Fresh Water Tank Fill Issues - Momentum 397TH

[TedS]

There would not be much pressure difference between two tanks at different elevations. 2.3 feet elevation equates to 1 psi.

[mhood]

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.

I appreciate the comment. I have slowed the rate down to as low as 1-2 gallons per minute, in an attempt to figure out the correct way to fill tanks to capacity given the design of the system, without success (it did take a long, long time to fill). The auxiliary tank always fills first and overflows. If I continue to let water flow through the inlet, the auxiliary tank water continues to dump out the overflow and eventually the main tank will fill and also overflow. My results were the same whether I used the main fill inlet, or the auxiliary fill inlet, located in the Nautilus bay. I cannot readily locate the comment I found out on the internet where someone had the same thoughts as you wherein they suggested a different fitting on the main tank to allow it to fill first. Howard's comments earlier suggested it might be better to simply have separate fills for both tanks with a separate draw line to the pump.

[TedS]

If one tank is mounted higher than the other, the high tank will drain into the lower tank until the water levels in both tanks are the same. That final level will be that of the vent/overflow port on the lower tank. Water seeks its own level.

[howson]

If one tank is mounted higher than the other, the high tank will drain into the lower tank until the water levels in both tanks are the same. That final level will be that of the vent/overflow port on the lower tank. Water seeks its own level.

Hmmm...could it be as easy as installing a shutoff valve on the Aux's overflow line and closing it once water starts to come out? This would force water into the main tank. The main's overflow stays open, so there's always a path for air to exit and enter the system. ( mhood -- I would NOT try this without physically watching the Aux tank to ensure it doesn't expand or contract leading to a failure with the overflow line closed. TedS ' inputs are always insightful and make me think! )

[TedS]

With the tanks interconnected, only one , the higher one, would need a vent/overflow. The aux vent could be valved closed or connected to the main tank vent.

On edit: the aux tank needs to vent, otherwise a bubble gets trapped and limits the fill volume.

[Jlawles2]

Why not vent the aux tank into the main tank? Any overflow of the aux tank then proceeds to fill the main tank. Just a thought. As mentioned one would need to watch the aux tank to ensure it is not getting too much pressure.

I wonder if the Tee is acting like a venturi and sucking water out of the main tank. Odd thought, but it did occur to me. If you have provisions to alter the plumbing, you may try changing the stem of the tee to the inlet and the flanges (straight through) to the branches feeding the tanks. That or a Wye. Don't see a good example but found this https://www.freshwatersystems.com/pr...UaAme_EALw_wcB

[TedS]

You could plumb the aux vent to the main vent.
There is not enough water velocity in the T to create a venturi.

[Cate&Rob]

Has anybody figured out where the large white hose goes? (Shown in the post 15 pictures) If this is a capped gravity fill, maybe the cap just needs to be removed to vent the tank while filling using either the main panel or aux fill sources?

Rob

[howson]

Cate&Rob and mhood -- confirmed the other end of the white hose is attached to the Main Tank as shown below.



Due to the size of that vent hose (and you can see there's a slight droop even in this example!) as I previously speculated (and now I am even more convinced is happening) is it's easier for the water to escape via the overflow than for water to move between tanks via the vent hose.

As I wrote in post #34 in response to TedS ' post, closing the Aux tank's overflow tube (install a pex cap or plug) should force all air and water from the Aux tank into the Main tank. The overflow in the Main should take care of letting air in (and out) of the tanks. But I'm not a plumber--remember I'm just an owner like you--so if you attempt this method and something goes sideways don't blame me! (But it's what I'd try if it was my Momentum, though I'd drop the coroplast and watch what happens with the Aux overflow plugged.) Note also that even though the path bypasses the filter I'd use the Aux Fill line and not the Main/Feed line to fill the tanks.

Please--any other forum participants--if you have an alternate viewpoint sound off!

Howard

[TedS]

howson I agree. That larger hose should be enough to connect the tops of both tanks and only the main needs the vent/overflow.

[howson]

mhood -- one more item to discuss. In the original post it states "Our Momentum has a front 98 gallon main tank with a rear 42 gallon auxiliary tank (according to GD parts listing)...".

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UPDATE: I entered erroneous data in the original post. It has since been updated and highly edited to reflect correct information. -Howard

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What I found on the parts lookup website (2022 model year):
98 gallon main: https://www.granddesignrv.com/owner-...s/detail/63182
48 gallon aux: https://www.granddesignrv.com/owner-...s/detail/63181

The reason I verified these specs is I wanted to check my assumption that the Aux was smaller than the main, thus even if they were fed simultaneously the Aux should max out first. Again, going back to the original post it states the trailer takes "around 80 gallons before it starts to overflow". That also makes sense because at that point the Aux is full and water is escaping out the overflow instead of "pushing" into the Main. In other words the Main and Aux are both holding the same amount of water (40 gallons each) and any additional water is just pushed out the Aux's overflow.

Bottom line: the most you'll get in the tanks, even if it's working correctly, is less than 140 gallons. The Main won't hold the entire 98 gallons due to it's overflow line. (I'd expect about 94 before it started to overflow if the Aux didn't exist.)

Howard

[mhood]

Note: The quote below was the original text that I've subsequently updated in the original post. I've left the quote below as originally entered so Mike's response makes sense. -Howard

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mhood -- one more item to discuss. In the original post it states "Our Momentum has a front 98 gallon main tank with a rear 42 gallon auxiliary tank (according to GD parts listing)...".

What I found on the parts lookup website (2022 model year):
56 gallon main: https://www.granddesignrv.com/owner-...s/detail/63182
48 gallon aux: https://www.granddesignrv.com/owner-...s/detail/63181

That's 114 gallons. Add in the 12 gallons for the hot water heater (per GD's spec sheet at https://www.granddesignrv.com/showro...s/397ths#specs ) and that's a total of 126 gallons of fresh water. (Don't ask me why the GD spec sheet shows 140 gallons...I don't know but something doesn't add up.)

The reason I verified these specs is I wanted to check my assumption that the Aux was smaller than the main, thus even if they were fed simultaneously the Aux should max out first. Again, going back to the original post it states the trailer takes "around 80 gallons before it starts to overflow". That also makes sense because at that point the Aux is full and water is escaping out the overflow instead of "pushing" into the Main. In other words the Main and Aux are both holding the same amount of water (40 gallons each) and any additional water is just pushed out the Aux's overflow.

Bottom line: the most you'll get in the tanks is slightly less than 114 gallons, even with the Aux overflow capped. The Main won't hold the entire 56 gallons due to it's overflow line. (I'd expect about 52 before it started to overflow.)

Howard

Howard - I appreciate you for all the followup on this; I also appreciate everyone's input and comments. The additional photograph was great; thank you again. Regarding the tank size, you have the correct links for the parts however the number you used is part of the tanks physical dimensions, not the gallon capacity. If you click on the links you provided, then look at the diagram underneath "ALPHA and Part Number", the gallon capacity of each tank is specified as 98 and 42 gallons; 140 gallons. I do have a warranty authorized appointment at a local independent shop however its not until late August; the information from the work you and others did in these threads should be helpful to them.

[Yoda]

howson I agree. That larger hose should be enough to connect the tops of both tanks and only the main needs the vent/overflow.

howson I also agree that a valve on the aux tank overflow would solve the problem. It would also help is the big hose could be tied up so there is not a low point in it to collect water. If water does collect in it the least path of resistance would be out the overflow too.

[howson]

Howard - I appreciate you for all the followup on this; I also appreciate everyone's input and comments. The additional photograph was great; thank you again. Regarding the tank size, you have the correct links for the parts however the number you used is part of the tanks physical dimensions, not the gallon capacity. If you click on the links you provided, then look at the diagram underneath "ALPHA and Part Number", the gallon capacity of each tank is specified as 98 and 42 gallons; 140 gallons. I do have a warranty authorized appointment at a local independent shop however its not until late August; the information from the work you and others did in these threads should be helpful to them.

Oops--I stand corrected. I'll fix the previous post. It does, however, exacerbate the issue with a 98 gallon main tank. When the Aux fills to ~40 gallons, the easiest path is then out the Aux tank's overflow. IMHO it's a design issue. Even with a "droop" (as previously speculated) if the Aux tank's overflow is open the Main will not fill over about 40 gallons.

Look forward to the follow-up.

Howard

[mhood]

Oops--I stand corrected. I'll fix the previous post. It does, however, exacerbate the issue with a 98 gallon main tank. When the Aux fills to ~40 gallons, the easiest path is then out the Aux tank's overflow. IMHO it's a design issue. Even with a "droop" (as previously speculated) if the Aux tank's overflow is open the Main will not fill over about 40 gallons.

Look forward to the follow-up.

Howard

Howard - Thank you again sir.