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  1. #1
    Setting Up Camp
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    Water coming out of overflow drain

    We own a 2019 Momentum 28G Toyhauler and since new I noticed we never have the 100 gallons fresh water tank the factory allegedly installed. When we are empty and water pump is pulling air I refill using my 65 gallon truck mounted tank when we are dry camping. Usually I get most of it in but then water starts running out of overflow tube. This last trip I filled using city water and by the time we got to our destination 2 hours later gauge showed 1/3 full. Sure enough we ran out of water the next day. I then refilled using my truck mounted tank but could only get 25 gallons in and water came running out of overflow tube for the next 1/2 hour. Rv is level by the way. Something is terribly wrong. Any ideas friends? Steve

  2. #2
    Long Hauler
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    Does yours have the double tanks? Some do, and they are connected by a relatively small gauge hose/tubing. So you can fill one until the overflow shows it will hold no more, then it will gradually fill the second tank and then you can add more water to the first tank. Or so I have been informed. I know our 351 does this and it can be very confusing if you're unaware of it's idiosyncrasies.
    Howard and Peggy
    2019 Momentum 351M, and 2018 RAM Cummins dually 6-speed.
    His: 1999 Honda Interceptor
    Hers: 2013 Spyder ST-S

  3. #3
    Site Sponsor TerryH's Avatar
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    Unfortunately it's a very common problem. The vent/overflow lines are notorious for siphoning water out of the tanks. A few different ways to deal with it. Grand Design has changed the way they vent now but it would involve dropping the chloroplast and replumbing the vent lines. I opted for adding 3/4" sharkbite ball valves to the overflows on our Momentum 31G. Readily available at lowers etc... There are 2 tanks. When filling once it starts siphoning from the front tank you can close that vent and that will allow the rear tank to fill. Once it starts coming out the rear overflow you can stop filling and close that valve. I can get over 90 gallons in mine using this method. It is imperative that the valves be opened once you are at your destination to allow the tanks to vent before using water or risk damaging the tanks. We're able go retain 100% of our water using this method.

    Last edited by TerryH; 02-22-2022 at 11:44 AM.
    2021 Momentum 31G
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  4. #4
    Setting Up Camp
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    Thanks to you both for responding! I noticed 2 quick drains and 2 blue pex overflow tubes under mine so I have 2 tanks also. Never even thought about that. Question is after installing shut off valves the rear overflow always starts spewing water first so I would then shut that valve to allow front tank to fill? It sound scary to close a overflow valve hoping that the front tank then fills!

  5. #5
    Site Sponsor TerryH's Avatar
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    Quote Originally Posted by Shaines1959 View Post
    Thanks to you both for responding! I noticed 2 quick drains and 2 blue pex overflow tubes under mine so I have 2 tanks also. Never even thought about that. Question is after installing shut off valves the rear overflow always starts spewing water first so I would then shut that valve to allow front tank to fill? It sound scary to close a overflow valve hoping that the front tank then fills!
    Perhaps it's plumbed differently on the 28G than on our 31G. Ours definitely fills the front first but we are within 10 gallons or so of capacity when it starts coming out the front overflow. Are you using a water meter? If not, I would highly suggest it so you know when you are at or near capacity. I'd guess that closing the rear until you get flow from the front is acceptable. We use this one from Amazon if you need to get a meter.

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    2021 Momentum 31G
    2021 20' Haulmark Edge race trailer
    '17 Ram CCLB 3500 dually 6.7 turbo
    '96 Chevy C3500 CC dually 454
    2019 Honda Talon X
    1985 Honda FL350R Odyssey

  6. #6
    Long Hauler
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    This is just me, but I wouldn't use a manual shut off for the overflow. I know if I have the chance of a mistake, I'll make it at some point.
    Howard and Peggy
    2019 Momentum 351M, and 2018 RAM Cummins dually 6-speed.
    His: 1999 Honda Interceptor
    Hers: 2013 Spyder ST-S

  7. #7
    Site Team Ynot4me2's Avatar
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    FWIW the 6 gallon that goes in the w/h counts in GD total fw calculation.

    Sent from my SM-P610 using Tapatalk
    Steph & Lise
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  8. #8
    Fireside Member KDutchover's Avatar
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    Quote Originally Posted by TerryH View Post
    Unfortunately it's a very common problem. The vent/overflow lines are notorious for siphoning water out of the tanks. A few different ways to deal with it. Grand Design has changed the way they vent now but it would involve dropping the chloroplast and replumbing the vent lines. I opted for adding 3/4" sharkbite ball valves to the overflows on our Momentum 31G. Readily available at lowers etc... There are 2 tanks. When filling once it starts siphoning from the front tank you can close that vent and that will allow the rear tank to fill. Once it starts coming out the rear overflow you can stop filling and close that valve. I can get over 90 gallons in mine using this method. It is imperative that the valves be opened once you are at your destination to allow the tanks to vent before using water or risk damaging the tanks. We're able go retain 100% of our water using this method.

    We too installed the Sharkbite ball valves on our trailer. We were tired of losing water while traveling to a dry camp destination.
    Dutch & Marlene
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  9. #9
    Long Hauler huntindog's Avatar
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    Quote Originally Posted by Hoopy Frood View Post
    This is just me, but I wouldn't use a manual shut off for the overflow. I know if I have the chance of a mistake, I'll make it at some point.
    There is another way. I and some others have installed shutoffs AND some more PEX using a "T" to make the vent higher. This is idiot proof. Could probably skip the shutoffs, but this way it can fill in the stock configuration by opening the valves.
    2021 398M Full Body Paint 8k axles. LRH tires. Disc brakes.
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  10. #10
    Setting Up Camp
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    Thank you again Terry H! I was hesitant on installing valves wondering if there might be a problem with the connection between both tanks. I asked my mobile RV tech to come over as he stops by my storage facility often. I purchased the shark bite valves like you mentioned yesterday. He met me at my storage facility this morning and we proceeded to try your suggestion. As I was pumping water in with my truck mounted tank he held his finger over the vent on the front tank and felt air coming out. This was after install of valves and we had already closed the valve on the rear tank when water started coming out of the vent. Sure enough as the front tank filled water started coming out of the vent which had never happened before. So obviously the rear tank was full and with this method it forced water into the front tank and filled it. We then closed the valve on the front tank. Then checked the water gauge and it showed full. It only showed half full before filling the front tank. Anyway long story short your method worked and I am very thankful for your suggestion! I will remember to open both valves when we reach our camping destination to allow for venting. Steve

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