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Old 10-18-2020, 03:23 PM   #1
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Fill, Vent, and Drain Questions

I am gun shy about filling my fresh water tank because I caused major damage by not opening the vent in my last toyhauler. I just purchased a 2016 Vengeance SS 320A Toy Hauler.

I went to fill the fresh water tank today after driving 200 miles with all bottom valves open, including the too in the picture below. One, valve I have marked "Drain?" and the other I have marked "Vent?". The one marked "Vent?" was in the open position as I filled it and it didn't start draining water until the tank was apparently full. I shut off the fill water and it keeps draining. I then ran water through all 3 faucets and it still keeps draining. I finally just closed the valve to stop from losing all of the water in the fresh water tank. Is this typical? Should I leave it open or closed while filling the tank. Should I close it and keep it closed once the tank is full, or should I open it back up at some point?

The other picture shows the docking station fill port, with another vent marked "Tank vent". Do both of the "Vents" fulfill the same purpose?
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Old 10-18-2020, 03:29 PM   #2
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The one underneath is an overflow. The other is the tank drain.

The one by the fill port is the vent.
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Old 10-18-2020, 04:30 PM   #3
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The one underneath is an overflow. The other is the tank drain.

The one by the fill port is the vent.
Thanks. It seems that the overflow creates a siphon once you get to overfull and the siphon won't stop dumping water out unless you shut the overfill valve. Is that typical? When should I be able to open that valve back up?
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Old 10-18-2020, 05:42 PM   #4
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What happens when you open the other valve you have marked "Drain" and close the open you have marked "Vent"? If water starts running out of the one you have marked "Vent?" as soon as you start filling then you probably have them reversed. The over flow should not have water coming out until the tank is nearly full. There is not any reason the overflow and vent cannot be the same hose other than to allow faster filling of the tank. If that is the case manufacturers should use a larger diameter vent.
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Old 10-18-2020, 11:54 PM   #5
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What happens when you open the other valve you have marked "Drain" and close the open you have marked "Vent"? If water starts running out of the one you have marked "Vent?" as soon as you start filling then you probably have them reversed. The over flow should not have water coming out until the tank is nearly full. There is not any reason the overflow and vent cannot be the same hose other than to allow faster filling of the tank. If that is the case manufacturers should use a larger diameter vent.
The one marked "Vent" is certainly the vent and the one marked drain is certainly the drain (for the fresh water tank). The water doesn't come out of the vent until the tank fills up. It just doesn't stop coming out when I turn the water off, causing me to believe that a siphon effect has been created and the water just continues to drain from the tank (possibly till empty or near empty).
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Old 10-19-2020, 06:15 AM   #6
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The one marked "Vent" is certainly the vent and the one marked drain is certainly the drain (for the fresh water tank). The water doesn't come out of the vent until the tank fills up. It just doesn't stop coming out when I turn the water off, causing me to believe that a siphon effect has been created and the water just continues to drain from the tank (possibly till empty or near empty).
Many folks seem to experience the siphoning/draining from the overflow and I suspect that's why someone put a valve on the overflow.

I'm not sure if it starts because the tank gets slightly ballooned when filling and pressure pushes some out once the filling stops and then keeps siphoning or what but it happens.

In theory, the overflow should stop draining once the level gets to the place where the overflow is attached to the tank but there have been LOTS of reports of the tank draining and being almost empty (or empty) when those folks arrive at the campground.

Since you also have a vent, I'd say shut the valve once full and don't open until you fill again.
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Old 10-19-2020, 06:46 PM   #7
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I wish...

Quote:
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Many folks seem to experience the siphoning/draining from the overflow and I suspect that's why someone put a valve on the overflow.

I'm not sure if it starts because the tank gets slightly ballooned when filling and pressure pushes some out once the filling stops and then keeps siphoning or what but it happens.

In theory, the overflow should stop draining once the level gets to the place where the overflow is attached to the tank but there have been LOTS of reports of the tank draining and being almost empty (or empty) when those folks arrive at the campground.

Since you also have a vent, I'd say shut the valve once full and don't open until you fill again.
I wish someone would look at one of these "siphoners" and report:
--Does the line come into the tank from the top?
--How far down does it penetrate the tank?

Some of these must be clear tanks, under a bed, easily checked by simple visual inspection. Our SOB is like that--I'll check it soon. The Cherokee's FW tank is opaque, HDPE, beneath the floor.

If this line ended flush with the top of the tank, it couldn't siphon the entire tank dry.
If it only penetrated 1", it could only take the first inch off the top of the tank.
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Old 10-19-2020, 11:05 PM   #8
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I wish someone would look at one of these "siphoners" and report:
--Does the line come into the tank from the top?
--How far down does it penetrate the tank?

Some of these must be clear tanks, under a bed, easily checked by simple visual inspection. Our SOB is like that--I'll check it soon. The Cherokee's FW tank is opaque, HDPE, beneath the floor.

If this line ended flush with the top of the tank, it couldn't siphon the entire tank dry.
If it only penetrated 1", it could only take the first inch off the top of the tank.
I agree, Larry. It is easy to forget to open that overfill valve. That is why I posted the 2nd picture of the port marked "Tank vent" next to the fill receptacle in the docking station. I would hope that that "Tank vent" serves as a vent also, and therefore would relieve pressure from the tank and keep it from ballooning and causing damage to the floor and other components in the interior of the RV if that tank gets overfilled. As a matter of fact, if it truly is a fresh water tank vent, and if one left the overflow which I called "Vent?" in my first photo, I would imagine that water could conceivably overflow through that vent also.

That is the $1000 question.
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Old 10-20-2020, 08:25 AM   #9
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I agree, Larry. It is easy to forget to open that overfill valve. That is why I posted the 2nd picture of the port marked "Tank vent" next to the fill receptacle in the docking station. I would hope that that "Tank vent" serves as a vent also, and therefore would relieve pressure from the tank and keep it from ballooning and causing damage to the floor and other components in the interior of the RV if that tank gets overfilled. As a matter of fact, if it truly is a fresh water tank vent, and if one left the overflow which I called "Vent?" in my first photo, I would imagine that water could conceivably overflow through that vent also.

That is the $1000 question.
The outlet of any vent has to be lower than the water level in the tank for siphoning to take place through that vent. Try this simple experiment:
  • Place a half-full glass of water at the edge of a table.
  • Take a piece of clear plastic tubing and put one end into the glass, to the bottom.
  • Hold the other end below the table edge and give it a suck to start siphoning.
  • Lift that end until it is above the waterline in the glass. The siphoning stops and might even reverse.
I couldn't tell from the illustration, but wonder if the vent in that bay is higher than the water tank.
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Old 10-22-2020, 01:19 AM   #10
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I couldn't tell from the illustration, but wonder if the vent in that bay is higher than the water tank.
I imagine it is higher than the water tank. I would need to actually get a tape measure out to confirm.
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