# Vented Loop Design?



## eherlihy (Jan 2, 2007)

While trying to route my new transducer cable, I encountered the vented loop from the heat exchanger before it enters the waterlift muffler. I had a choice, undo the two 3/4" hoses, or undo the one ¼" hose.

I decided that rather than messing with two hose clamps, one would be easier - that was a mistake. After I removed the clamp, I could not get the hose to let go of the barb. A little more pressure, and snap, off came the hose with the barb still inside.

So off to WM to get a new one. No joy at the closest WM, so 55 min drive in the other direction where they had 5 in stock. The new vented loop is one with the duckbill valve, and the old one had a barb with a hose. 
The original looked more, or less, like this (except it was old - 1987 black plastic);









Here is a pic of the new one;









Because the loop is mounted HORIZONTALLY (not vertically) under the cockpit floor, I was unsure if this new fangled style would work. Fortunately, I found a brass barb fitting that fit, so I bought that too.

I put a little pipe dope on the threads, installed the brass barb, and installed the new vented loop in the old location - after correctly routing the transducer cable.

So, the question for the collective; would the new-fangled style have worked?


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## mitiempo (Sep 19, 2008)

They are comparable but should be mounted vertically.


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## eherlihy (Jan 2, 2007)

...and that's why I removed the duckbill and modified it to match the original design. I can see where the vent tube eventually goes vertical, and it has worked this way since 1987. There is no way that the duckbill could be mounted vertically unless I change a bunch of 3/4" exhaust hose. 

I guess that I made the right decision.


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## VallelyJ (Nov 21, 2008)

The duck bill is just a check valve. It's orientation isn't important; only the direction of flow (and that it's kept clean so that it doesn't stick shut). If the loop is the highest point on the line, and above the heeled waterline, the vacuum isn't going to know whether the plumbing is laying on its side or upright. If the hose starts to siphon seawater into the engine, the valve will break the vacuum.
If you aren't confident in the duckbill, or you won't be able to access it to check it every now and then, you can just get a 1/4" barb with a female threaded end that fits where the cap threads on, and put the 1/4" hose back on. I assume it exits through the cockpit coaming or someplace. Then you can leave the duckbill out.


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## eherlihy (Jan 2, 2007)

VallelyJ said:


> The duck bill is just a check valve. It's orientation isn't important; only the direction of flow (and that it's kept clean so that it doesn't stick shut). If the loop is the highest point on the line, and above the heeled waterline, the vacuum isn't going to know whether the plumbing is laying on its side or upright. If the hose starts to siphon seawater into the engine, the valve will break the vacuum.
> If you aren't confident in the duckbill, or you won't be able to access it to check it every now and then, you can just get a 1/4" barb with a female threaded end that fits where the cap threads on, and put the 1/4" hose back on. I assume it exits through the cockpit coaming or someplace. Then you can leave the duckbill out.


Thanks!

That confirms what I believed, and did.

The issue for me is that I believe that the duckbill is designed to be installed at the absolute peak of the loop when the vented loop is vertical. Turning the vented loop on it's side effectively moves the duckbill to a point slightly less (1 or 2 mm) than the peak, because of the difference in the circumferences of the vent and the loop. My thought is that this may have prevented the vent from opening when it should.


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## Maine Sail (Jan 6, 2003)

eherlihy said:


> ...and that's why I removed the duckbill and modified it to match the original design. I can see where the vent tube eventually goes vertical, and it has worked this way since 1987. There is no way that the duckbill could be mounted vertically unless I change a bunch of 3/4" exhaust hose.
> 
> I guess that I made the right decision.


Just be sure your "exit" hose does not have any dips in it that can collect and hold water. With a bad impeller even a small "trap" can cause a siphon.

For engine vacuum breakers I prefer the ones made by the Scot Pump Co. as I find they are far more trouble free than a duck bill....

You can buy just the vacuum breaker:








Or the whole thing:








I tend to just make custom ones:









This was Ericson's idea for a siphon break and it sprayed salt water all over the engine when the brazed copper tube failed. It was also mounted under the cockpit:









A few dollars and 30 minutes later a new one that is significantly more robust replaced it.


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## eherlihy (Jan 2, 2007)

Maine Sail said:


> This was Ericson's idea for a siphon break and it sprayed salt water all over the engine when the brazed copper tube failed. It was also mounted under the cockpit:


Fire suppression system?


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## VallelyJ (Nov 21, 2008)

> My thought is that this may have prevented the vent from opening when it should.


Nah. You're talking a fraction of an inch. If you're worried about the joker valve malfunctioning, just use a small open tube vented outside of the boat.
Recall that the siphon break works because it takes less force to suck air through it than it does to lift a hose full of water out of the sea. I can't see how moving it a half a hose diameter below the peak of the hose ID is going to make drawing air harder then lifting all that water.


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## Jgbrown (Mar 26, 2012)

Maine sail: How do those vented loops do with the head? Any smell? I'm just cautious because my head hoses are exposed inside the main cabin, nothing I can do about that. I have no vented loops but would like to add them for safety and those look great.


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## bvander66 (Sep 30, 2007)

Dont recommend marelon vented loops in the exhaust water, should be metal. Exception is Vetus makes an exhaust quality plastic/grp one.


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## casey1999 (Oct 18, 2010)

Question on the vented loop:
The vented loop on the water line to my engines water injection elbow has the vent line exiting through the hull of the boat at the stern. It has worked this way for 30 years. The vent line is somthing like 1/4 inch plastic tube. The through hull fitting has no seacock. 

I am thinking of fitting a seacock but does the vented loop need to run out the hull? At some points of sail and sea state, seems the through hull could be under water.
Regards


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## casey1999 (Oct 18, 2010)

Maine Sail said:


> Just be sure your "exit" hose does not have any dips in it that can collect and hold water. With a bad impeller even a small "trap" can cause a siphon.
> 
> For engine vacuum breakers I prefer the ones made by the Scot Pump Co. as I find they are far more trouble free than a duck bill....
> 
> ...


Main Sail,
In the last (bottom pic) where does the vent hose go?


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## Maine Sail (Jan 6, 2003)

casey1999 said:


> Main Sail,
> In the last (bottom pic) where does the vent hose go?


Out the transom. Gives a nice "tell tale" spit....


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## Maine Sail (Jan 6, 2003)

bvander66 said:


> Dont recommend marelon vented loops in the exhaust water, should be metal. Exception is Vetus makes an exhaust quality plastic/grp one.


Vented loops/siphon breaks/vacuum breakers are inserted into the raw water intake side of the system not generally into the "exhaust" system.. The water is still cool before it enters the RW exhaust injection elbow. There are literally thousands upon thousands of vessels out there with Marelon siphon breaks to protect the RW circuit from a siphoning event. While I am not a huge fan of the "duck bill", and generally prefer the Scot break, they work fine for an engine siphon break and are specifically sold for that as one of the intended uses...

Beyond that Marelon is the only plastic I know of to pass the stringent USCG fuel valve burn test. A Marelon valve sustained temps to 1275 degrees with the majority of the burn test time being between 850f & 980F. The valve did not leak any gasoline and passed the tests. If you hit anywhere close to these temps in a wet exhaust system, especially on the RW injection side, you have much bigger problems to worry about..


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