallons of water and empty a bag of ice into black tank just before leaving. the ice breaks up the solids so the flushing system works better.
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Old 08-25-2022, 07:07 PM   #19
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I had the same problem and found if I very slowly turned the water on it would work
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Old 08-26-2022, 09:03 AM   #20
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Flush Won't Work

I couldn't believe we had that problem too a few weeks ago. We've had our unit 8 years and never had a problem. First of all, we had a hose hooked up to the flusher off a Y at the hookup and kept it there, not knowing the shut-off for that hose was leaking slightly. This caused the water to build up and come into the RV via that vacuum breaker. The water pressure wasn't great enough to close it, so it just dripped under the bathroom sink. Replaced that with plumbing and stopped leaving the hose hooked up. Then the same thing happened as you are talking about. Coincidence? There are none. Filled the black tank with 1 cup Dawn and 1 cup fabric softener and left that for a few days. That did the trick. The sprayer head inside the tank got clogged and this treatment worked for us.
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