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Old 04-23-2013, 08:36 AM   #1
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Dedux: Water tank venting

I've seen lots of good ideas over the years to "solve" the Alpine issue of siphoning fresh water out of the vent pipes. Most solutions focus on installing check or turn-off valves and pipe extensions in/around the propane tank compartment. My initial solution was to put a checkvalve on the front vent pipe but I soon found the back vent pipe was also prone to siphoning--especially once the front check-valve was installed.

Recently, I had a problem with the "fresh water fill" valve leaking city water into/overfilling the fresh water tank. Rather than replace the defective valve, I eliminated the fill valve by installing a separate inlet for the fresh water tank [$10 at CW]. Since I had an extra valve [semi-functional], I rerouted the rear vent pipe thru this valve and extended the outlet back thru the sub-floor, using clear plastic [5/8"] tube [capped the end with some window screen and a cable tie]. I now have a vent tube valve that is handy in the sewer compartment when I fill the fresh water tank.

PS--my initial thought was to place check-valves on both vent tubes but was concerned the check-valves would restricted/block venting when the tank is being emptied.
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2003 40' MDTS
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Old 04-23-2013, 10:14 AM   #2
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The only problem with having manual valves on the vent tubes is if they are left closed while filling the tank from city water, the huge pressures inside the tank can do a lot of damage to the tank and the mountings.
My preference is to have a vent pipe at least twice the diameter of the fill pipe (or if that isn't possible, a restriction in the inlet pipe) and run it up somewhere where it can't syphon.

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Old 04-23-2013, 12:56 PM   #3
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No problem--front vent is a checkvalve, so its always open against the pressure. The new valve on the rear vent is always open except immediately following tank fill and then its in the sewer/water fill compartment so easy to remember....
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Old 04-23-2013, 09:27 PM   #4
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Old Scout your math is good and I bet you could build a fine still.
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Old 04-24-2013, 03:28 PM   #5
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what were they Thinking

Reading about all the issues on the siphoning fresh water tank ,, After I bought my Coach from Saddleback RV 2005 in Irvine AKA "King of the RV Scams" They told me that Alpine has never heard of this problem before . That was back in 2004/2005.
The dealership put some Mickey Mouse triple loop on top of the tank.. LMAO.. I had to fix the issue myself a year later.
What were they thinking ??

I have the same problem with the rear overflow tank for my Coolant. Everytime i drove somewhere i left a line of coolant spraying in the wind

HAD TO INSTALL A overflow tank for the overflow tank.....
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Old 04-24-2013, 07:17 PM   #6
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The coolant tank shouldn't do that (duh, but what I mean is there is or at least was something specific to your coach that is wrong). Could be bad coolant cap not making a seal or something else.
FYI- most folks have added an aftermarket coolant overflow as the upper "sight glass" so as to have a positive ID that coolant is full. But that's w/a good radiator cap on the surge tank.
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Old 04-25-2013, 08:22 AM   #7
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EM makes a great point--believe the OEM cap is 15-16 lbs and probably should be replaced every 5-6 years as preventive maintenance. Another issue is just how full to keep the coolant level. The key here is always check level in the sight glass when engine is running at operating temperature. If you check and fill it cold, you are putting in too much. Similar thing for checking engine oil--my 03 calls for 28 quarts which fills it to the "full" line on the dip stick. However, if I do that I get alot of blow-by out the slobber tube. By keeping the engine about 2-3 quarts low [25-26qts], I have almost no blow-by. Seems each engine has its own operating characteristics. If its constantly belching fluids or smoke, something is wrong--kinda like us!

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tank, water

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