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Old 09-06-2013, 12:33 PM   #1
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Black tank flushing

OK I have read several dozen threads on this but here is my question. If I only have a 1/4 tank of black and I am getting ready to hit the road why would I not just fill the tank with a hose and dump then?? From what I read the less weight in the tt the better,so with only say 5 gallons of clean water in it it will beat say 25 gallons of black water. I see the newer tt have a connection to spray water ito the black and since I would be hooked up with water seems like the right thind to do. Well????
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Old 09-06-2013, 01:02 PM   #2
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If I am at a full hook up camp site and have a 1/4 full black tank I'll generally dump it before I leave. I fill it full with at hose and then pull the valve. I always do this if my next stop is at a campgound without full hookup. The black tank on my 68 holds only about 16 gallons so my issue is not so much weight when traveling but rather not having to use my blueboy to dump at the next site if I am staying a while. We always try and stay at a full service campsite the last night of our trips so I can dump the tank and then fill the black tank again with water and dump again. It is much easy to do this at the campsite than at a dump station with folks lined up behind you wainting.
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Old 09-06-2013, 01:09 PM   #3
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You might want to take a short local trip with more water in the tank (maybe about half full so it will slosh around) over a local bumpy road to shake "things" up a bit from the bottom of the tank before trying to dump. The greater volume of fluids will cause a higher velocity discharge that will help drain "everything" out of the tank.

We put about five gallons of water and a tank chemical in both the gray and black water tanks before departing on a trip along with a full fresh water tank.

If the trailer has been parked for quite awhile, I would also consider sterilizing the fresh water tank with some laundry bleach and flushing at least twice before getting on the road.
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Old 09-06-2013, 04:36 PM   #4
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An idea that a friend gave us was to put in about 1/4 cup of Twenty Mule Team Borax in the black tank along with some added water and drive around a little while or between campgrounds. The mixing with the borax does a good job of washing the tank and the old style tank sensors.
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Old 09-06-2013, 05:09 PM   #5
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An idea that a friend gave us was to put in about 1/4 cup of Twenty Mule Team Borax in the black tank along with some added water and drive around a little while or between campgrounds. The mixing with the borax does a good job of washing the tank and the old style tank sensors.
Also works with Comet cleanser, Tide laundry detergent, or just about any other powdered cleanser. Especially if you add ice, and drive around (with lots of turns and speed changes to slosh the ice around) long enough for the ice to melt.

Cleanser + ice is an old restaurant trick for cleaning glass coffeepots where the shape doesn't allow you to insert a hand holding a brush.
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Old 09-06-2013, 05:33 PM   #6
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Ift is very unlikely you will ever have 25 gallons in your black water tank. More apt to have 25+ gallons in the gray water tank and 5 or 6 in the black.

Now that said i just spent 14 days at Edisto with a bad water valve on the toilet. We did fill the black tank with the kids dumping water into the toilet to flush.

What I generally do while dumping is dump the black tank. When the flow slows, and while that valve is still open, I lift the drain hose so it is above the tank level and open the gray tank valve for about 10 seconds. Close the gray valve and drop the hose. This produces a large back flow of gray water into the black tank. Much more than any hose flushing would provide.

Once that flow stops I close the black tank valve and open the gray.
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Old 09-06-2013, 06:30 PM   #7
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What I generally do while dumping is dump the black tank. When the flow slows, and while that valve is still open, I lift the drain hose so it is above the tank level and open the gray tank valve for about 10 seconds. Close the gray valve and drop the hose. This produces a large back flow of gray water into the black tank. Much more than any hose flushing would provide.
Strictly speaking, once you cross-connect the black and gray tanks through the discharge hose, you don't have a black and gray tank anymore. You have two black tanks, because you've potentially contaminated your gray tank with black water.
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Old 09-06-2013, 07:22 PM   #8
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Now if you go back and follow the sequence closely you will see that there is No Cross Contamination.

Even if there were who cares and what difference would it make. It all ends up in the same place.

It is based on the first principle of plumbing, which I will not quote here.
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Old 09-06-2013, 08:09 PM   #9
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Now if you go back and follow the sequence closely you will see that there is No Cross Contamination.

Even if there were who cares and what difference would it make. It all ends up in the same place.

It is based on the first principle of plumbing, which I will not quote here.
I know the first principle. Stuff flows downhill. Well, actually down-gradient, which isn't always downhill or there would be no such thing as high tides. But for plumbing flowing due to gravity and not due to a pump, it's downhill. Anyway…

I also said "potential" contamination. If you follow your procedure precisely every time, you should be good. But not every reader who decides to try it will necessarily understand the need to be as conscientious, so a word of caution to them is warranted.

And as for it all ending up in the same place, that's not true in places where you're allowed to discharge graywater onto the ground.

I apologize if I offended you, but I do stand by what I wrote.
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Old 09-06-2013, 09:23 PM   #10
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When dumping your tanks, dump black, then leave the valve open and dump the grey. Close the grey valve, and while still hooked up to the drain line, go inside and run about 15 seconds of water into the toilet through a continuous flush. It washes out the tank and the drain hose. No residual odors, and it doesn't take much time at the dump station.
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Old 09-11-2013, 07:08 PM   #11
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Greetings


Tonight I watched some videos on YouTube about RV care and they mentioned dumping the black tank first, leaving the valve open and pulling the gray tank with both valves open. I was shown to do each one separately but now I wonder as I also found this thread. Would it differ depending on the design? I do not have a flush system but one example on the video was a motor home and they did pull each tank separately then flushed it via a connection with a clear elbow and a hose.
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Old 09-11-2013, 09:21 PM   #12
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The one thing that they may have left out when opening both valves is to lift the drain hose, for about 10 seconds, as you open the gray water tank and then dropping the hose and closing the gray water valve.

This puts a Large amount of gray water into the black tanks and give it a much better flush than and flush system ever could.
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Old 09-11-2013, 11:20 PM   #13
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We pull the dumps separately, though if you do it like HowieE suggests, that method should aid in the back-flushing of the black tank somewhat.

We do have a flush system in our Bambi, but we don't think it's very efficient, so we basically use it refill the black tank (after the initial release) to 3/4 full or so and letting it rip again (with the grey valved closed). We also use a clear elbow between the dump valve and the drain hose...you'd be amazed at how much "stuff" still comes out on the 2nd and 3rd flush. If we are dumping between campgrounds, we don't bother with this since we're going to use the black tank again right away ... rather, we try to do it at the last dump before we leave for home.
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Old 09-13-2013, 09:37 PM   #14
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There is always something more to learn. I have a clear elbow and you are correct about how beneficial the visual is for cleaning. That motor home flush segment was great on the video bur it got me thinking about what was really better on a system without a flushing method other than running a hose to the toilet. Howie, what you mention about a rush of gray water makes sense.
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