Vented Loop Controversy

[Ron Dague]

This should be good for some strong opinions! I know the reasons for a vented loop, the anti-siphoning. But, why does the intake water to the head normally go straight from the thru-hull to the head pump, then have a vented loop between the pump and the bowl water supply. I've referenced Jabsco, and Calder's "Boatowner's Mechanical and Electrical Manual". My boat is also "plumbed" that way. Wouldn't it make more sense to put the vented loop between the thru-hull and the manual pump? By the time the water gets past the pump, it has alrady gone past 2 valves (thru-hull and head valve) that should both be closed when not being used. I'm sure there is some obvious reason and I'll will subsequently be humbled by the responses! PS: Thanks Ernie and others for advice. I've finished the install of my macerator. I went straight from the head to the holding tank, then a y-valve between tank and either macerator or pump-out. Since I plant to never leave the waste thu-hull open, unless "macerating" the waste tank, I skipped the vented loop on the output, to minimize back flow of waste, less hose to permeate, etc. I know, I'm a bad boy! (I've installed the macerator pump on/off switch right next to the macerator pump, y-valve and thru-hull, as a reminder!)

 

[Peggie Hall/HeadMistress]

Because...

A vented loop breaks a siphon by introducing air in a line. Pumping a toilet would suck air into the intake hose through the vent in the loop if you put it between the thru-hull and the pump...so the pump can't prime. If it didn't prevent flush water from coming in altogether, it would drastically reduce the amount. Putting the loop AFTER the pump allows the pump to work, but still breaks any siphon that could fill or--worse yet--overflow the bowl. A word of caution however: a vented loop breaks a siphon, but it cannot stop water going through it under pressure. So a vented loop in the intake does not prevent "ram" water--water forced up up the intake line (or any other hose) while underway. It's still necessary to keep all seacocks closed except when the device connected to 'em is actually in use.

 

[Jerry Villines]

Vented Loop Wheezing

Both of my vented loops (head intake and tank pump out) wheeze (leak when pumping). I have changed the joker valve but still only get positive suction by placing my finger on the loops when flushing or pumping out. What can I do?

 

[Peggie Hall/HeadMistress]

Vented loops need air valves, Jerry

A vented loop should only let air in...it's not supposed to let air--or anything else--out. That's accomplished by a one way air valve that goes in the nipple at the top of the loop. If you can put your finger over the hole, your loop is missing the air valve. It's not uncommon...the valves are sold separately because they're replaceable parts that wear out, but few people--including 99.9999% of the people who work in boat supply stores--know that, so when they buy a vented loop, they don't ask for one and nobody tells 'em they need one--if they even stock air valves, cuz the buyers often know even less than the people who work in the stores. But air valves can be ordered from the loop mfrs...and that should cure your problem.