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Do I need a vented loop?


ab1974

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I have a Porta Pottie (Dometic 974MSD) that has a discharge / pump out fitting that allows me to pump the waste from the inbuilt holding tank out via the thru hull. Gives me the advantage of a Porta Pottie (lightweight, no separate holding tank, small space wise and cost effective) without having to take the holding tank off and either empty off the back when in open water or walk down the marina to the toilet.

 

The waste is going to be pumped out from the holding tank via a macerator pump, which is mounted just under deck level, then to the thru hull (which has a seacock). The holding tank is also ventilated.

 

Question is do I need to have a vented loop in the output line? Is there a chance of water syphoning back in? I guess not if the seacock is closed at all times except when pumping out? But is it sensible? Will the macerator pump stop the syphoning - as they have an input and output does that mean they have an inbuilt non return valve?

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You only need a VL if the toilet is below the water line. And bare this in mind, that below the water line could be when you are heeled over, not at rest.

The pump itself has two seperate rubber impellors. One for the waste and one for the clean water. The impellor itself tends to be a bit of a valve, but as they wear, you can get stuff seeping back past. If you have a seacock, then that will shut everything off, but you have to ensure that happens. So really, you have to weigh all that up. Is toilet ever going to be below water line when it gets emptied. Is seacock ever going to be accidently left open. If yes, get a vented look. If no, then you will be fine.

Oh by the way, opnce you have emptied the toilet and the pump pumps air into the line, the syphon is pretty much broken anyway.

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One of me mates boats sunk due to no loop nor, the stupid prick, seacock on the dunny. Syphoned back in and down she went.... while on the marina.

 

I have a loop and a valve. Once when sailing in slop, water did try to come back in. I knwo close the valve when sailing in yucky... just in case.

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in the last yacht I had anti siphon loops installed on both inlets and outlets. One day while out racing in heavier conditions, the head tried to sink us. From then on I have been pedantic about keeping the dunny seacocks closed.

 

As it turned out we had the siphon loops but no vents at the top. That in conjunction with the fact that we were washing the windows meant that the top of the siphon hose was below the water line at times.

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Guest georgeboscon

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