Fresh Water Tank Fill Issues - Momentum 397TH

[OffToHavasu]

The advantage to the fresfhill vs auxiliary (you got a typo in there) fill is that the freshfill will go through the filter.

Thanks for the photos and diagram!

It's also interesting that they did the new water break setup on the main tank, but not the aux tank.

[mhood]

Howard - I appreciate you for the diligent followup with Grand Design re this issue; thank you sir. Followup questions if you have a minute: 1) The white vent tube you refer to, visible connecting to the auxiliary tank and suspected of perhaps a droop, do you know or could you find out where the other end terminates? 2) Could your source at Grand Design provide you with the proper methodology they see as proper to fill both tanks to near capacities given the design illustrated (the answer to this question perhaps would also provide an explanation as to the reason there are two fill inlets in the nautilus, one main and one auxiliary) without excessive filling of the auxiliary tank resulting in excessive spillage?

[howson]

Howard - I appreciate you for the diligent followup with Grand Design re this issue; thank you sir. Followup questions if you have a minute: 1) The white vent tube you refer to, visible connecting to the auxiliary tank and suspected of perhaps a droop, do you know or could you find out where the other end terminates? 2) Could your source at Grand Design provide you with the proper methodology they see as proper to fill both tanks to near capacities given the design illustrated (the answer to this question perhaps would also provide an explanation as to the reason there are two fill inlets in the nautilus, one main and one auxiliary) without excessive filling of the auxiliary tank resulting in excessive spillage?

The other end of the vent must be connected to the main tank. The purpose of that line is to allow the tanks to equalize. I'd expect you'll find it on the right side of the tank (not shown in the pic of the main tank above). If that vent is working correctly, it shouldn't matter which method is used to fill the tank--they should equalize before water comes out of either overflow.

IMHO at this point with the information at hand what's left is to pull down the coroplast and do a physical inspection. Hopefully with what is now known finding the culpritshouldn't be difficult.

Cate&Rob or any other forum member--anything I missed?

Howard

[Country Campers]

I do not see the other end of the larger "vent" attached to the main tank.
is there an external fill port for the auxiliary tank on the outside wall near that tank?
This "vent" line looks more like a external fill line to me.

Brian

[Cate&Rob]

It's also interesting that they did the new water break setup on the main tank, but not the aux tank.

Hi Curtis,

I noticed this as well and mentioned it to Grand Design. The aux tank would be just as vulnerable to slosh pumping starting a siphon action through a single overflow line. They are looking into why this tank was missed in addition of the second overflow to all water tanks.

Rob

[Cate&Rob]

I do not see the other end of the larger "vent" attached to the main tank.
is there an external fill port for the auxiliary tank on the outside wall near that tank?
This "vent" line looks more like a external fill line to me.

Brian

I would agree with Brian on this . . . the picture of the main tank does not show attachment of this larger hose. It looks like a “gravity fill” type of thing.

Rob

[Yoda]

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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

Howard.
Just to point out - the fill line to the main tank also feeds the aux tank at the same time from the bottom . I.E. both thanks will fill when the main tank is filling from the bottom connections. If the AUX is smaller and the fill rate being close to the same (rear tank will be a bit slower) the Aux will always overflow first. Also depending on if the trailer is not level or if the rear AUUX tank is set lower that could be a contributing cause. Also as the AUX fill line is connected to both tanks too the same occurs when using that fill port. GD is spending a small fortune on Pex lines they don't need and I agree the vent on the Aux tank needs to be corrected to the anti siphon design.

If I am not seeing what I think I see in the pictures, please let me know.

[howson]

@yoda--valid observations IMO.

[OffToHavasu]

I like the idea of using the main water connection for filling the tanks because, as mentioned, it utilizes the filter. I'm not great at hydrodynamics, but it would seem a larger than 1/2" pex is needed at the bottom of the tanks, connecting the two, to allow a quicker balance as the tanks fill.

Also it appears, for example, if the rear tank is elevation higher than the front, and if the rear tank empties, it will allow air to be drawn in to the system.

[mhood]

I appreciate everyone's input; participants here have provided more insight than I have been able to acquire elsewhere. I thought my question was simple however after asking several folks at Grand Design, if they can either get me an explanation on how this water fill design is supposed to work, or put me in touch with someone who knows, I have not yet found someone there to explain this. I have only heard speculations on how it is supposed to work. My cursory search on the internet has located numerous postings where others have experienced the same issues filling these two tanks, as far back as 2016.

[howson]

mhood -- the more this is discussed the less I comprehend, too. I'm the last to proclaim any plumbing expertise, so the doubts of my fellow moderators on how this system functions gives me pause.

Given my simplistic view and knowledge of plumbing systems, it sure seems like each tank should have it's own discrete fill line while sharing the main water supply line (that goes to the pump). This would allow individual loading of the tanks while allowing water to be drawn from both tanks as needed by the main system. A check valve in the feed line between the main and aux would keep water from backfilling from main to aux but allow the aux tank to supply water as needed. In this arrangement, no "vent" line would be needed between the two tanks (which seems redundant anyway, given air can enter through the overflow line). Again, no expert so there's something obvious I may be missing...but the current arrangement sure has me scratching my head.

Howard

[mhood]

mhood -- the more this is discussed the less I comprehend, too. I'm the last to proclaim any plumbing expertise, so the doubts of my fellow moderators on how this system functions gives me pause.

Given my simplistic view and knowledge of plumbing systems, it sure seems like each tank should have it's own discrete fill line while sharing the main water supply line (that goes to the pump). This would allow individual loading of the tanks while allowing water to be drawn from both tanks as needed by the main system. A check valve in the feed line between the main and aux would keep water from backfilling from main to aux but allow the aux tank to supply water as needed. In this arrangement, no "vent" line would be needed between the two tanks (which seems redundant anyway, given air can enter through the overflow line). Again, no expert so there's something obvious I may be missing...but the current arrangement sure has me scratching my head.

Howard

Howard - I am certainly no expert in plumbing but what you described is what I was also thinking. I do not know why they would originally attempt to fill the two tanks, through the main tank fill inlet, by running this line to the bottom of each tank where the incoming water would push against the fluid pressure already in the tank. I realize in your previous explanation this same line to each tank is also used to suction water from the tanks via the pump; I suppose maybe it was easier to run one line with a multiple purpose? The auxiliary tank fill inlet was apparently added at some later point, perhaps to address this, and allow filling of both tanks through a point near the top of the tank. Separating the fill lines and leaving the suction line might have been a better solution as you explained. It would be nice if someone at Grand Design would simply explain their design intentions.

[Cate&Rob]

It is "RV normal" to fill water tanks from the bottom, because this same line is then used to draw water out of the tank. The "auxiliary" fill system seems redundant and looks to have been added when the second water tank was added. Is there also a "gravity fill" opening on the side of the RV (not at the water panel) that the larger white hose might go to? This looks to be a 3rd way to fill this tank. I agree that it would be ideal if you could get through to the designer who created this system . . . keep pushing Grand Design on this.

Rob

[mhood]

It is "RV normal" to fill water tanks from the bottom, because this same line is then used to draw water out of the tank. The "auxiliary" fill system seems redundant and looks to have been added when the second water tank was added. Is there also a "gravity fill" opening on the side of the RV (not at the water panel) that the larger white hose might go to? This looks to be a 3rd way to fill this tank. I agree that it would be ideal if you could get through to the designer who created this system . . . keep pushing Grand Design on this.

Rob

The two water fills on the 397th are both in the Nautilus wet bay; one marked main and one marked auxiliary; no side fill. Howard replied earlier that he thought the white larger hose you refer to is simply connected between the two tanks and used to 'equalize' them however this does not show in the provided photographs he was able to acquire.

[Jlawles2]

Reviewing the pictures posted by Howard, the Aux tank will probably fill first. The Tee fitting in the fill line while intended to equalize the 2 tanks will most likely let the aux tank receive more pressure / flow as it's ont straight through connector. Even though based on hydrostatics the tank with the highest water level should have more pressure on the fill line forcing water into the lower level tank. HOWEVER, with a high flow situation, the straight through connector will have a higher pressure resulting in that tank filling first due to the low overall height of the tanks thus resulting in a very low pressure rise between the 2 tanks.

To the OP. Next time you fill the tanks, slow down the fill rate. Yes this will take longer, but try say 1/4 turn open vs full open on a standard hose bib valve. Hopefully the lower flow rate will allow the tanks to equalize properly.

I am not a plumber and this is just my observation based on several years of strange things happening with fill lines.