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Old 03-11-2013, 10:07 AM   #15
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Yes, a valve on both can be dangerous.

A valve, either check or ball, on the font is a very good way to keep the tanks from siphoning or overflowing on downhill stretches when full and is perfectly safe as our Alpines have 2 vents on top of the tank, one a short one in front which is an overflow, another in back which loops up quite high (at least in our '99) and is a vent.
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Old 03-11-2013, 11:01 AM   #16
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Originally Posted by alpinedrvr View Post
Yes ,yes,yes, Dave I had not thought of that at all. I kept thinking of the city water pressure as I was filling and not the air needed as the water pump is pumping out of the tank. Hmmmmmm I'll do some more thinking (which is dangerous!)

Geoff
Goeff
Can you explain the reason for the CV and where/why you added it?
My coach has the same siphon problem and lost water out the overflow when stopping.
I added a 1/4 vent line with a check valve, (like this http: //www.usplastic.com/catalog/item.aspx?sku=57176&gclid=CMXcl7b89LUCFe4-MgoduVUAsg), which prevents water from flowing out but allows makeup air in when pumping water out.
To prevent pressurizing/damaging the tank when filling I added a ball valve in the 1/2" overflow pipe.
However I MUST remember to open the ball valve when filling.

BTW, IMO the best solution would be an overflow/vent hose that terminates 2 or 3' higher than the top of the tank, (no check or ball valve needed), but I can't find a good way/path to run one in my coach.
Mel
'96 Sahara
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Old 03-11-2013, 11:21 AM   #17
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A number of us have had this problem plus fresh water dumping when first driving when the tank is full. A number of us have attached a plastic about 1/2" diameter hose to the outflow and looped over the propane tank and then down. This way the outflow is not obstructed, you can see when the tank is full as the water siphons down the hose and is easy to see and most important the hose can be easily sealed off in a number of ways, I merely loop it thru the pneumatic door strut, so the water won't siphon.
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Old 03-11-2013, 11:27 AM   #18
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The front vent line is attached to the tank along the side as opposed to the top. If you fill the fresh water tank to "full" then water will start dripping out of the front vent line. The problem is that there's still room for 10 - 15 gallons of water and water can continue to drip out if you move the coach (siphoning). Many owners have added a check valve or a shut-off valve (what I did) to load the tank with more water. Eventually, the rear vent line will start dripping. I either watch the tank from inside the basement or simply let it fill for a few minutes. It makes a HUGE difference when boondocking!
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Old 03-11-2013, 01:32 PM   #19
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Hi Mel

this is the link explaining the CV :
Full Fresh Water Tank

This is exactly the same CV I installed....1/2" adjustable (5-10psi) from King Bros. The Alpine has 2 vents 1 front 1 rear.

Bob I too am going boondocking and would like the extra 10-15 gals. I just installed a shutoff valve on the rear vent. Yesterday when I filled the tank until the rear vent overflowed I was not quick enough to shut off the water and i lost a lot of water through siphoning. It was probably more than 5 minutes that it was flowing before I shut off the water. Today after I installed the shutoff valve i filled the tank again and watched carefully for the rear vent to overflow. As soon as I saw water coming out of it I shut off the water. It stopped right away before I had a chance to close the aforementioned shutoff valve and I still have a full tank! I leave tomorrow and will leave the shutoff open to see if I lose any water while travelling. If not maybe my siphon problem is solved

Thanks everyone for the help

Geoff
2000 38 FDS
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Old 03-11-2013, 02:21 PM   #20
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An easy way to stop "syphoning" is to have the outlet of the overflow pipes above the top of the tank. Can usually run it up inside the wheel well.

Sometimes the syphoning happens because when you fill the tank from the city water, it fills the tank totally full to the extent that the tank swells, especially if you aren't watching and don't turn the fill valve off. Once the water starts flowing down the overflow tube, that extra water swelling the tank will continue to flow for quite a while. Trouble is, then the suction caused by the overflow tube being below the tank, causes the tank to start collapsing and that continues until the suction pressure equals the elasticity of the tank top trying to stay flat. At that point there will be a big suck of air back into the tank and the tank will show up as being less than full. This whole process can take 10 or 15 minutes.


As for damaging the tank while ffilling - figure out the total pressure swelling the top of a tank at even 1psi - which is a water head of 2'. Say 50" x 30" x 1psi = 1500 pounds. Even that force will break mounting straps. Allow mains water to roar into a tank with the typical 1/2" overflow pipe and you could easily pressurise the tank to 10psi or even more. Do the sums, but rectangular tanks are typically rated to handle a few inches of water pressure, not 30 feet of pressure.
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Old 03-11-2013, 03:07 PM   #21
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Hi Mel

this is the link explaining the CV :
www.irv2.com/forums/f104/full-fresh-water-tank-27633-2.html

This is exactly the same CV I installed....1/2" adjustable (5-10psi) from King Bros. The Alpine has 2 vents 1 front 1 rear.

Geoff
2000 38 FDS
Goeff
thanks
Mel
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