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Old 01-05-2009, 17:35   #1
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Vented Loop Question

i am restoring an allied seawind 30 and currently all it has for a bathroom is a porta potty, i will be adding a proper head/holding tank and i keep reading that i need VENTED loops where the water comes in and goes out. i understand the need for a raised loop (as with bilge pumps that have thruhulls near the waterline) well above the waterline but why must it be vented? im finishing up my engineering degree but im scratching my head on this one...

it seems that a simple rasied loop would be sufficient to keep the boat from flooding itself? would it not?, please enlighten me?
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Old 01-05-2009, 17:41   #2
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A vented loop is an anti-siphon device. Same as a vacuumn breaker in a plumbing line.
A simple raised loop will not prevent a siphoning action.
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Old 01-05-2009, 18:14   #3
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Just to expand a bit on what captain465 said - The whole point of a siphon is you can get water to flow through a loop that goes higher than the open body of water the hose is in. The closed system creates a vacume that sucks the water out. This is exactly what you don't want in your head. Having a hole (venting) at the top of the loop prevents the vacume from forming. Having a small flapper type valve can close that vent for some applications like the intake side when you need to suck water in.
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Old 03-05-2009, 21:02   #4
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Originally Posted by pressuredrop View Post
i understand the need for a raised loop (as with bilge pumps that have thruhulls near the waterline) well above the waterline but why must it be vented? im finishing up my engineering degree but im scratching my head on this one...

it seems that a simple rasied loop would be sufficient to keep the boat from flooding itself? would it not?, please enlighten me?
I mostly go along with what you say. It seems to me most unlikely that the low head loss side of the siphon (which is the sea side due to valves, etc on the boat side of the loop) would not just drain down without the vent. That especially so for the larger diameter, typically 1-1/2 inch discharge loop such as on marine toilets - in fact Jabsco in their current instructions only mention the use of a vent in the discharge loop as being optional.

However, I would consider it good practice to fit the vents even though they can be a pain if they leak (especially the pooey side one for the toilet ). What I try to do is to remove the vent cap and innards and use small stainless steel pipe fittings with hose tails, etc to pipe the cap and innards to a wet area where leakage is not a problem - for example, in my own boat's case that was done for the toilet ones so that any leakage would drain to the shower tray. This is easy to do with the likes of the simple Forespar duckbill vents in their loops.

In some instances I don't consider even a loop is required if backflow can be reliably prevented in another way. A common area is for showers where the tray is usually below the waterline and any sump and pump in the bilge. If the sump pump is one whose valving prevents back flow (such as the multiport diaphragm ones) and backed up with an industrial quality non return valve I have no problem with that going straight to the sea without a loop, although, as always, one should close the sea cock if the boat is unattended for more than the same day. If the sump pump is left on that provides even more security.

Dribbles from the vents can be problem on a loop in the raw water circuits of the engine if they are exposed to exhaust pulsations as may be so if with a wet exhaust system. Routing the vent away to a safe area in the above way is useful then too.
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Old 04-05-2009, 02:58   #5
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... in fact Jabsco in their current instructions only mention the use of a vent in the discharge loop as being optional...
Jabsco actually caution against a vent in the water intake side (their toilets).
http://www.ittjabsco.com/files/29015_vented_loops.pdf
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Old 04-05-2009, 04:10   #6
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Yes, but they do advise putting the loop between the pump and the bowl and venting that - that location so the pump suction is not broken.

However, a simple reliable method does exist allowing the loop and vent to be put on the suction side of the pump which I have explained elsewhere. This allows a much tidier arrangment insofar as far as pipework is concerned as only the pump suction hose and the waste hose need to be exposed in the toilet compartment (in the case of the Jabsco, for example, the short white hose between the pump and the bowl is left as it is delivered rather than substituted for by the loop as in the common installation).

But someone with no experience of the method but long on arm chair reading always comes along hot under the collar to tell me I don't know what I'm talking about, boats sinking, etc, etc so I will avoid a conflagration by keeping my head down on that . So dies the sharing of some kinds of knowledge .
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Old 04-05-2009, 05:15   #7
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... However, a simple reliable method does exist allowing the loop and vent to be put on the suction side of the pump which I have explained elsewhere...
... But someone with no experience of the method but long on arm chair reading always comes along hot under the collar to tell me I don't know what I'm talking about ...
Please remind me where that explanation & debate took place. Thanks.
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Old 04-05-2009, 06:50   #8
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I have seen a head with the a vented anti-siphon loop in the intake line. The vent had a small flapper type valve that closed with suction created from the suction of the pump.

I certainly agree putting it between the pump and bowl makes more sense - a lot more sense. It's always easier to push water through an imperfect system that draw it. It's one of the reasons I prefer a foot pump at a sink to a rocker pump at the spigot.

Not to mention the potential consequences of that flapper valve getting stuck in the shut position and a few other things coming together if you are off the boat.....
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Old 04-05-2009, 07:41   #9
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As long as you guys are talking about this let me ask a question. I am restoring an 'Islander 37' which has no holding system. I am installing a 19 gallon tank in the center compartment under the v-berth. (there was no other place) The old thruhull has been glassed in and I want to put the discharge out next to the tank. It will probably never be used. I'm using a whale gusher MK5 pump for the discharge. I want to put it strait down to the seacock without a loop. Seeing as how it will stay closed is there anything wrong with this? Thanks, Noel
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Old 04-05-2009, 16:23   #10
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Please remind me where that explanation & debate took place. Thanks.
Have responded to this request privately.
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Old 04-05-2009, 17:57   #11
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As long as you guys are talking about this let me ask a question. I am restoring an 'Islander 37' which has no holding system. I am installing a 19 gallon tank in the center compartment under the v-berth. (there was no other place) The old thruhull has been glassed in and I want to put the discharge out next to the tank. It will probably never be used. I'm using a whale gusher MK5 pump for the discharge. I want to put it strait down to the seacock without a loop. Seeing as how it will stay closed is there anything wrong with this? Thanks, Noel
Not an expert but thinking it thru. If the holding tank is below the waterline and you do not have some method of stopping water from coming in the thru hull won't the tank fill with water anytime the thru hull is open? If you have the pump in between then the pump may stop the water from coming in but you would be trusting the pump to do somehting it wan't designed to do.
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Old 04-05-2009, 20:02   #12
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I recently overhauled sanitation in my boat. Instead of using heads pump I installed diaphram pump with coiled hose and hose nozzle next to head, also td off and ran salt to galley to aid in dishwashing. The pressurised water does much better job of rinsing bowl and uses less water extending tank empty interval. For plumbing I used sched 40 pvc and have no odor problems and cost less. I did no direct discharge all goes through holding tank. Authorities like to see that when inspecting. Out throughhull only is opened while macerating. For lockout I bought ignition switch brass for 12 bucks and hooked up so macerator works when holding key in start position. Passed fwc inspection this week. Standing rules on our boat all throughhulls closed whenever leaving boat.
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Old 04-05-2009, 20:09   #13
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[QUOTE=MidLandOne;279739]

However, a simple reliable method does exist allowing the loop and vent to be put on the suction side of the pump which I have explained elsewhere.

I'd be interested in this as well.

Thanks !
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Old 12-06-2009, 00:36   #14
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Hi guys, Not sure if i should have started a new thread or not but i have a "Head"problem and thought i could jump in on this one - apologies if i'm wrong - I have a vacuflush toilet on my 40'Rivera and its working great ( touch wood, hope i dodn't just jinx myself) but I do have a problem with my waste going overboard even though I have the valve set for it to go to the holding tank. I have turned the valve back and forth a few times but still didn't fix. Any suggestions.
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Old 17-06-2009, 15:26   #15
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HI MichaelB
I am wondering if your diverter valve is the Y plastic type, I have seen one where you operate the handle and the ball valve inside doesen't turn, I sugest first thing is strip the valve.
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