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Old 08-02-2018, 07:55   #1
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Placement of Vented Loop and Y-Valve

Iím replumbing the head and trying to determine the correct placement of the vented loop in the system. Two optionsÖ

1. 1. From toilet directly to a y-value, with one branch going to a vented loop and then a thruhull and the other going directly to the holding tank
2. 2. From toilet to vented loop, then y-valve with one branch to thruhull and another to holding tank

Option two doesnít seem to work for the obvious reason that the y-valve would be below the water line and unprotected by the vented loop. If it failed, the holding tank would flood. I wouldn't think I need to protect backfilling from the holding tank to the toilet with a vented loop because the holding tank is deep in the bilge in the center of the boat.

Is option 1 the right way to go?
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Old 08-02-2018, 08:16   #2
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Re: Placement of Vented Loop and Y-Valve

The best way to do it depends on a couple of things...whether the boat is a sailboat or a powerboat and whether it's an uphill run from the toilet to the tank.

The vented loop needs to be at least 6-8" above waterline AT ANY ANGLE OF HEEL...which on most sailboats puts it 2-3' above the bowl...In which case, the best place for it is immediately after the toilet. Whether it's sailboat or not, if it's an uphill run to the tank, putting a loop immediately after the toilet that's at least slightly higher than the top of the tank means you'd only have to flush long enough to push the bowl contents over the top of the loop...gravity will get it the rest of the way to the tank. You can put the y-valve anywhere in the line coming down from the loop that's accessible and keeps it well above the waterline.

If it's a sea water toilet, you'll also need a vented loop in the flush water inlet ...NOT in the line between the thru-hull and the pump, but in the line connecting the pump to the bowl...which means replacing the short piece of hose the mfr uses to connect 'em with lines long enough to put it the required 6-8" above the waterline at any angle of heel, which means that on a sailboat it also will be 2-3' above the bowl. The attached photo shows the typical installation for both loops on most sailboats.

"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein
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© 2019 Peggie Hall
Specializing in marine sanitation since '87.
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