Originally Posted by Triton318
Seven years ago, when rebuilding my Triton, I installed a Blake Lavac marine toilet with an intake hose and a hose that goes from the toilet to the diaphragm pump and then to the discharge through-hull/seacock. I never installed vented loop fittings for either the intake or discharge side (partly because its function seemed to elude me and partly because the fittings were so expensive). Each hose does, however, have a loop that is situated above the waterline. Still...I often wonder if something catastrophic is going to happen because I don't have the vented loop fittings. To date, I've never had a problem with water coming in when it shouldn't. (Also, I always close both seacocks when I leave the boat.)
It only matters if the top of your head is below the waterline at any time, any heel angle.
Loop a hose from a seacock to above the waterline and back down below the waterline in your boat. If you started with air in the hose when you open the seacock, nothing happens. If you suck on it so water is in the entire hose, now water will come into your boat. (Sorry if you were real clear on what a siphon was.) Now plug
the end of that hose into your head that has had a few years of service
such that the valves in it don't seal 100% and the water will continue to come in if the top of the bowl is below the waterline. If the top of your bowl is above the waterline, water will come in until it is level with the water outside the boat. Actually you have no valves on the intake side of your Lavac, so either your head is above the waterline, or you have an air leak in the intake hose that you aren't aware of. Are you sure you didn't install the air bleed valve that comes with the head? The air bleed valve is an anti-siphon break. Any sort of hole in the hose near the top of the loop will allow air into the hose breaking the siphon. A more complicated siphon break has a one way valve that when the water in the hose is under pressure the valve closes to prevent contents of the hose from coming out. When the contents are trying to siphon into the boat the pressure in the hose at the top of the loop is less than surrounding, the valve will open allowing air in and breaking the siphon.
If you do have the air bleed in the intake hose, and therefore there is a possibility that your head is below the waterline, and the check valves in your diaphragm pump leak and you somehow leave the outlet hose full of water, then you will benefit from an anti-siphon valve in your outlet hose.
Many years ago on a trip from Hawaii
, the boat I was crewing
on had a Lavac head. On the tack that put the head on the high side of the boat, normal pumping action did not produce enough vacuum to lift
the water to the top of the loop. The air bleed let air in too fast. We resorted to bringing pots of water from the galley
to pour into the head. One day I planted my feet in the corners of the room, my back against the bulkhead and pumped as fast as I could. Once I got the water around the corner (over the top of the loop), I got water to flow into the head.