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  • Safe use of black tank flush

    This came up on another forum, as it does from time to time when someone suggests installing a backflow preventer on the black tank flush inlet or keeping a separate hose attached to the black tank flush inlet. Both are really bad ideas. In the interest of health and safety DO NOT put a backflow preventer on your black flush inlet, or leave a hose attached to it.

    Some of the most plumbing-knowledgeable GD owners around including Cate&Rob have made a good case against doing it, and against leaving the black flush water supply connected to the inlet. Here it is.

    The black flush water supply rises from the hose inlet through a plumbing line to high up inside your trailer, probably up behind your shower wall. It connects there, feet above the black tank, to a valve that creates a vacuum break. From there it continues back down from the valve and into your black tank.

    When you send pressurized water into the black tank flush inlet it creates a feet-long column of fresh water up to the high-up valve before it continues flowing down into the black flush mechanism in the tank. That column of fresh water is a deliberately-designed buffer between the inlet and any bacteria-laden water in the tank and black flush mechanism.

    Each time you finish flushing the tank you should remove the black flush supply hose and allow that column of fresh water to flow out. The feet of fresh water on the inlet side of the valve flows back out the inlet and the feet of mostly-fresh water on the other side of the valve flows down into the tank, carrying any bacteria on that side of the system with it.

    Trapping that water in the system - by installing a backflow preventer or by leaving a hose full of water attached - defeats the purpose of the vacuum break safety system. It leaves a column of water - a pathway for pathogens - all the way from the tank to the inlet (and into your hose if left attached).

    On the other hand, allowing the fresh water to flow back out of the system drains the pathway from both the up and down sides of the vacuum break system. You can then reattach the black flush hose if you want to.

    Understanding this leads to another bit of advice that may surprise some people. You can use the SAME HOSE for black flush and the trailer's city water supply! As long as you disconnect from black flush immediately after you turn off the water to it, you can simply move the hose to city water and turn it back on again. Nothing but fresh water has passed through the black flush inlet. However if you trap water in there, that's when it can get risky.

    The system works like this for a reason: health and safety. Grand Design doesn't generously load up our trailers with unnecessary things, especially things hidden inside walls like the black flush vacuum break system that most people don't understand. It's there for a reason.

    Hope this helps.
    Mark - 2018 Solitude 310GK - Ford F-350 SRW diesel short box - Pullrite Superglide hitch

  • #2
    Originally posted by boyscout View Post
    This came up on another forum, as it does from time to time when someone suggests installing a backflow preventer on the black tank flush inlet or keeping a separate hose attached to the black tank flush inlet. Both are really bad ideas. In the interest of health and safety DO NOT put a backflow preventer on your black flush inlet, or leave a hose attached to it.

    Some of the most plumbing-knowledgeable GD owners around including Cate&Rob have made a good case against doing it, and against leaving the black flush water supply connected to the inlet. Here it is.

    The black flush water supply rises from the hose inlet through a plumbing line to high up inside your trailer, probably up behind your shower wall. It connects there, feet above the black tank, to a valve that creates a vacuum break. From there it continues back down from the valve and into your black tank.

    When you send pressurized water into the black tank flush inlet it creates a feet-long column of fresh water up to the high-up valve before it continues flowing down into the black flush mechanism in the tank. That column of fresh water is a deliberately-designed buffer between the inlet and any bacteria-laden water in the tank and black flush mechanism.

    Each time you finish flushing the tank you should remove the black flush supply hose and allow that column of fresh water to flow out. The feet of fresh water on the inlet side of the valve flows back out the inlet and the feet of mostly-fresh water on the other side of the valve flows down into the tank, carrying any bacteria on that side of the system with it.

    Trapping that water in the system - by installing a backflow preventer or by leaving a hose full of water attached - defeats the purpose of the vacuum break safety system. It leaves a column of water - a pathway for pathogens - all the way from the tank to the inlet (and into your hose if left attached).

    On the other hand, allowing the fresh water to flow back out of the system drains the pathway from both the up and down sides of the vacuum break system. You can then reattach the black flush hose if you want to.

    Understanding this leads to another bit of advice that may surprise some people. You can use the SAME HOSE for black flush and the trailer's city water supply! As long as you disconnect from black flush immediately after you turn off the water to it, you can simply move the hose to city water and turn it back on again. Nothing but fresh water has passed through the black flush inlet. However if you trap water in there, that's when it can get risky.

    The system works like this for a reason: health and safety. Grand Design doesn't generously load up our trailers with unnecessary things, especially things hidden inside walls like the black flush vacuum break system that most people don't understand. It's there for a reason.

    Hope this helps.
    Mark - excellent post ! Great idea to share this level of detail.

    Dan
    Dan & Carol
    2014 303RLS Reflection #185 (10/2013 build)
    2012 Silverado LTZ Crew Duramax 2500HD - 2700/16K Pullrite Superglide

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    • #3
      Agree completely with the post. However I use a small length of garden hose for my black tank flush even though its not technically needed. I will not even fill my water tanks at the dump station as well. Its just wrong for parks to have fresh water located at the dumping station. Too many opportunities for cross contamination IMO. I can partially blame my RN wife for me being a germaphobe.

      Jim

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      • #4
        Originally posted by boyscout View Post
        The system works like this for a reason: health and safety. Grand Design doesn't generously load up our trailers with unnecessary things, especially things hidden inside walls like the black flush vacuum break system that most people don't understand. It's there for a reason.
        Excellent post, Mark. Owner's manuals often fail to provide the "why" behind the "what". Your post should be included in the plumbing section of something that could be called, New Owner's 101.

        Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

        2017 Ford F-350 DRW, '19 315RLTSPlus

        Comment


        • #5
          Originally posted by MidwestCamper View Post
          I use a small length of garden hose for my black tank flush even though its not technically needed.
          A small length of hose from the black tank flush connection on the water control panel, to outside of the RV, is a great idea! This means that the water dropping back from the vacuum break valve lands on the ground rather than in your pass through storage or water panel space. This is a great application for water quick connect fittings. You can disconnect the water hose from the short black tank flush hose without shutting off the water pressure. The black tank flush line will immediately drain to the ground and you can immediately connect this fresh water hose back to the fresh water inlet.

          Rob

          Cate & Rob
          (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
          2015 Reflection 303RLS
          2022 F350 Diesel CC SB SRW Lariat
          Bayham, Ontario, Canada

          Comment


          • #6
            This is one thing I was going to try to do this winter and with boyscout providing this post and information I will just add this to it.

            Here is a picture of the vacuum breaker that is generally used for the black tank flush system.
            Click image for larger version

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            As you can see , hopefully , the flow is from the bottom and out the right side , as pictured.

            I have always wanted to cut one apart to see what is inside , here it is.
            Click image for larger version

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            In this picture above you can see the white plastic piece , pointing up , this is pictured with the valve in the up position when water pressure is applied to the inlet. There is a rubber gasket on the top of this "float valve part" that seals from water spraying out the top cover. Also behind this valve you can see another white object , next picture please.
            Click image for larger version

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            This is yet another valve. This ball check valve has a spring that forces it back when there is no water pressure in the flush line. This ball is located at the connection of the "out flow" and the "in flow" , reference the first photo.

            So how does it work?

            When water pressure is presented into the inlet side it pushes up the flat valve and this allows water pressure to push on the ball spring valve in the outlet end to go to the black tank flush nozzel. When water pressure is stopped the flat valve falls down to a rested state allowing air to enter the system. This allows the water to drain , one way to your garden hose and the other way to the black tank.

            Hope this is helpful in understanding how this valve works as well as what it looks like.

            Brian
            Brian & Michelle
            2018 Reflection 29RS
            2022 Chevy 3500HD

            Comment


            • #7
              Originally posted by howson View Post
              Owner's manuals often fail to provide the "why" behind the "what". Your post should be included in the plumbing section of something that could be called, New Owner's 101.
              I'll wait for the rich royalty checks to begin appearing in my mailbox.
              Mark - 2018 Solitude 310GK - Ford F-350 SRW diesel short box - Pullrite Superglide hitch

              Comment


              • #8
                Originally posted by boyscout View Post

                I'll wait for the rich royalty checks to begin appearing in my mailbox.
                Sorry, but Chiefblueman already has dibs on "being blue". (You can't hold your breath that long anyway.)
                Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

                2017 Ford F-350 DRW, '19 315RLTSPlus

                Comment


                • #9
                  I’ve been using a typical pressure water regulator on a separate hose that I use for the black tank flush is this wrong?

                  Comment


                  • #10
                    Originally posted by Rolltide2524 View Post
                    I’ve been using a typical pressure water regulator on a separate hose that I use for the black tank flush is this wrong?
                    I do not use a pressure regulator on the black tank flush, I do not believe it is needed, and also do not believe using one is wrong.Most camp ground water pressure's are safe , around 40 to 50 psi , unfortunately some are higher. I do always use a regulator on the city water side though.

                    Brian
                    Brian & Michelle
                    2018 Reflection 29RS
                    2022 Chevy 3500HD

                    Comment


                    • #11
                      Originally posted by Rolltide2524 View Post
                      I’ve been using a typical pressure water regulator on a separate hose that I use for the black tank flush is this wrong?
                      I do the same as Brian and agree with him--nothing wrong with using one if you choose.
                      Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

                      2017 Ford F-350 DRW, '19 315RLTSPlus

                      Comment


                      • #12
                        Rolltide2524 , like Brian and Howard I don't use a regulator and have no concern about it breaking anything. Plus, I want good pressure spraying out of the fitting inside the black tank to maximize its ability to stir up the contents of the tank. The regulator is not required and maybe not a great idea.

                        Using a separate hose is certainly not wrong. Grand Design's lawyers have ensured that product documentation advises owners to do this; it covers the company against any tiny risk that someone might get sick by using the same hose and try to go after GD. However on reading the post at the top of this thread describing how the system works you might agree that the risk is pretty small.

                        I've been using the same hose for fresh water and black rinse for about a year, probably 30 or so black tank rinses and lots of time with the trailer sitting around in hot weather allowing any bacteria that wants to flourish in the black tank rinse system to do so. I'm still here, and except for a growing nastiness in my personality have not suffered any ill health in the past year.
                        Mark - 2018 Solitude 310GK - Ford F-350 SRW diesel short box - Pullrite Superglide hitch

                        Comment


                        • #13
                          Although infrequent, vacuum breakers can fail. For that reason and the fact that at several campgrounds I frequent only have non-potable water available for black tank flushing, I will always use a seperate hose. So I can't send you that check .
                          Dave and Susan
                          Imagine 2250RK
                          2019 Ford Ranger

                          Comment


                          • #14
                            Originally posted by S&D's Immy View Post
                            Although infrequent, vacuum breakers can fail. For that reason and the fact that at several campgrounds I frequent only have non-potable water available for black tank flushing, I will always use a seperate hose.
                            It would be unwise to run non-potable water through a drinking water hose for ANY purpose or reason, of course! That's not what has been suggested here.

                            I don't share your concern about the unlikely failure of the vacuum breaker. Flushing means sending pressurized fresh water one way through the length of the system to the flushing fixture in the tank. Turning off that pressure, removing the supply hose, and allowing the fresh water in the flush system to drain back out does mean that in the event of a vacuum breaker failure some of the fresh water on the down-side of the flush system might be siphoned out and drain on the ground. However any traces of bacteria it *might* contain would then have to survive on the soon-dry brass end of the input hose to the flushing system (like others I have a 3-foot hose on the input to the black flush) for 5-8 days until the next dump and flush. The bacteria can't do that. Not concerned.

                            Thanks for your thoughts.
                            Mark - 2018 Solitude 310GK - Ford F-350 SRW diesel short box - Pullrite Superglide hitch

                            Comment


                            • #15
                              Originally posted by boyscout View Post

                              However any traces of bacteria it *might* contain would then have to survive on the soon-dry brass end of the input hose to the flushing system (like others I have a 3-foot hose on the input to the black flush) for 5-8 days until the next dump and flush. The bacteria can't do that. Not concerned.

                              Thanks for your thoughts.
                              Mark,

                              Not to prolong this thread. I agree that as designed and as you pointed out in your first post, people should have no issues using the same hose. I wasn’t suggesting that you thought it would be okay to run nonpotable water through a drinking hose just pointing out why I have a separate hose dedicated to black tank flush.

                              In the event of a vacuum breaker failure bacteria could be flushed back to the end of the hose as you pointed out. As a biologist I will say that bacteria, unlike a virus that only survive a few days, can survive months on dry surfaces. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1564025/ .

                              Still I don’t believe your process would be an issue just that there is a extremely remote possibility under the correct conditions to contaminate drinking water with the same hose. Just as there is being struck by lightening. The silver lining is that when a vacuum breaker fails it usually leaks, thus letting you know it failed. Then you can sanitizing surfaces to be safe prior to using the hose again.
                              Dave and Susan
                              Imagine 2250RK
                              2019 Ford Ranger

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