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Old 23-02-2019, 18:41   #1
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Head vented loop question

Hey guys! Just bought a 1981 Pacific Seacraft 37, and am diving right into the fun stuff--the head! We were having trouble getting raw water into the bowl to flush. There is a vented loop under the sink cabinet, between the seacock and the pump. I am guessing this was an incorrect installation, since it prevents the pump from generating suction and drawing in seawater. We covered the valve with a finger while pumping and it draws water! From my research it looks like the vented loop needs to go between the pump and the toilet bowl, so that it is pushing water past the siphon loop, rather than trying (unsuccessfully) to SUCK water past the loop.

Here is my plan: leave the siphon loop where it is. Disconnect the black intake hose from the siphon break fitting, and have it fun instead directly from the seacock to the pump. Drill 2 holes in the side of the cabinet. Reroute the hose between the pump and the bowl to go through the newly drilled hole, up to the siphon loop, back down, out through the cabinet, and connect to the bowl (whew).

Right as I'm about to fire up the drill I can't help but think---why hasn't somebody done this before? I can't see anywhere else that a siphon loop may have gone in the past. There are no other holes to route the hose into a cabinet, and the fold-down shower seat precludes it from being mounted outside a cabinet. Am I missing something here? I am almost positive that the head is below the waterline. Why would I be the first person in almost 40 years to see the need to drill exra holes in the cabinet...I must be missing something
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Old 23-02-2019, 19:00   #2
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Re: Head vented loop question

A lot of boats don't have a siphon break anywhere on the raw water intake. It may not be necessary if the top of the bowl is above the waterline. The siphon break could have been added before the sale to pass an inspection. You are correct that the siphon break should go between pump and bowl inlet.
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Old 23-02-2019, 19:47   #3
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Re: Head vented loop question

From my research it looks like the vented loop needs to go between the pump and the toilet bowl, so that it is pushing water past the siphon loop, rather than trying (unsuccessfully) to SUCK water past the loop.


That's correct. And...it needs to be at least 6-8'above waterline AT MAX HEEL...not just when the boat's at rest, which on most monohull sailboats puts it 2-3 FEET above the bowl... making its current location under the vanity way too low. Most people mount it on the bulkhead behind the toilet. If your heart is really set on drilling holes, and if there's a hanging locker behind the head, you can hide it in there.


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Old 23-02-2019, 20:21   #4
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Re: Head vented loop question

In a lot of boats, plumbing the head correctly means exposed hose. For me, I chose not to plumb by the standard suggestions and guidelines but have operating procedures to minimize risk. In any case, the boat is plumbed better and safer than originally done by Bristol. It looks like Pacific Seacraft chose aesthetics also.
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Old 23-02-2019, 20:58   #5
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Re: Head vented loop question

Quote:
Originally Posted by peghall View Post
From my research it looks like the vented loop needs to go between the pump and the toilet bowl, so that it is pushing water past the siphon loop, rather than trying (unsuccessfully) to SUCK water past the loop.


That's correct. And...it needs to be at least 6-8'above waterline AT MAX HEEL...not just when the boat's at rest, which on most monohull sailboats puts it 2-3 FEET above the bowl... making its current location under the vanity way too low. Most people mount it on the bulkhead behind the toilet. If your heart is really set on drilling holes, and if there's a hanging locker behind the head, you can hide it in there.


--Peggie
Yikes!!!

Diagram from Vetus instructions below..
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Old 24-02-2019, 00:20   #6
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Re: Head vented loop question

I hope you mean 6-8 inches, not 6-8 feet. I don’t have any boat that’s 6-8 feet above waterline at max heel
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Old 24-02-2019, 06:36   #7
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Re: Head vented loop question

Related to the Vetus illustration above: I think in an ideal installation the outlet would be located below and away from the inlet to reduce the amount of overboard discharge being taken up by the intake.
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Old 24-02-2019, 06:44   #8
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Re: Head vented loop question

Boatbuilders don't give a damn about safety, they just want to sell boats They began choosing aesthetics over safety when they began turning boats into floating condos...often not even bothering with an inlet vented loop at all. Safety standards used to call for putting all seacocks in easily accessible places, but because things like seacocks and vented loops are too "ugly" to be decor, builders now put them in such INaccessible places that make it all but impossible to close 'em when leaving the boat...more than a few owners who've come to me for help don't even know where their head intake seacock is. They're also the worst sanitation system plumbers on the planet because they don't design sanitation system...holding tanks are just added expense that adds -0- value to the boat. So they'll put 'em anywhere. To save money they'll put only one tank on a boat with two toilets that are at opposite ends of the boat, creating a run to it from one of 'em that can be 20 feet, or even longer...and hide vented loops in locations that are too low to be of any value. I shouldn't complain about 'em...they created a need that's provided me with a career that's lasted 30 years!

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Old 24-02-2019, 06:50   #9
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Re: Head vented loop question

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Originally Posted by pmagistro View Post
Related to the Vetus illustration above: I think in an ideal installation the outlet would be located below and away from the inlet to reduce the amount of overboard discharge being taken up by the intake.

You're correct...the toilet discharge thru-hull should not only be below the inlet thru-hull but should always be well aft of it.


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Old 24-02-2019, 10:21   #10
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Re: Head vented loop question

Not to hijack this thread, but since you all seem to have it sorted.... I've got one I have never seen before.

The vent on a Forespar 1 1/2" vented loop leaks black water when pumping holding tank overboard. The loop is between the discharge pump (Sealand bellows) and the seacock. I replaced the small duckbill valve in the loop vent but it still leaks. It makes a squealing noise as air leaks by and then we get some black water seepage. Seems like the discharge line is over pressurized?? I can hold a thumb over the vent while it's pumping and it will build up pressure.

The only thing I can think of is a partial blockage in the discharge line between the vent and the seacock. I have a diver coming tomorrow to check for growth at the thru hull before I start pulling hoses.

I've back flushed the holding tank vent and also have tried pumping it with the holding tank deck fitting open. Still leaks.

The toilet is electric macerating and nothing but human waste has ever been put in it (just the wife and me, retired yacht crew..... I know... we should be smart enough NOT to buy a boat!!)

Any ideas would be much appreciated!

Hubs
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Old 24-02-2019, 10:48   #11
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Re: Head vented loop question

Never assume that any system seen in any boat has EVER worked correctly in the past. Often things get installed in the distant past and never worked right from day one. Other things could have been installed (especially with septic systems) to pass some marina rule or condition of sale/insurance and just do not or can not work as intended.

One could easily blame it on clueless DIY-ers but just as possible it could have been done by a "professional" in the marine industry.
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Old 24-02-2019, 11:13   #12
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Re: Head vented loop question

The vent on a Forespar 1 1/2" vented loop leaks black water when pumping holding tank overboard. The loop is between the discharge pump (Sealand bellows) and the seacock. I replaced the small duckbill valve in the loop vent but it still leaks.


Unless there's a buildup of sea water minerals or sludge the tank discharge line that's reduced its diameter--which is entirely possible if you never flush out the tank--you may need to replace the entire air valve instead of just the diaphragm.


I'll be very surprised if your diver finds anything...


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Old 24-02-2019, 12:05   #13
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Re: Head vented loop question

Thanks for the quick reply, Peggie. I appreciate your taking the time to engage.

I did replace the entire vent housing in addition to the tiny duckbill (just a Marelon threaded cap). It seems like there is just to much pressure for the duckbill. When I replaced the duckbill with a small piece of rubber sheet to seal off the vent, everything works fine. Aside from the obvious concerns of defeating the anti siphon, I'm concerned that if there is some physical blockage, I may overpressurize the system and blow black water into the bilge.

There doesn't seem to be any resistance from the holding tank through the pump to the vented loop, so if the diver finds nothing, I'm really at a loss of where to look next??
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Old 24-02-2019, 13:45   #14
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Re: Head vented loop question

Quote:
Originally Posted by Jhubs81 View Post

I did replace the entire vent housing in addition to the tiny duckbill (just a Marelon threaded cap). It seems like there is just to much pressure for the duckbill. When I replaced the duckbill with a small piece of rubber sheet to seal off the vent, everything works fine. Aside from the obvious concerns of defeating the anti siphon, I'm concerned that if there is some physical blockage, I may overpressurize the system and blow black water into the bilge.

There doesn't seem to be any resistance from the holding tank through the pump to the vented loop, so if the diver finds nothing, I'm really at a loss of where to look next??
I don't think there is a need for a vented loop between the holding tank and the through hull. Open the valve when pumping and close it when not. Worst case some salt water gets in the holding tank.
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Old 24-02-2019, 14:45   #15
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Re: Head vented loop question

The vented loop is fail-safe protection in case the thru-hull is left open. Actual worse case would be a thru-hull left open while no one is aboard the boat to see that sea water has filled the tank enough to overflow it back to the bowl, causing it to overflow. A real mess at best, a boat on the bottom in its slip at worst. A remote possibility, yes...but those of us who are are old enough to have experienced Murphy's Law (anything that can go wrong, will go wrong) enough times know that Murphy was actually an optimist.


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