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Old 08-04-2021, 08:55 PM   #1
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Black tank flush

First post so hopefully I don't get bashed too hard. I did try to search every possible way I could think of.

I am borrowing my stepdad's 2019 25RQGMX and am trying to figure out if what I'm looking at is a black tank flush hookup. There are a total of 3 water hose connections on it, 2 over on the left (street) side appropriately labeled "fresh water" and "city water" but there is one on the right (curb) side with no labeling whatsoever. They use the 3' wand attachment to clean the tank but I can't think of anything else that this port would be for...other than a flush but every trailer I've had/used with one has it clearly marked either "black tank flush" or "flush valve non-potable only"
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Old 08-04-2021, 09:06 PM   #2
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Very possible it is the black tank flush.
There have been several posts on this and other R/V forums where the ports were mislabeled or not labeled at all.

Look inside the R/V where the port enters and see where the line goes.
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Old 08-04-2021, 09:42 PM   #3
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Every blank tank flush that I have ever seen has had a threaded collar so that the supply hose could be screwed on (just like city water). That connection does not look like it has threads in order to screw a hose onto it.
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Old 08-04-2021, 09:58 PM   #4
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Quote:
Originally Posted by NavyLCDR View Post
Every blank tank flush that I have ever seen has had a threaded collar so that the supply hose could be screwed on (just like city water). That connection does not look like it has threads in order to screw a hose onto it.
If you blow up the photo, it does have a round center. I think it's threaded and the angle makes the threads not really visible. It's probably the black tank flush inlet. If the black tank is empty, hook up a water hose to it and listen for the flow into the tank (follow the dump line to get an idea where the black tank is).
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Old 08-04-2021, 10:36 PM   #5
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It is threaded and comes in here next to the feed lines for the sink across from the bathroom...presuming that's not a a feed line for a tank sprayer

Edit: not sure why my pictures are posting sideways
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Old 08-04-2021, 10:47 PM   #6
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Or maybe? Comes down and goes into the floor underneath the refrigerator, perhaps on track to the tank

Screw it, I'll pull the liner for the underbelly

Edit: nevermind, no I won't, liner may have bolts across the center but it has what appear to be those like concrete nails that are shot in with a .22 along the edge rail.

Staying in the trailer currently so I'll just hook up a hose and check next time I drain the tank.
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Old 08-05-2021, 04:05 AM   #7
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If it is the short hose with the two white fittings, and the picture is sideways so the hose actually goes up into the bottom of the black plastic item, and the elbow fittings comes out of the side of the black plastic item then....

The black plastic item is a vacuum breaker check valve which gets installed in the black tank flush line. When you apply pressure to the inlet, the check valve closes and water flows into the black tank flush nozzle(s). When you stop flushing the remaining water flows into the black water tank, the pressure on the check valve releases and it opens the top of the vacuum breaker valve to atmosphere to break the vacuum so there is no siphon effect.

So, that would be the black tank flush connection. Even though mine is different trailer, my black tank flush connection also comes in very close to the bathroom sink and the vacuum break anti-siphon check valve is under my bathroom sink.
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Old 08-05-2021, 06:04 AM   #8
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Originally Posted by NavyLCDR View Post
The black plastic item is a vacuum breaker check valve which gets installed in the black tank flush line. When you apply pressure to the inlet, the check valve closes and water flows into the black tank flush nozzle(s). When you stop flushing the remaining water flows into the black water tank, the pressure on the check valve releases and it opens the top of the vacuum breaker valve to atmosphere to break the vacuum so there is no siphon effect.
Actually when pressure is applied the check valve OPENS and at the same time the vacuum breaker is sealed (held sealed by pressure) and water flows to the tank nozzle. When pressure is removed the check valve CLOSES and the vacuum breaker seal opens and allows the remaining water in the valve cavity and input plumbing line to drain back out the input port when the port is opened to ambient (hose removed). Additionally, if factory installed, the check valve/vacuum breaker is supposed to be installed at least 6" above the toilet rim height no matter where it is located.
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Old 08-05-2021, 06:05 AM   #9
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Quote:
Originally Posted by Brewsky88 View Post

I am borrowing my stepdad's 2019 25RQGMX and am trying to figure out if what I'm looking at is a black tank flush hookup.
isn't it easier to ask him?
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Old 08-05-2021, 06:38 AM   #10
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Originally Posted by Mike134 View Post
isn't it easier to ask him?
He did say he was borrowing his stepdad’s rig, I’m not sure that means his stepdad actually knows that though.

Shhhhhhh!
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Old 08-05-2021, 07:36 AM   #11
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Quote:
Originally Posted by Brewsky88 View Post
It is threaded and comes in here next to the feed lines for the sink across from the bathroom...presuming that's not a a feed line for a tank sprayer

Edit: not sure why my pictures are posting sideways
Yes, that is for the black tank flush.
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Old 08-05-2021, 08:03 AM   #12
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Quote:
Originally Posted by NavyLCDR View Post
Every blank tank flush that I have ever seen has had a threaded collar so that the supply hose could be screwed on (just like city water). That connection does not look like it has threads in order to screw a hose onto it.
My black tank flush looks exactly like that. The threads are on the inside and connects to the male end of a hose.
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Old 08-05-2021, 08:37 AM   #13
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Based on the second picture you posted, I can say with certainty, that connection on the outside is the black tank flush connection.

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Old 08-05-2021, 03:41 PM   #14
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Originally Posted by Mike134 View Post
isn't it easier to ask him?
As I mentioned, he uses the 3' wand connected to a hose because I even asked about if it had a flush and that was his answer. This is his first trailer in a long time and may not even know that these types of systems are available, just doing it how he always has, and has only had it out a few times.

It peaked my interest as a possibility because I had never seen an inlet completely separate from all of the other hook ups like this is that wasn't a flush, but had also never seen one not labeled. With how loaded this trailer is (fueling station, outside shower, electric bunks, electric rear door, generator) I'm making notes of all the perks and quirks I'm finding about it
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Old 08-05-2021, 04:08 PM   #15
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What's the worry about flushing the black tank? Just drain it, it doesn't need to be clean, just empty. If you poisoned the tank (anything blue) it will stink regardless.

An enzyme and Odorlos(tm) will continue to digest any small amount remaining after dumping leaving the tank cleaner than any flush. Been leaving waste in the tanks for decades with those treatments. Heck my boats required pumping to empty and we sure didn't flush them, just pumped every few months.

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