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Vented loop vent confusion/waste system sanity check

6744 Views 26 Replies 13 Participants Last post by  svHyLyte
I could use advice from people who know about holding tank systems.

I'm aiming to install a new holding tank system from scratch. For whatever reason my 1976 Ericson 29 does not have one and does not appear to ever have had one... just two thru-hulls, one for intake and one for output.

I've done hours and hours of research and come up with this diagram of the waste system I intend to implement:

I think it should be OK, provided I can get everything to fit together and muster up the courage to buy 100's of dollars of components and drill a couple above-waterline holes through my hull. Though if anyone has any feedback or suggestions on how I am approaching this I'd be glad to hear it.

My biggest confusion right now is coming from how I'm supposed to connect the vented loop to the vent line. I'm planning on trying to use 1.5" sanitation hose for everything (including the vent line, which as I understand it, is better off thick to allow the holding tank more oxygen). However, the biggest hose any vented loop will accept on its vent is 1/2" as far as I can find, and the cheap one I would prefer to buy, has a measly 1/4" port with its adapter.

On top of that, it states that the valve has to be removed to insert the adapter. I researched and found that if you attach to a vent line, the loops are more reliable without the valve anyway. This is all fine and good, except that as far as I can tell there will now be sewage splashing up into the vent line every time I pump the head. This means, if I don't want to deal with terrible smells, I have to somehow find a 1/4" ID marine sanitation hose (which I can't find anywhere), AND adapt it to a tee connector on the 1-1/2" marine sanitation hose I plan on using for the vent.

I cannot find an adaptor anywhere that will let me adapt a 1/4" hose to 1 1/2", much less one designed for sewage. I can't even find a combination of adapters that would work; the closest I could get is this tee connector, and it's still three times too big.

Did part manufacturers as a whole really overlook a standard, widespread head configuration, or am I just blind, crazy and overthinking things (I suspect the latter)?
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You are over thinking things. There is no need to connect the vent on the "anti-siphon loop" to the vent from the holding tank. The "duck bill" valve in the vent is designed such that neither water nor air can pass the vent, out of the loop. It will only open to admit air into the loop when there is "suction" set up by a siphoning effect. That is why one must "push" effluent through the loop with one's pump rather than attempting to "pull" it through.

The following is a somewhat clearer illustration of your arrangement:

This arrangement, however, relies on two Y-Valves which can be problematic unless they are exercised routinely. Note, too, that one would always want to maintain the deck pump-out/through-hull selector Y-valve in the deck position against the possibility of a back-flow valve failure/leakage in the holding tank pump (it does happen!).

Some advocate a 'small' vented loop (the vent valve on the vented loop being 'small') on the inlet to the toilet head line; but, that has to be carefully and accurately sized so that the vacuum generated by the heads manual water pump exceeds the in-rush air leakage of the vented loop - this is to prevent flooding though the toilet in case the toilets dry/wet valve is faulty; if faulty the head will flood into the boat especially if the top of the toilet bowl is below the waterline. The inlet line vented loop height is dependent on how far below the static waterline the toilet is mounted and how far the top of the vented loop is above the static waterline when the boat is well heeled over.
Its very simple, If the vent valve on the water inlet to the head allows a greater volume of air than the pump can move water, you will only pump air and not much water to the head/bowl. The air flow through-put of the that vent valve must be sized/selected so that the volumetric flow of air does not reduce what the hydraulic static head requirement to pump water to the bowl. Usually a small fixed diameter orfice is added to the vent valve to control the amount of air flow and still allow enough flow to 'break' any suction that would allow siphoning of water into the boat if that bowl is below the waterline.
Simple hydrostatics.
Most definitely this the absolute SAFEST way to plumb an onboard toilet

Actually I am correct. Here is a link to Jabsco's instructions for their most popular manual head:

And here is the pertinent section:

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