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Old 10-20-2017, 09:20 PM   #1
Senior Member
Brand: Thor Motor Coach
Model: outlaw 37ls
State: Europe
Posts: 230
THOR #6831
losing fresh water from tank?

LOSING FRESH WATER? my 37LS has a 100 gal fresh water tank, being a newer owner i was concerned with high water usage and poor reading tank gage.
well here is another design issue, the 100 gale fresh water tank can and will start a siphoning action losing 25-30gal under these conditions.

when the tank is filled to overflow ( i like to do this) and or just about any level over 75 gal. the tank will begin to siphon water from the tank, driving and sloshing the water to the over flow, filling the tank to overflow,
this is aggravated by the fact that the top of the water tank will collapse inward to keep pressure on the siphon action.

it seems the top of the tank is not strong enough to resist the vacuum of the siphon action. ( i have heard that some thor models and others may have a piece of wood shoved in the water tank to support the roof of the tank
would i want a piece of wood in my water tank?

i solved this loss of water by installing a air vent in the overflow hose.
it is a cheap fix some pek tubing and hose clamps. i connected the 2 overflow tubes into a tee fitting with a short upward vent ( about 4") this has stopped all loss of water.
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Old 10-20-2017, 10:22 PM   #2
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Travelin' Texans's Avatar
Brand: Redwood
State: Texas
Posts: 1,225
THOR #3610
If it's like most fresh water tanks, one of those lines is a vent, the other an overfill. On most the vent is supposed to be attached to the small screened port next to the fill port, but Thor engineers in their infinite wisdom choose to run it out below so as not to make a mess in the convenience center. Their fix for siphoning was to add pipe to that line & loop it up higher, it works. Mine both go out the bottom so I just added pex valves to both lines & have remember to open both while filling, when it starts to siphon close that valve,
continue filling til it comes out the vent, once I've used a bit out of the tank I open the vent.

Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless
Replacement undetermined
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Old 10-20-2017, 10:45 PM   #3
Senior Member
Brand: Thor Motor Coach
Model: Challenger 37KT
State: Texas
Posts: 235
THOR #1183
Re poor tank reading gauge. Redneck workaround if side of tank can be seen - fill tank - drain into 5 gal bucket - stop and mark new water level with sharpie and repeat till empty. Start at bottom mark and label 5 gal. next mark 10 gal etc.
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Old 10-20-2017, 11:41 PM   #4
I Think We're Lost!
Bob Denman's Avatar
Brand: Still Looking
Model: Tiffin Wayfarer 24 BW
State: New York
Posts: 16,980
THOR #8860
Glad to hear that you found a good fix for this! Congratulations!
Good Sam Member 843599689
Current coach: Tiffin Wayfarer 24 BW
"History shows again and again; how Nature points out the folly of Man!"
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