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Old 15-03-2012, 04:31   #1
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Head to Holding Tank Vented Loop?

My older Bristol has a vented loop between the head itself and the holding tank...is this necessary? In this system, discharge must be pumped upwards from the head and I wonder if flow would be improved by removing the vented loop and running hose directly and downward to the holding tank. There is also a vented loop in the holding tank-to-direct sea discharge, which I understand to be useful and important to keep. But what about a vented loop between head and holding tank? Would appreciate some advice about whether I ought to remove this in favor of a more direct route to the holding tank.
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Old 15-03-2012, 05:57   #2
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Re: Head to Holding Tank Vented Loop?

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My older Bristol has a vented loop between the head itself and the holding tank...is this necessary? In this system, discharge must be pumped upwards from the head and I wonder if flow would be improved by removing the vented loop and running hose directly and downward to the holding tank. There is also a vented loop in the holding tank-to-direct sea discharge, which I understand to be useful and important to keep. But what about a vented loop between head and holding tank? Would appreciate some advice about whether I ought to remove this in favor of a more direct route to the holding tank.
I assume your head it higher up than the holding tank? If so then I too can't see why the vent loop is needed. But if the holding tank fluid level can get close to or be higher than the head (probably the lowest point in the head) then back siphoning could occur which would not be a happy thing.
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Old 15-03-2012, 06:23   #3
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Re: Head to Holding Tank Vented Loop?

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I assume your head it higher up than the holding tank? If so then I too can't see why the vent loop is needed. But if the holding tank fluid level can get close to or be higher than the head (probably the lowest point in the head) then back siphoning could occur which would not be a happy thing.
Same here...My aft head is about 20 cm above the top of its holding tank with no loop...I haven't had any trouble with back flow (of course there is a joker valve) I do tend to keep my tanks pumped out.
Perhaps Peggie will enlighten us.
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Old 15-03-2012, 06:32   #4
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Re: Head to Holding Tank Vented Loop?

I assume your head it higher up than the holding tank - when heeled!
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Old 15-03-2012, 06:47   #5
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Re: Head to Holding Tank Vented Loop?

Don't take down a fence until you are sure why it was put there in the first place.
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Old 15-03-2012, 08:40   #6
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Re: Head to Holding Tank Vented Loop?

Thanks for making it clear about looking at the height of the head and holding tank. The tank is molded into the forepeak, beneath the bunks and is certainly lower than the head bowl-- though of course I'll look at that carefully as soon as I get back down to the boat tommorow. My guess the vented loop was included in the original design not so much to guard against holding tank backup as to guard against suction inflow through the direct discharge outlet-- which I have closed off and pegged. I've got a good head-- a WC Skipper-- which works ok, but seems to me it would work even better if the path didn't include the lift up through the loop. It's tempting to make the change...
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Old 15-03-2012, 10:36   #7
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Re: Head to Holding Tank Vented Loop?

You might want to consider (if you haven't already done so) that your boat isn't always level. Is there anytime (heeling, hobby horsing in rough sea) that the levels could be even close? If so, you should leave the siphon break in the line.
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Old 15-03-2012, 11:23   #8
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Re: Head to Holding Tank Vented Loop?

Yeah...a couple of people have mentioned that and I get it, though I am not 100% clear on if the risk of backup exists any time the boat is heeled and the holding tank is higher than the head or if the risk of backup is more present when using/pumping the head under heeling conditions. (Or both?) Obviously, my grasp of fluid mechanics is pretty weak.
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Old 15-03-2012, 11:32   #9
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Re: Head to Holding Tank Vented Loop?

My head was below my holding tank with no vent, no problems. Your tank fills from the top right? I had no vent on the hose going out the seacock either...but my tank was above waterline..... maybe your tank is low in the boat?
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Old 15-03-2012, 11:39   #10
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Re: Head to Holding Tank Vented Loop?

Risk analysis. This isn't a sinking issue. It's a mess issue. The hose from the head should go into the top of the tank, so the tank has to be full to have the possibility of a siphon as well as the head below the top of the tank, then you have to have two valves in the head fail as well. It will only siphon until the level drops below the inlet in the tank.

Seems to me the danger would be a tank top below the waterline with a non vented overboard discharge line so the tank can stay full from the ocean. Then put your non vented line back to the head that's lower than the tank, then when the valves in the head leak and any valves in the overboard discharge line fail you will sink your boat.

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Old 15-03-2012, 11:51   #11
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Re: Head to Holding Tank Vented Loop?

The risk is that 'IF' the holding tank liquid level is higher than the lip of the toilet bowl 'THEN' you could conceivably induce a reverse siphon of the liquids in the holding tank back into the bowl. (Most marine toilets use a 'duck' valve to prevent backwards movement of the liquids (well, and solids...)) if this were to happen you would be wiping and wiping and wiping - and that's just the disgusting part...

My boat is a trimaran so I don't get the severe heeling ('severe' is admittedly a relative term here, one man's ceiling, another man's floor...) you might/do.

But on Ceil, the head (toilet bowl lip-line) is unfortunately AT the normal waterline - so when I was at your same point, (tired of pumping the flow UP hill into a tank) I removed the anti-siphon loop...

Luckily I was on-board two days later when a storm came through - the duck valve in the head probably slowed thins down but did not stop the flow... I have a shower well built-in to the head so the waste was 'only' 3 inches deep but it was still pretty '^^&itty' if you get my drift. I woke to the smell but also heard the sloshing. If I had not been on board the boat would have been host to the entire contents of the holding tank because once a siphon starts it doesn't stop until the liquids are all down-hill...

Lessons learned:
1. check the anti-siphon loop valve (if it's not clear it will not stop a siphon starting)
2. don't remove the loop for any reason.
3. '$$%%@ it' stinks, but it stinks WAY more when mixed with salt water....
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Old 15-03-2012, 11:54   #12
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Re: Head to Holding Tank Vented Loop?

Why does the hose from the head to the holding tank go to the bottom of your holding tank? That's the only reason the entire contents of the holding tank would siphon out.

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The risk is that 'IF' the holding tank liquid level is higher than the lip of the toilet bowl 'THEN' you could conceivably induce a reverse siphon of the liquids in the holding tank back into the bowl. (Most marine toilets use a 'duck' valve to prevent backwards movement of the liquids (well, and solids...)) if this were to happen you would be wiping and wiping and wiping - and that's just the disgusting part...

My boat is a trimaran so I don't get the severe heeling ('severe' is admittedly a relative term here, one man's ceiling, another man's floor...) you might/do.

But on Ceil, the head (toilet bowl lip-line) is unfortunately AT the normal waterline - so when I was at your same point, (tired of pumping the flow UP hill into a tank) I removed the anti-siphon loop...

Luckily I was on-board two days later when a storm came through - the duck valve in the head probably slowed thins down but did not stop the flow... I have a shower well built-in to the head so the waste was 'only' 3 inches deep but it was still pretty '^^&itty' if you get my drift. I woke to the smell but also heard the sloshing. If I had not been on board the boat would have been host to the entire contents of the holding tank because once a siphon starts it doesn't stop until the liquids are all down-hill...

Lessons learned:
1. check the anti-siphon loop valve (if it's not clear it will not stop a siphon starting)
2. don't remove the loop for any reason.
3. '$$%%@ it' stinks, but it stinks WAY more when mixed with salt water....
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Old 15-03-2012, 12:09   #13
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Re: Head to Holding Tank Vented Loop?

You are correct in that, the system was installed with the inlet at the bottom (before I bought the boat). It doesn't connect to the bottom any more. I moved it after this incident. But the fact remains that if the direct disposal seacock is open and the anti-siphon valve is blocked you could still bring sizable amounts of black-water back into the bowl (just think about how long the hose is and what diameter) and then into the boat. You can in fact sink the boat under these circumstances. While the discussion was not centered on the sea-cock being open, if you are on a tack for a time and the anti-siphon doesn't work you will in fact empty at least the contents of the tank that come in contact with the black water inlet at the top of the tank. The ocean I have sailed in seems to be able to make the boat do what it wants it to, so I choose to keep the safety systems in the loop.
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Old 15-03-2012, 12:40   #14
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Re: Head to Holding Tank Vented Loop?

Yeah... I see no real problem keeping the siphon loop inthe system... it'll be plugged up with yuck anyway!
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