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  1. #11
    Seasoned Camper Luv2Ski's Avatar
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    klemdotter, I think the Nautilus line that connects to both tanks near the bottom will act as an equalization line coupling the two tanks. So you can't drain the 40 gallon tank and expect it to stay that way because the water in the 117 will drain through the Nautilus line back into the 40 gal tank until the level in the two tanks is equalized.
    Steve and Cheryl

    2017 Momentum 328M w/Dual Pane Windows and 3rd A/C. Aftermarket mods: Titan EOH Disc Brakes, MORryde IS suspension and Reese 5th Airborne Sidewinder pin box
    2014 Ram 3500 Longhorn Megacab 4x4 DRW with 6.7 HD Cummins Turbo Diesel, AISIN trans, 3.73 axles and a Reese 20K puck mount hitch
    Call sign: AAØSB, Class: Extra



  2. #12
    Seasoned Camper Luv2Ski's Avatar
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    Quote Originally Posted by jenamiles View Post
    I can't figure out how to upload pics to here from my mobile and they aren't on my computer yet. I'll try and move them over and upload tomorrow. In the meantime, quick answers to the new questions. The overflow valves are exact center on the side of the tank front to back so in theory, you'd need a pretty significant grade to push water out of the overflow. It also rises 2" past the top of the tank or so before coming back down. The tank bottoms are offset by one inch. The larger tank sits one inch lower and one inch higher (so total it's 2 inches taller than the small tank). The adapter that they are using is a Tee.
    Thanks Jenamiles. Interesting what they did with the 40 gallon tank. Wonder why they couldn't align the tops (i.e. boost the 40 gallon tank up by the full two inches. Then both overflows would be at about the same height. Heck, they could even run the nautilus to just the 117 gal tank and use a separate, larger diameter tie line between the two tanks at the bottom. If the larger diameter can equalize at the GPM rate that the Nautilus line is filling the 117 gal main tank, then all is good and there's no need to futz with fill rates. Both tanks would hit their overflow at essentially the same time on level ground,

    Nice to hear that they centered the overflow drains on the tanks. The rise in the overflow drains will only help out on level ground. If the RV has both FW tanks at capacity and begins ascending a steep 8% grade, the front 117 gal tank will be at a higher level than the rear 40 gal one. Water would flow from the higher tank to the lower tank through the Nautilus line trying to reestablish surface equilibrium. This would drive up the surface in the 40 gallon tank causing it to overflow until the 117 gal tank is drained to the point of water surface equilibrium.

    Regarding the pics, I think you have to be a site supporter to post them. http://www.mygrandrv.com/forum/payments.php if you're interested. It's only 12 bucks a year. Trial members can see pics for like 30 days then that gets disabled. The purpose of the site support fee is to cover Chris' expenses - trust me, he's not getting rich running this place. I would highly recommend becoming a supporter.
    Steve and Cheryl

    2017 Momentum 328M w/Dual Pane Windows and 3rd A/C. Aftermarket mods: Titan EOH Disc Brakes, MORryde IS suspension and Reese 5th Airborne Sidewinder pin box
    2014 Ram 3500 Longhorn Megacab 4x4 DRW with 6.7 HD Cummins Turbo Diesel, AISIN trans, 3.73 axles and a Reese 20K puck mount hitch
    Call sign: AAØSB, Class: Extra



  3. #13
    Setting Up Camp
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    Well, I read your post and went out to pull the drain on the 40 gallon tank. Steve, you are right. A nice stream of water started draining from the auxiliary tank... So it looks like there is no way to fill just the main tank the way this system is plumbed. I guess when I travel I will just fill for about 12 to 15 minuets and go with about 35 gallons dispersed into both tanks. Does anyone know the real reason that the fill system for the two tanks is designed this way? And what exactly is the purpose and desired result in reengineering the plumbing system? I have to admit that with all the talk of installing more Tees or Ys and another crossover etc. has got me confused. I am not into pulling the belly cover and plumbing a new water system on a new trailer with a MSRP of 107,000 dollars. Now if GD is going to offer a warranty upgrade to solve a problem, count me in.
    I guess the bottom line is this: is this just a glitch in the water system that we will all have to learn to live with or is it a major problem that has to be fixed and I mean by Grand Design?

  4. #14
    Seasoned Camper Luv2Ski's Avatar
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    I was told by GD customer service yesterday (8/11) to shut off water pressure when the small tank overflows and the big tank is only at maybe ⅓ full. Just walk away for several minutes and let the tank levels equalize through the fill line. Come back and fill again till the small tank overflows. Walk away and let it equalize. Now do it again - over and over until my 117 gallon tank is full. That's just nuts and totally unacceptable to me. It's one thing to do this technique in front of my house and another thing entirely to do it at the park service's fill station with 4 RVs lined up behind me.

    Here's my suggestion for engineering to correct this:

    Goals:

    1. Maximize usable tank capacities for extended stay dry camping. This is the purpose of ordering an extra tank!
    2. Avoid issues with rates of fill not matching thereby causing one tank to overflow before the other is adequately filled.
    3. Ensure when tanks are filled to capacity, that travelling up and down steep grades (such as those often found near dry campsites) doesn’t cause water to gush out the overflow lines and muddy up the dirt road for the next camper that comes along.
    4. Continue to use the single Kantleak/Nautilus line for both fill and draw.


    Proposed modifications:
    1. Vertically align tank tops by elevating the 40 gallon tank. This puts the overflow taps at the same vertical level. The old design had the 40 gallon tank top 1" lower than the large tank top per jenamiles' investigation. If I understand correctly, the difference in overflow tap heights was compensated for with a riser on the overflow tap on the small tank thereby retaining the large tank's original high water mark. Without the riser, the high water mark would be at the lower tank's overflow level. Dynamic forces encountered during driving could cause a siphon to start however - especially on the low tank side when ascending a steep grade.
    2. Tap Kantleak/Nautilus feed/draw line into low point of main 117 gal tank only. No need to tap into both tanks. Again, according to jenamiles' research on this thread, the 40 gallon tank bottom sits 1" higher than the 117 gallon tank bottom. At some point, that tap will be “sucking air” from the smaller tank and this will potentially break the siphon needed by the water pump to draw water from either tank. If that happens, we've lost 10 gallons of usable capacity because we can't get to the last inch of the 117 gallon tank. Note: I'm assuming a level RV when dry camping.
    3. Relocate overflow taps and lines on each tank as shown below (or at the side-top corner if top clearance is inadequate). This is done to avoid unnecessary spillage on steep road grades when the tanks were initially filled to capacity on level ground.. Leaking water on a highway is probable cause for getting pulled over for suspicion of leaking sewage.
    4. Provide an independent water equilization line not affected by fill flows or water pump draw. Right size the line diameter through trial and error. Test use cases should include both pressure filling as well as towing dynamic forces such as rapid change of grade encountered in normal driving and camp site access (e.g. ascent to crest to descent) and braking on ascent and descent.
    5. Provide an optional high tie line between tanks to permit air migration from lowest tank to highest tank when travelling on steep grades. This helps avoid airspace compression in the lower tank when water blocks the overflow line tap point.
    6. Avoid creating a siphon through the overflow line when decellerating suddenly (breaking hard on an ascending grade for example).


    Click image for larger version. 

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    Steve and Cheryl

    2017 Momentum 328M w/Dual Pane Windows and 3rd A/C. Aftermarket mods: Titan EOH Disc Brakes, MORryde IS suspension and Reese 5th Airborne Sidewinder pin box
    2014 Ram 3500 Longhorn Megacab 4x4 DRW with 6.7 HD Cummins Turbo Diesel, AISIN trans, 3.73 axles and a Reese 20K puck mount hitch
    Call sign: AAØSB, Class: Extra



  5. #15
    Left The Driveway
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    I feel like the name says it all, it's an aux tank. It should be used as a spare, a warning that your low on your main tank and you have to switch to your aux tank letting you know you only have 40 gallons left.
    The simple fix is to add a valve between the two tanks. You can chose to not fill your aux tank if your only going for a short trip and rely on the 117 gallons or less if you chose. If your going for a longer stay fill the aux by either using your power fill or the aux fill. If you use the power fill wait till the aux starts venting water then close the valve and continue filling your main tank, at this point both tanks are 100%.
    If you run out of main tank simply open the aux valve and your pump can consume from the aux now .
    Seams like GD just forgot the valve. They both have vents,they both have fill points.
    Just add the valve with a cable handle or use an electric valve .

  6. #16
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    Quote Originally Posted by Luv2Ski View Post
    I was told by GD customer service yesterday (8/11) to shut off water pressure when the small tank overflows and the big tank is only at maybe ⅓ full. Just walk away for several minutes and let the tank levels equalize through the fill line. Come back and fill again till the small tank overflows. Walk away and let it equalize. Now do it again - over and over until my 117 gallon tank is full. That's just nuts and totally unacceptable to me. It's one thing to do this technique in front of my house and another thing entirely to do it at the park service's fill station with 4 RVs lined up behind me.

    Here's my suggestion for engineering to correct this:

    Goals:

    1. Maximize usable tank capacities for extended stay dry camping. This is the purpose of ordering an extra tank!
    2. Avoid issues with rates of fill not matching thereby causing one tank to overflow before the other is adequately filled.
    3. Ensure when tanks are filled to capacity, that travelling up and down steep grades (such as those often found near dry campsites) doesn’t cause water to gush out the overflow lines and muddy up the dirt road for the next camper that comes along.
    4. Continue to use the single Kantleak/Nautilus line for both fill and draw.


    Proposed modifications:
    1. Vertically align tank tops by elevating the 40 gallon tank. This puts the overflow taps at the same vertical level. The old design had the 40 gallon tank top 1" lower than the large tank top per jenamiles' investigation. If I understand correctly, the difference in overflow tap heights was compensated for with a riser on the overflow tap on the small tank thereby retaining the large tank's original high water mark. Without the riser, the high water mark would be at the lower tank's overflow level. Dynamic forces encountered during driving could cause a siphon to start however - especially on the low tank side when ascending a steep grade.
    2. Tap Kantleak/Nautilus feed/draw line into low point of main 117 gal tank only. No need to tap into both tanks. Again, according to jenamiles' research on this thread, the 40 gallon tank bottom sits 1" higher than the 117 gallon tank bottom. At some point, that tap will be “sucking air” from the smaller tank and this will potentially break the siphon needed by the water pump to draw water from either tank. If that happens, we've lost 10 gallons of usable capacity because we can't get to the last inch of the 117 gallon tank. Note: I'm assuming a level RV when dry camping.
    3. Relocate overflow taps and lines on each tank as shown below (or at the side-top corner if top clearance is inadequate). This is done to avoid unnecessary spillage on steep road grades when the tanks were initially filled to capacity on level ground.. Leaking water on a highway is probable cause for getting pulled over for suspicion of leaking sewage.
    4. Provide an independent water equilization line not affected by fill flows or water pump draw. Right size the line diameter through trial and error. Test use cases should include both pressure filling as well as towing dynamic forces such as rapid change of grade encountered in normal driving and camp site access (e.g. ascent to crest to descent) and braking on ascent and descent.
    5. Provide an optional high tie line between tanks to permit air migration from lowest tank to highest tank when travelling on steep grades. This helps avoid airspace compression in the lower tank when water blocks the overflow line tap point.
    6. Avoid creating a siphon through the overflow line when decellerating suddenly (breaking hard on an ascending grade for example).


    Click image for larger version. 

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    Hi Steve - The POST you and others commented on was way back in 2016 - I hope this message reaches you. Its now 2022 and my new Momentum still has the same problem you and others worked to correct. Was there ever a resolution to the two fresh water tank issues I am experiencing: unable to fill two tanks to advertised capacity and the siphoning issue while traveling? Another post in this thread with you referred to someone working with a Raymond, at Grand Design, on this issue. Do you know how to contact this Raymond or anyone else that worked on this fix at GD? Thank you sir. Mike

  7. #17
    Fireside Member Rich38's Avatar
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    When I had the belly open to repair a leak at the aux tank, I had the same idea We2RV2 has mentioned. I kick myself for not following thru.

    I would like the option of either filling the front or rear tanks, not always having water in both.
    2020 Dodge Ram 3500 Dually
    2018 351M Momentum Toy Hauler
    2015 Can Am Turbo Maverick - sold
    2019 Can Am X3 XRS

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