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Filling Fresh Water Tank - Overflow Not Working

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#1 Smoker



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Posted 08 September 2012 - 10:33 AM

Help. When filling my fresh water tank it should flow out the overflow vent once filled right? It would always do this. Recently when I filled it now it starts to expand the tank and practically lift it off it’s rest. Water does not flow out vent hose. I tried blowing into the tank thru the vent hose and it feels not plugged. I felt no restrictions. What could be a matter ? As a note my vent hose comes out of tank at the top on the passenger side end and then goes up about 12” around some parts of RV before coming back down thru a hole in floor. This is on a Forest River Berkshire.

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#2 wolfe10


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Posted 08 September 2012 - 11:01 AM


Strange that blowing in the overflow showed it clear/unobstructed, yet it does not function as an overflow. Only explanation I can think of is that you have an anti-siphon valve at the top of the loop that allows you to blow air, and may have a blockage between tank and anti-siphon valve.

And, be VERY careful filling until this is remedied. Hydraulic pressure in a fully full tank can do tremendous damage-- easily lifting the floor of the motorhome.
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Posted 29 September 2012 - 07:09 PM

Just wondering if you have a manual fill for your fresh water. Not the one off your "city water" system. I opend mine and used it as a way to tell if the tank is full.
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#4 Smoker



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Posted 28 June 2013 - 11:19 AM

I'm still having this problem and have not figured it out yet.  ...Ron

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#5 desertdeals690


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Posted 28 June 2013 - 11:49 PM

Take the hose off the tank and try to fill it.  If the tank has a fitting that the hose goes on see if the fitting will unscrew. 

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#6 Briarhopper


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Posted 29 June 2013 - 01:18 AM

You should be able to feel air flow from the vent when you start filling the tank.  If not there may be some sort of one-way blockage.


Our tank has two 1/2" vent lines. There may be a second vent that is plugged and the single remaining vent is not enough to allow a high flow city water fill-up.  


Do you fill with a hose stuck loosely into a port or do you have a valve that allows you to fill the tank while the hose is still connected?


If you fill while connected, is there a separate fill port for filling with a open ended hose? If so, you can open this port for venting.

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#7 KajunTaz



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Posted 16 August 2013 - 11:24 PM

Had the same problem with our 96 Mirada... turned out to be BOTH ventlines clogged with mud from wasps... Took an old wire hanger and ran it through the tubes and problem solved. Ended up sealing the ends with a small piece of screen and haven't had a problen since.

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