Fresh Water Tank Fill Issues - Momentum 397TH

[mhood]

Our Momentum has a front 98 gallon main tank with a rear 42 gallon auxiliary tank (according to GD parts listing); there are two fill inlets on the Nautilus system, one marked for each of these two tanks. When attempting to fill the two tanks to advertised capacity of about 140 gallons (not including hot water tank), I am able to fill about 80 gallons before the rear auxiliary tank begins to overflow (this is the same whether I use the main tank fill or the auxiliary tank fill inlet). If I keep filling, eventually the front main tank will also overflow; the rear auxiliary tank does not stop overfilling while the main tank continues to capacity. I understand this is not good for the auxiliary tank as it may cause it to expand too much; it is also time consuming and wastes water. I have tried, on many occasions, to pause filing while the tanks 'equalize'; this does not help. I have contacted GD customer service without them having a solution other than taking it into the shop; they could not explain the questions posed stating engineering does not always keep them informed about these things. As far as I know, this is not a siphoning issue; just a trying to fill the tanks issue.

If anyone can be of help:

Why are there two inlets, one marked Main, the other marked Auxiliary?

What is the solution to fill both tanks close to their advertised capacity without all of this overflowing of water?

Thank you.

[howson]

mhood
First -- welcome to the forum. For your first post you've done an excellent job describing the problem and the steps you've taken to resolve it.

From the limited information made available to the moderators on the 397TH there is not enough detail showcasing how the two tanks are interconnected. Unfortunately the AUX tank is not even shown on the water plan drawing. (!)

From the tank layout what I could see is the black tank is between the main tank and the aux tank. This leads me to suspect, based on the information in the post, that there's a soft water line that interconnects the two tanks. This line could be "drooping", thus creating an inaccurate balance point between the two tanks (I hope that makes sense).

Is this something you want to try and dig in to fix yourself or are you just looking for info? If you're a DIY'er (like many of us here) what I'd do is drop enough of the underbelly to see how the tanks are interconnected. (The aux tank is aft of the bathroom and the main tank is forward of the bathroom -- the black tank is likely right under the toilet if it's configured like most GD trailers.) Once you can peek underneath try to find the interconnect between the two tanks. If I'm right you'll find a soft hose that can either be rerouted or replaced with pex to alleviate the symptoms.

Another option -- maybe -- would be to put a shutoff valve between the two tanks to isolate them so they can be filled independently. If the tanks have separate overflows and fill ports, isolating the two from each other shouldn't cause any harm. With the valve in place and closed, once the main's water has been used to some extent open the valve to let the AUX balance the two tanks. You'll only know if that's a feasible plan by looking at how the two tanks are interconnected (and with what).

Keep in mind the above are educated guesses based on what I've seen in other units and my own 315RLTS, but I'd have to actually look at the 397TH to be more specific. It's going to take some investigation to actually determine how the tanks are interconnected. Unfortunately at this point if DIY is not feasible then having your dealer look at it may be the best option.

Please report back when / if you discover the issue! And if you do drop the underbelly to look take copious pictures and post them. An owner-to-owner forum's power is from sharing information.

Howard

[Cate&Rob]

OffToHavasu
Hi Curtis,

Does your Momentum have an auxiliary water tank? Your thoughts on this discussion?

Rob

[OffToHavasu]

I do have a second tank. I have NOT monitored how many gallons I've put in, so I can only advise on what I know.

The two tanks are connected by a small hose at the bottom of the tanks. The front tank is is filled through the Nautilus panel (and filtered) and the rear tank is filled through the "Aux" port (unfiltered).

Obviously due to the connection between the tanks, you can fill both tanks from either port.

In all the times I've filled the tanks, I've always done the main tank first, then the Aux tank, and in those times, I've never had the Aux tank overflow before the front tank. So that's very odd to begin with.

Now, putting some thought in to this, what appears to be happening, and I'm slinging you know what against the wall here to see what sticks, that there is a possibility they used a larger connection between the tanks, allowing water to flow faster to the Aux tank.

So a suggestion to try, once the Aux tank starts to overflow, throttle back your water flow and see if it balances and the overflow stops. Continue at that rate until both start overflowing.

That's all I have.

[mhood]

mhood
First -- welcome to the forum. For your first post you've done an excellent job describing the problem and the steps you've taken to resolve it.

From the limited information made available to the moderators on the 397TH there is not enough detail showcasing how the two tanks are interconnected. Unfortunately the AUX tank is not even shown on the water plan drawing. (!)

From the tank layout what I could see is the black tank is between the main tank and the aux tank. This leads me to suspect, based on the information in the post, that there's a soft water line that interconnects the two tanks. This line could be "drooping", thus creating an inaccurate balance point between the two tanks (I hope that makes sense).

Is this something you want to try and dig in to fix yourself or are you just looking for info? If you're a DIY'er (like many of us here) what I'd do is drop enough of the underbelly to see how the tanks are interconnected. (The aux tank is aft of the bathroom and the main tank is forward of the bathroom -- the black tank is likely right under the toilet if it's configured like most GD trailers.) Once you can peek underneath try to find the interconnect between the two tanks. If I'm right you'll find a soft hose that can either be rerouted or replaced with pex to alleviate the symptoms.

Another option -- maybe -- would be to put a shutoff valve between the two tanks to isolate them so they can be filled independently. If the tanks have separate overflows and fill ports, isolating the two from each other shouldn't cause any harm. With the valve in place and closed, once the main's water has been used to some extent open the valve to let the AUX balance the two tanks. You'll only know if that's a feasible plan by looking at how the two tanks are interconnected (and with what).

Keep in mind the above are educated guesses based on what I've seen in other units and my own 315RLTS, but I'd have to actually look at the 397TH to be more specific. It's going to take some investigation to actually determine how the tanks are interconnected. Unfortunately at this point if DIY is not feasible then having your dealer look at it may be the best option.

Please report back when / if you discover the issue! And if you do drop the underbelly to look take copious pictures and post them. An owner-to-owner forum's power is from sharing information.

Howard

I appreciate the reply!. I have two drawings, which I received from GD, one of which shows the aux tank was added to the Momentum in 2016; they are attached if you find them helpful in the future. I can fix this myself however the trailer is seven months old and still covered under warranty; I have told GD about the issue and the shop I would like to take it to. I think their blessing on the repairs would be prudent should I have further issues, like maybe a tank dropping (I've heard that happens once in a while).

I appreciate the suggestions to fix the issue and will pass them on to the shop. There are two inlets in the Nautilus Bay, one for each tank. I just don't know their purpose or how they are plumbed; no guidance on this in the volume of paperwork received with the trailer or from GD to date. The repair could be as simple as fixing a drooped line like you also suggest may have occurred. I was looking for someone who may have also experienced this issue so I am better informed. Thanks again.

Edit: Drawings removed. Sorry Mike . . . but the agreement we have with Grand Design for this forum is the same as their requirement everywhere else . . . their drawings cannot be posted to any social media or forum site.

Rob (Moderator)

[mhood]

I do have a second tank. I have NOT monitored how many gallons I've put in, so I can only advise on what I know.

The two tanks are connected by a small hose at the bottom of the tanks. The front tank is is filled through the Nautilus panel (and filtered) and the rear tank is filled through the "Aux" port (unfiltered).

Obviously due to the connection between the tanks, you can fill both tanks from either port.

In all the times I've filled the tanks, I've always done the main tank first, then the Aux tank, and in those times, I've never had the Aux tank overflow before the front tank. So that's very odd to begin with.

Now, putting some thought in to this, what appears to be happening, and I'm slinging you know what against the wall here to see what sticks, that there is a possibility they used a larger connection between the tanks, allowing water to flow faster to the Aux tank.

So a suggestion to try, once the Aux tank starts to overflow, throttle back your water flow and see if it balances and the overflow stops. Continue at that rate until both start overflowing.

That's all I have.

I appreciate your reply and thoughts. I have unfortunately tried throttling way back, to as low as one gallon a minute, in an attempt to figure this out. You sharing your main tank always overflows first is helpful; all my experiences and efforts have always resulted in the aux tank overflowing first, and continuing to overflow while the main tank fills. This, I believe, helps substantiate the repair shop taking a look at it. Thanks again.

[Country Campers]

mhood

Please remove those drawings from the above post. They are not to be posted in public, Grand Design requires this.

Brian

[Cate&Rob]

[QUOTE=mhood;n86843]. . . there are two fill inlets on the Nautilus system, one marked for each of these two tanks. /QUOTE]

Hi Mike,

Perhaps if you post some pictures, this would help us understand the tank fill provisions on your Momentum. From the Grand Design Parts Lookup information, the Nautilus panel for your model appears to have only one city water connection. Is the second fill "elsewhere" as described by Curtis in post #4?

Rob

[Cate&Rob]

mhood
Another option -- maybe -- would be to put a shutoff valve between the two tanks to isolate them so they can be filled independently. If the tanks have separate overflows and fill ports, isolating the two from each other shouldn't cause any harm.

Building on howson Howard's suggestion of isolating the two tanks with a valve . . . the connecting water line will be buried within the underbelly, so a manual valve would be difficult to access on a regular basis. A 12V electric solenoid valve would allow you to isolate one tank from the other with the flip of a switch. I use the valve shown in the attached pictures in a different application (to turn off city water) but it would work well for this application. It draws power only when moving the valve and can remain open or closed for extended periods of time.

Rob

[familytruckster4]

I dont have a solution but please allow me to throw out a couple of points to ponder.
I dont believe that every RV is built to the actual engineering plans, My opinion is that often theres some creative license taken during assembly (acknowledging the assemblers are paid by piece work)
I totally understand why you would not want to work on a new trailer.... But, you may want to consider at least looking into the underbelly so that you have a better understanding of the problem and its potential solution. I think that even with a "pre authorized" repair the dealer tech may not really investigate as thoroughly as needed. RV dealers lose money on warranty repairs, the labor times and rates really do not cover the cost of a competent tech. The tech likely will not get paid for the investigation/analysis of whats needed. I can envision it taking an hour or 2 to find the issue and then the tech only getting paid for the 15 minutes for replacing a hose. This may sound cynical but I have seen it first hand. The tech may see this as a rabbit hole and decide they just down want to go down it. In the end no one wants to work for free.
Just my 2 cents worth of free advice, worth exactly what it cost you.

[howson]

familytruckster4 -- you nailed it. I have the privilege of reading emails exchanged between independent RV mechanics and your analysis echoes their sentiments about warranty work almost word-for-word. My first response to an individual that asks me about purchasing an RV is, "Are you willing to work on it yourself?"

[JBill9694]

If I was a betting man, which I'm not, I would bet that the overflow on the auxiliary tank was installed lower than it should be.

Flow is related to pressure, which can be expressed as difference in head or water surface. When filling the main tank there is a larger head difference between this tank and the city water, so a lot of flow. The head difference between the main tank and the auxiliary tank is a lot less so less flow out of the main tank into the auxiliary tang than coming into the main tank. So, if the overflows are set at the same height the main tank will fill first. The second fill port would then be used to top off the auxiliary tank without adding water to the main tank.

[mhood]

[QUOTE=Cate&Rob;n86893]

. . . there are two fill inlets on the Nautilus system, one marked for each of these two tanks. /QUOTE]

Hi Mike,

Perhaps if you post some pictures, this would help us understand the tank fill provisions on your Momentum. From the Grand Design Parts Lookup information, the Nautilus panel for your model appears to have only one city water connection. Is the second fill "elsewhere" as described by Curtis in post #4?

Rob

The attached photo is from the internet as our trailer is not available today. The second white fill, to the right of the water filter canister, is labeled aux fresh water fill. I have not found anyone who can explain how this fill is different from the fill labeled main tank. which is located just below the selection knobs.

[Country Campers]

Mike,

The aux tank full connection will have its own hose that goes to that tank, at least I would hope so.

Brian

[howson]

mhood

---

EDIT: To any forum member that may have a concern about the pictures below--they were provided by and approved to post here by a Grand Design representative. -Howard ( Moderator)

---

Took a bit but there's more information I can share on the tank setup.

First picture, of course, is how the Nautilus panel is configured (for readers following along that won't know) :



The AUXILIARY FRESH WATER TANK FILL connection goes to the top of MAIN and AUX tanks. (Note the following pictures were taken from above prior to the floor being installed). Also take note of that large white vent line in the upper left corner (I'll return to that later in this post). Look closely at the two pictures below and you'll see how the lines interconnect.



The MAIN tank looks similar but note how the fill line is at the bottom of the tank. (That line is not only the fill line it is the draw line.) The conclusion is when using the Fresh Water Fill the source must "push against" the water in the MAIN tank to fill it. (Seems to me only the AUX line should be used?)




Based on what I see in the pictures, my guess is that large white vent line on your Momentum is too long and droops, kind of like a sink's trap. Water makes it's way into that vent line effectively sealing the two tanks from each other preventing equalization. Only way to know for sure is to drop the coroplast and do an inspection.

Hope this helps--please let me know what you find if you do the inspection (or have someone else do it). Pictures, too! (Please!!!)

Howard