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Water Siphoning from Full Fresh Water Tank

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  • Water Siphoning from Full Fresh Water Tank

    See photo below.

    Saw a solution to the problem of water siphoning out of the overflow tube when the fresh water tank is full that didn’t look like a potential disaster some day when I was in a hurry. Adding a plug or valve to the overflow line has scared me. I can easily see my self forgetting to open the valve/plug when filling the tank or when in use.

    The owner had extended the overflow to the side of the unit in the area under a slide with the open end of the extension above the top of the fresh water tank.
    Colan Arnold
    2016 Momentum 397TH
    Full time since 2016

  • #2
    Putting the outlet of the vent/drain above the tank will remove any chance of siphoning. Adding a screen will keep bugs out.
    It also keeps the water from sloshing out while traveling.
    Ted
    2021 Reflection 310RLS
    2020 F350 PS,CC,LB,SRW

    Comment


    • #3
      colan
      Hi Colan,

      It would be interesting to know if this extension actually stops siphoning. As built, the vent line already rises to this level . . . but drops down again to clear the coroplast. I wonder if taking the pipe up again might actually work . Why are there two vent lines in your picture?

      Rob

      Click image for larger version

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      Cate & Rob
      (with Border Collies Molly & Angel and their kitty Gracie)
      2015 Reflection 303RLS
      2014 Ecoboost F150 with Heavy Duty Payload Package
      Aylmer, Ontario, Canada

      Comment


      • #4
        Most vent lines drop straight down from a 90deg fitting near the top of a side of the tank. Water will drain until the water level reaches the vent port. My previous trailer had two vents. Don't know why. An ideal location for the vent port would be the center of the tank top.
        The inverted U will siphon water as long as the tank water level is above the vent port. So an extension to take the outlet above tank level will stop the siphon potential.
        Ted
        2021 Reflection 310RLS
        2020 F350 PS,CC,LB,SRW

        Comment


        • #5
          Originally posted by colan View Post
          See photo below.

          Saw a solution to the problem of water siphoning out of the overflow tube when the fresh water tank is full that didn’t look like a potential disaster some day when I was in a hurry. Adding a plug or valve to the overflow line has scared me. I can easily see my self forgetting to open the valve/plug when filling the tank or when in use.

          The owner had extended the overflow to the side of the unit in the area under a slide with the open end of the extension above the top of the fresh water tank.
          Colan for your TT that solution should work. Be sure you have a drain valve at the low points as you don't want water siting in the tube for a long time. A bug screen is a must. I suspect that during turn some water will still get out due to tank sloshing. BTW - does your rig have 2 fresh water tanks with separate vents?

          Hope this helps
          2018 Reflection 150 Series 220RK 5th wheel. Reese R20 Titan hitch, Steadyfast system, 2004 F350 King Ranch dually

          Comment


          • #6
            Yoda - not my trailer. On a fifth wheel I worked on late last week. Always on the lookout for new ways to tackle problems.

            Rob - the reason for two lines is the RV had two fresh water tanks with a vent for each tank. Not a Grand Design.
            Colan Arnold
            2016 Momentum 397TH
            Full time since 2016

            Comment


            • #7
              I can see how ending the vent pipe in a horizontal run at a level above the tank would stop the siphon action that is possible through the original loop above the tank and back down. This arrangement will still not stop the "slosh pumping" loss that occurs while travelling with a full tank . . . although the extra length of small diameter pipe may slow this down.

              Rob
              Cate & Rob
              (with Border Collies Molly & Angel and their kitty Gracie)
              2015 Reflection 303RLS
              2014 Ecoboost F150 with Heavy Duty Payload Package
              Aylmer, Ontario, Canada

              Comment


              • #8
                I went round and round with this. I have a terrible memory and would always forget to open my vent. I was able to extend my vent out the rear of the rig keeping it pitched up the whole way, and ran it into a roof vent on the rear wall. I put some window screen over it to keep the bugs out. It works really well, no loss from sloshing or siphoning.

                I detailed it in this thread:

                https://gdrvowners.com/forum/operati...evisited/page2
                Neil Citro
                2018 Reflection 28bh
                2019 F350 6.7L Long Bed Crew Cab

                Comment


                • #9
                  ncitro, good way to do it.
                  That is similar to venting black and gray tanks. They don't siphon or slosh out.
                  Ted
                  2021 Reflection 310RLS
                  2020 F350 PS,CC,LB,SRW

                  Comment


                  • #10
                    Originally posted by ncitro View Post
                    I went round and round with this. I have a terrible memory and would always forget to open my vent. I was able to extend my vent out the rear of the rig keeping it pitched up the whole way, and ran it into a roof vent on the rear wall. I put some window screen over it to keep the bugs out. It works really well, no loss from sloshing or siphoning.

                    I detailed it in this thread:

                    https://gdrvowners.com/forum/operati...evisited/page2
                    Just so I am clear, you expand from 1/2" to 2 1/2" at some point?
                    Mike
                    2017 Imagine 2800BH pushing a 2012 Toyota Tundra

                    Formerly: 2002 Rockwood Popup
                    Location: Massachusetts

                    Comment


                    • #11
                      Originally posted by mpking View Post

                      Just so I am clear, you expand from 1/2" to 2 1/2" at some point?
                      My vent was 3/4". I was able to run 3/4 PEX to the rear, and adapt it to the inside of the roof vent fitting. I forget exactly but the fitting had a smaller connection on the inside and a larger on the outside. It was pretty straightforward. Looking at the cap online it looks like they take 1-1/4 or 1-1/2" pipes. I just used a plumbing reducer.


                      Neil Citro
                      2018 Reflection 28bh
                      2019 F350 6.7L Long Bed Crew Cab

                      Comment


                      • #12
                        @Cate&Rob and @howson I think you 2 are the resident plumbing experts on the GD stuff. Would it be possible for GD to install a Vacuum breaker like they use on the blackwater flush tank? Technically it would need to be installed backwards treating the vent line as the supply to work properly. This way when the discharge line starts to siphon water out of the fresh water tank, it opens a vent higher than the tank letting air in which in turn will stop the siphoning. This also keeps the tank from sucking back any road spray if its wet when the tank suction gets larger than the siphoning suction. Also, why does GD not put a side fill for the fresh water tank? I understand the idea of power filling, but it's a bit of a hassle to hook the unit up to the supply hose in order to fill the tank.
                        Tow Vehicle: 2018 GMC K2500 Denali Diesel
                        Coach: 303RLS Pending Delivery week of March 1st.

                        Comment


                        • #13
                          Jlawles2 ,
                          I assure you, 99% of all I know about plumbing in an RV was from reading Cate&Rob posts or asking him questions. But thank you for the compliment--(and I still hate plumbing).

                          I'll leave the response to Rob or I'll reveal my ignorance on vacuum breakers.

                          Howard
                          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.

                          Howard, 2017 Ford F-350 DRW, '19 315RLTSPlus

                          Comment


                          • #14
                            A valve like the one used in the black tank flush might work. Its intent is to close when water flows and open to break the siphon so that black water is not siphoned back to the fresh water plumbing.
                            I'd prefer the simplicity of keeping the vent outlet above tank water so that a siphon cannot start and have no moving parts.
                            Ted
                            2021 Reflection 310RLS
                            2020 F350 PS,CC,LB,SRW

                            Comment


                            • #15
                              https://www.hodgesmarine.com/fsp9030...QaAg43EALw_wcB

                              This vacuum break loop has a duckbill valve at the top. The valve is effectively a check valve that stays closed to not allow water out the top , but opens to allow air to enter killing the siphon suction.
                              Ted
                              2021 Reflection 310RLS
                              2020 F350 PS,CC,LB,SRW

                              Comment

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