Auxiliary water tank question

[klemdotter]

Hello forum members,

I just purchased a 2016 Momentum 397th. It has a 117 gallon main fresh water tank with a 40 gallon auxiliary fresh water tank. My question is: when you fill the tank does it fill the main tank first then the auxiliary tank or fill them both at same time. Also when the fresh water gauge says for example 1/3, is it 1/3 of 117 gallons or 1/3 of 157 gallons? No info about the extra tank in owners manual and dealers have no idea. BTW we love the trailer. The quality and fit & finish is excellent. Also, does anyone have a portable reverse osmosis system that they use. If so where do you put the 1/4 inch fill line from the RO system?
Thanks,
Scott








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[duc809]

I had the same question on how the auxiliary water tank really works. We are looking pretty hard at Momentums.

[Bob & Joni]

I have the same question we have a 350m with the same tank set up. When filling the indicator gets to about 2/3's and then at some point the water starts coming out the rear tank over flow if I let it set for 15 or 20 minuets I can add more water doing this 3 or 4 times I can get the indicator to read full.
Then when driving to or camping spot in the North Cascades I notice water coming out the overflow and when we finally get up the mountain we are back to 2/3 on the indicator. So far hasn't been an issue the two trips we have made we dump water after dry camping for 5 to 6 days.

[b727capt]

I have a 398M with the aux tank. As far as I can tell they are all connected. The standard 117 gallons is actually 2 tanks. They all seem to equalize to the same level.

[jenamiles]

We just had a tech out this week to address our fresh water tank never showing a 1/3 even though water was flowing out the overflow. We thought that maybe the sensors weren't working. The tanks are not plumbed how our PDI tech told us and, at least on ours, the way they are plumbed, the first tank would never get above 40-45%. I will say that it was a "design flaw." I spent roughly 6 hours in our underbelly and now know our plumbing and wiring very well. These frames are well built. After having to deal with faulty tanks in several rigs, I can tell you that this underbelly is the best designed one I've seen in 5 years.

First, how ours was plumbed. For simplicity sake, I'm going to say there is a 117 gallon (tank 1 over axle 1) and a 40 gallon (tank 2 over 3rd axle).
There is one long pipe that comes from the Nautilus system down to tank 1 (connects in line with the 1st fresh tank drain and about 6" closer to the front axle). There is a three way adapter installed there. The right angle goes in to tank 1. The straight thru goes straight (past the rear black tank which sits over second axle) and then right angles in to tank 2 (which sits over the third axle more or less). The same pipe used to fill the tanks is also the same pipe used to pump water back out so there were some limitations in coming up with a fix that would work with the nautilus system.

Here is what we found once we exposed the underbelly and could troubleshoot.
1) One, the water would run down the pipes, split and fill tank 2 at roughly 2x the rate of tank 1. So when tank 2 was overflowing, tank one had maybe 30-40 gallons of water in it. (Thus never reading above a third.) This meant that my 117 plus 40 gallon topped out at roughly 70-75 gallons before overflowing.
2) The top of the tank on tank one sits 12" above the frame and the top of the second tank sits 11" above the frame. This means that no matter what you do, unless you isolate each tank, you will never fill tank one to capacity. My quick math was somewhere between 9.5 and 12 gallons of space unavailable just based on physics. Now, I'm not so worried about losing @10-12 gallons if I'm gaining 40 and given the limitations to fix it, I'm going to just live with the loss. (Fixing it results in a 30 gallon gain over just tank one.)

Fix that Grand Design recommended:
1) After talking to Ray, design came up with the following fix. Create a second right angle so that the main line that runs from the nautilus right angles, then goes straight into tank 1 and right angles off of that line in to tank 2. (Thus slowing down the flow in to tank 2.)
**I've implemented this fix but am not at a place where I can fill tanks for less than $20 and since I paid out of pocket for all of the troubleshooting with a mobile tech, I wasn't going to make him sit for another hour and spend more on water to try it. I'll test it next week. (And yes, I'm hopefully I'll be reimbursed the time under warranty but doubtful.)
**If you have a tech come down to do this, we dropped the underbelly because we thought it was a bigger issue - sensors/tank two. We needed to access all of it. In reality, this fix requires a small, 6x6 or so, access panel directly parallel with the tank 1 drain about 6" closer to the tires.

If you have this issue, please contact service and let them know. According to them, I am the only person who has ever reported this issue with an auxiliary tank. They don't know things are an issue unless you report them. I had to "prove" based on engineering and science that my issue was a real issue and not just a blocked pipe. If something doesn't seem to be working right, ask. The more people who provide feedback, the better Grand Design can meet our needs and provide a better quality product.

If this doesn't work, we have two other ideas that we'll talk to Ray about and see which one (or another from design) would be allowed under the warranty and not void it. We have gone thru every design option we could possible think of and given tank heights and plumbing limitations, we could only come up with two other feasible options without more significant rework. One, we could isolate both lines at the right angle with shut off valves and just fill and use one tank at a time. (Not great given that the line sits just behind the first axle so you'd be climbing under a slide and reaching around the tire. Two, adding in an equalizer pipe that screws in where the drain valves are now that sits just below the underbelly (would need to be wrapped in foam to weatherize it) and then add the drains back either just under the connections or at the back where it could angle down slightly and be easier to reach by the black 2 pull. Fix one could be applied thru the same panel cut to change the right angles. Fix two nothing needs to be taken down.

[Luv2Ski]

jenamiles, thanks for the research and information. We have a 2017 328M with the exact same problem. I wanted to get a water flow sensor to measure the actual capacity of the tanks next time we hit the road. Then report the problem to GD. Like you, our aft FW tank overflows first. I have taken to plugging the drain with a wine bottle cork and waiting for the other tank to fill. Based on your description above, that's not so smart. Here is how I would have designed this system but I'm a software engineer so what do I know?



Maybe the coupling pipe on the bottom could be shorter due to space restraints. I moved the overflow up into the vents on draft 2 to allow for a steeper assent and descent angle on the road without spilling water. I suppose you could move the Nautilus fill down to the "water pump tap" since it's used in both directions.

[jenamiles]

Your drawing would absolutely work in an ideal world; however, you'll some space limitations in the underbelly limitations. The top of tank one sits at the base of the frame with maybe an inch or two to spare on top and the bottom of the tank is flush with the underbelly plastic. Essentially, you really have no workable space above/below the tank itself. All plumbing in and out are on the sides of the tank except the gate drain valves. I have pics on my phone and can post images from there; however, the fill/pump out hose is roughly 1/2"-1" from the bottom of the tank on the side of both tanks. Then the overflow is roughly 1/2" - 1" from the top of the tank on the side, the plex extends up about 2 inches, right angles about an inch, right angles down to then come out of the underbelly.

[Luv2Ski]

Thanks. Would love to see the pictures.

The way they have it plumbed, they're using the overflows to double as air vents. The only cross tie (tank to tank equalization) is at the Nautilus feed/supply line. This works just fine if the tanks fill at the same vertical rate. This of course isn't happening. It sounds like the tank bottoms are at the same height (when level) and the tops are at different heights (meaning the overflow taps are also at different heights). This would explain why I get water out the overflow at the rear 40 gal tank long before I get anything out the main 117 gal tank. They probably only have level sensors on the main tank because they erroneously assume the water in the two tanks will find equilibrium during the pressure fill. The 3-way adapter you refer to, is it a tee or a wye? If a tee, it's always easier for water under pressure to go straight to fill the smaller rear tank than turn 90 degrees to fill the larger tank.

When I corked the small tank overflow once it was full, I was effectively pressurizing the small tank and forcing all the water flow into the main tank. Once it reached overflow level, I removed the water pressure and switched the Nautilus over to dry camping mode. The feed/draw line ties the two tanks together so they try to find equilibrium but the small tank overflow is lower than that of the larger tank. When I pull the cork, the small tank overflow starts spewing water because it's indirectly draining the large tank through the tie line and thereby wasting some of the large tank's capacity. Not very well thought out if my guesses are correct.

The really crappy thing about this situation is that water can easily overflow when driving on steep grades like those here in the Rockies. 6% or 7% grades on the interstates are common (Floyd Hill, Eisenhower/Johnson approaches, Vail Pass, etc.). 10% grades are found on some state highways. I know of a couple of campsites that require navigating 13%+ grades on county roads. I guess we're supposed to fill up the tanks and cork the overflows while driving the last bit to the campsite, eh? Do you know if the overflows are tapped in at each tank's fore to aft centerline as would be viewed from the side? If not, that would make the situation worse on steep grades.

[jenamiles]

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.

[klemdotter]

I spent some time on the water tank issue today. I first filled the tank from home outside faucet that runs free at 5 gallons/min. Filled the tanks through the nautilus system for 9 minuets. I then drained each tank into 5 gallon bucket. Got 14 gallons out of auxiliary tank and 17 gallons out of main tank. There is obviously some restriction due to plumbing on the trailer. I then filled the tanks until the over flow started on the auxiliary tank and shut it off. This took 22 minuets. The fresh water gauge was at 1/2 tank. So I figure that in that time I flowed 40 gallons into auxiliary tank and about 55 gallons into main tank. My theory now is if you want to fill the main tank you need to drain the auxiliary tank then start filling. When the over flow starts out of the auxiliary tank you will have filled both tanks full. Now if you want to just have some water in main tank for travel. Then on the first fill you could fill until the water starts overflowing from the auxiliary tank, then drain auxiliary tank and maybe a little out of main tank so you have 30 to 35 gallons only in main tank. That way you are only hauling 300 lbs or less of water. I am assuming that almost everyone will not be into re engineering and spending a large amount of money to solve the tank fill problem. I know that this seems to be a simplistic solution and does waste some water, but it does work. You could also just fill for about 12 to 15 minuets and call it good with water in both tanks totaling about 30 to 35 gallons for travel. Most regular exterior faucets will only fill 5 to 7 GPM max. With the friction loss in plumbing your probably getting 3 to 4 gallons per min.