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The holding tank input line is usually plumbed at the top of the tank... so that incoming waste doesn't have to force its way in against the pressure of the existing contents of the holding tank. It is pretty unlikely for a holding tank to every siphon back in to a head for this reason....unless you've massively overfilled it. Also, the tank's vent line acts as a siphon break.
My preferred way of plumbing a head is to have the head discharge directly into the tank. Then to have the pumpout line from the tank equipped with a diverter valve, one side of which goes up to the deck pumpout fitting and the other of which goes to a diaphragm pump or macerator pump and then to the seacock and through-hull.
This setup minimizes the plumbing but maximizes the flexibility of the system. Some say you lose capability, but in reality, you don't, since anywhere you could have discharged the head directly overboard, you can dump the tank out.
With the diverter valve, it is unlikely that you need a vented loop in the discharge line, since the head can't siphon through the diverter valve.
Originally Posted by jarcher
If a toilet discharge is piped directly to a holding tank, is a vented loop necessary? If not, what keeps the tank from spilling or otherwise backing into the toilet, or even into the discharge tube?
Currently I have the toilet discharge going to a diverter valve, then to either the seacock or the tank. The seacock is never used, and the diverter valve is never changed, always leading to the tank. I could take it out, but the hose from the valve to the tank is in place, so I don't want to fool with it.
When I replace the toilet, I plan to just run the discharge to the existing valve. I am trying to figure out of I need a vented loop.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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