Fresh water tank

Mmnsc · 05-16-2014, 10:18 PM

When filling my fresh water tank I noticed as it got to full the top of the tank started to "dome up". I got to thinking and wondering is the some kind of vent to allow the air out. I am not sure my tank has one as this is the first time I filled it all the way up. Also is there some kind of whistle to let you know when you are approaching full. Had I not had the door open where the water pump is I would not have known when the tank was approaching full. If there is a vent tube where does it exit? Appreciate any help. Thanks

Superchief · 05-16-2014, 10:32 PM

Marty,

I have thought the same thing when filling the tank. I usually fill it within a 1/2 inch of the top, just past the top of the last sensor on the tank. I believe that they put the sensors so that you will only fill it to that level, and not to the very top of the tank. Then sensor light on the panel in the coach on mine indicates when it is full. I have not seen or heard an air relief valve go off. I don't believe there is one. Also, if you did fill it to the top, I would think the back pressure of the compressed air in the tank would be greater than the pressure of the incoming water, and the flow of the water would stop. Kind of like kinking your hose, the water stops flowing, but doesn't explode the hose. That's my theory, and I'm sticking to it.

Craig

NO3putt · 05-16-2014, 10:34 PM

Ours did the same thing, haven't looked into why yet, but think there is a vent tube on the passenger side forward of the water pump. My thinking that the vent tube is not working as designed. Would like other to chime in.

beipers · 05-16-2014, 10:35 PM

On mine, looking in the pump door, it is plumbed in the top right corner of the tank. The Pex tube goes up above the grey tank then back down and exits the cabinet at the front right corner. My FW tank does the same thing and you need to be careful because it will pull the corners off the bottom also. 2 weeks ago i forgot i had the tank filling and i was finally able to get water to push out the vent. By then the tank was more bulged than I thought it could stand.

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NO3putt · 05-16-2014, 10:39 PM

Would think the vent tube would allow air to exit the tank when filling?

Mmnsc · 05-16-2014, 10:44 PM

I still had about 3/4 inch of dead space to the top of the tank but the bulging had me concerned. I was surprised that the top of the tank would bulge from the water pressure. Seems to me there should be some form of overflow I will try to investigate further as there are several tubing lines in the passenger side of the tank that I have no idea of their purpose.

beipers · 05-16-2014, 10:57 PM

It bulges when the water gets to the top of the tank and covers the vent port. It takes more pressure to push the water through the vent tube than it takes to deform the tank. Until the pressure equalizes the tank will bulge. Because the vent is not in the center of the tank, air is trapped and will become compressed. If you watch as it fills, you'll see that some air will bubble across the tank and out the vent for some time as the tank grows. It will look like you still have a gap at the top but it is a 1/2" vent fitting and the water actually climbs the side of the tank due to water tension and the port is cut off before the water gets to the top.

My tank only has 1 vent. There are 2 other tubes going into the top of the tank. 1 is the fill tube and the other is the return from the water heater bypass for winter operation.

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Superchief · 05-16-2014, 11:15 PM

Marty, I am changing my theory! As beipers has noted there is a vent tube near the. Top of the tank. Not sure where it is, but I "You Tubed" water tanks on Motorhomes and they show a vent tube on the tanks. Craig

beipers · 05-17-2014, 08:40 AM

I should clarify. The vent port is actually on the sidewall of the tank, but near the top. It is not actually on the top side. That is why it gets covered before the water fills to the top corner.

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Noneyerbusiness · 05-17-2014, 08:49 AM

I can't even see my tank .... but it has a vent tube coming right back up the fill port. I "hear" it filling and the water will start overflowing. If I allow it to settle, I can put gallons more water in there ....

Mmnsc · 05-17-2014, 11:18 AM

Emptied the tank and the bulge disappeared. In looking at how the tank is made the top has deep ridges that allow for the expansion. On refilling the tank the vent tube on the upper left side of the tank allows for some of the air to escape but the force of the incoming water over rides the vent causing the tank to expand. I learned a lesson on RV water tanks. Thanks for all the replies.

Campin Cajun · 05-17-2014, 02:20 PM

Mine does the same thing. I was thinking about shortening the vent line but then the tank wouldn't hold as much water. I may shorten it in stages, a little at a time, until I get my desired outcome.

Campin Cajun · 05-17-2014, 02:22 PM

I would also like to take the suction line out the tank and put an elbow (90) on the inside the tank with the opening facing down toward the bottom of the tank. This would enable me to use more water out of the tank instead of the water pump catching air at 1/3 level.

dan-nickie · 05-17-2014, 07:15 PM

Quote:

Originally Posted by Campin Cajun

I would also like to take the suction line out the tank and put an elbow (90) on the inside the tank with the opening facing down toward the bottom of the tank. This would enable me to use more water out of the tank instead of the water pump catching air at 1/3 level.

So, mine's not the only one that does that.
Guess I need to get under there and see where those lines are.

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