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Old 30-06-2012, 21:19   #1
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Holding Tank Venting

Another topic that never dies. After considerable forum scanning and research I still have a couple of things I am unclear about.

I have commenced a complete DIY overhaul of the sanitation system - head, plumbing and holding tank. Many choices are a compromise dictated by available space. The new holding tank is plastic, about 20 gallons and located near to the waterline. I have had a 1 1/2 inch vent outlet fitted to the tank with the only viable vent outlet to the stern through the transom about 5 to 6 feet away. The closer route through the topsides would place the outlet below the heeled waterline.

The vent pipe run to the transom would be fairly straight with only a slight incline from tank to outlet. Outlet would be a 1 1/2 inch nylon through hull fitting high up on the transom which I would protect from pooping seas with a SS clamshell cover (if I can find one this size).

My question: is the transom a suitable place for the vent, given it is often the part of the boat most protected from air movement.

And one more question if I may re vented loops as part of the installation: I am installing a vented loop between the head discharge and both down stream outlets - direct overboard or to holding tank (Y valve diversion). The question is, do I need another vented loop between the holding tank and the direct overboard discharge (underwater through hull fitting). The discharge pipe from the top of the tank can run above the waterline before dropping to the through hull outlet. Discharge will be by manually operated diaphragm pump (Whale toilet pump).
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Old 30-06-2012, 21:51   #2
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Re: Holding Tank Venting

Suggest you install some type of stink filter on the vent line. And be sure that vent works! If you ever have your tank pumped and the vent is not venting, You could find your holding tank collapsed.
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Old 30-06-2012, 22:11   #3
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Re: Holding Tank Venting

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Originally Posted by GypsyHawk View Post
Another topic that never dies...

I have had a 1 1/2 inch vent outlet fitted to the tank with the only viable vent outlet to the stern through the transom about 5 to 6 feet away.
Why such a big vent? It should be closer to a 3/4" size.

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The vent pipe run to the transom would be fairly straight with only a slight incline from tank to outlet. Outlet would be a 1 1/2 inch nylon through hull fitting high up on the transom which I would protect from pooping seas with a SS clamshell cover (if I can find one this size).
It's not going to be easy to find one this size and if you did, it wouldn't protect you well in a following sea. A scupper valve would be a better idea.

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My question: is the transom a suitable place for the vent, given it is often the part of the boat most protected from air movement.
See above. My vent is on the side of the boat and I haven't had any issue with the heel of the boat and water coming in the vent.

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Originally Posted by GypsyHawk View Post
And one more question if I may re vented loops as part of the installation: I am installing a vented loop between the head discharge and both down stream outlets - direct overboard or to holding tank (Y valve diversion). The question is, do I need another vented loop between the holding tank and the direct overboard discharge (underwater through hull fitting). The discharge pipe from the top of the tank can run above the waterline before dropping to the through hull outlet. Discharge will be by manually operated diaphragm pump (Whale toilet pump).
If your head (or holding tank) is mounted below the waterline, or if it moves below when the boat heels, you must have a vented loop in a discharge line that connects to a through-hull fitting. Otherwise, if the head's internal valves are held open by debris--an inevitable occurrence--water will siphon back into the boat.

Putting the Y-valve after the tank instead of before it allows you to empty the tank offshore when a pump-out station is unavailable.

It is possible to combine these last two systems into one that provides total flexibility for the varying circumstances you encounter. With a Y-valve before the tank and one after, you can direct head discharge into the holding tank or directly overboard. And you can empty the tank at a pump-out station or offshore. A simple Y-connector allows both overboard discharge hoses to share the same through-hull fitting.
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Old 30-06-2012, 22:18   #4
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Re: Holding Tank Venting

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Originally Posted by foggysail View Post
Suggest you install some type of stink filter on the vent line. And be sure that vent works! If you ever have your tank pumped and the vent is not venting, You could find your holding tank collapsed.
If your holding tank is operating correctly, you should have minimal stink. I use K.O. for treatment and C.P. for cleaning the toilet. No vent, no stink, no problems. Unlike chemical products, it breaks down and neutralizes odor-causing waste materials. Adding a vent into the solution typically causes more problems than it's worth.

Just my $0.02
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Old 30-06-2012, 22:25   #5
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Re: Holding Tank Venting

1-1/2" vent You might as well use it for the pump out. I believe the vent is only required to be 5/8". I placed my vent straight up from the tank just below deck level coming out the side of the hull. Also I installed one of these, and angled it so a wave or rain could not enter into it.

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Old 01-07-2012, 03:15   #6
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Re: Holding Tank Venting

From my research including what Peg Hall writes it seems that a 1 1/2 inch vent is optimal, with 1 inch being a workable compromise. Also, that a filter was not a necessary component if the outlet was of sufficient diameter with optimal run and lay. I do worry about a 1 1/2 inch outlet on a bluewater sailing boat where any part can be underwater in given conditions. A vent outlet direct to the topsides seems very likely to spend a lot of time underwater when heeled. Does a clamshell cover or vent fitting as pictured in the above response really stop water ingress in this situation?

Also, the question remains, is the transom a suitable place for a vent outlet, whether inch and a half or inch?

Thanks for the considered responses so far.
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Old 01-07-2012, 03:52   #7
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Re: Holding Tank Venting

The vent should have a flame gauze in it no different to a fuel vent. Cheers Frank
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Old 06-07-2012, 08:58   #8
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Re: Holding Tank Venting

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The vent should have a flame gauze in it no different to a fuel vent. Cheers Frank

Why? So bugs can clog the screen and stop up the vent causing you to collapse your tank when pumping out next time? I would prefer and vent without a screen that could be reamed with a piece of wire periodically to keep clear, but Im open to new ideas.
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Old 06-07-2012, 09:29   #9
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Re: Holding Tank Venting

Water entering through the vent line should not be a disaster.
I had an 1-1/2" vent fitting high on the topsides that would occasionally be submerged; Worst case, the holding tank gets some water in it.
I think the shorter route would be best.
Perhaps you can run the hose upward or inward (toward the center-line) from the through hull vent fitting before routing it to the tank, to get the hose above the heeled waterline?
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Old 06-07-2012, 15:16   #10
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Re: Holding Tank Venting

Ah theres always more to things than first appears especially on boats.

Brass vent gauze prevents flashback explosions from fume/methane build up. Ignition can be from a lamp flame match cigarette anything. You may not have realised this.

A good vent loops and goes down to just above the waterline. No gauze needed then but a seacock is good practice... Cheers Frank
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Old 06-07-2012, 15:48   #11
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Re: Holding Tank Venting

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Why? So bugs can clog the screen and stop up the vent causing you to collapse your tank when pumping out next time? I would prefer and vent without a screen that could be reamed with a piece of wire periodically to keep clear, but Im open to new ideas.
Well for one thing, litttle bees love to get inside your vent lines and build little clay clogging nests. ALOT harder to get them out of the line then the exposed vent. Dont ask how I know!
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Old 06-07-2012, 17:19   #12
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Re: Holding Tank Venting

the longer the run of the pipe from the tank to the fitting the less venting you will get. You also want to be SURE there are no low spots in it - if water gets in (and it will from time to time) then you will no longer have a vent if it fills the low point.

Also, even if you use only KO and CP (which is what I do) there are occasionally foul smells, especially if the boat has been sitting for a while (and yes, I pump out before leaving it but you never can get every last drop out). I wouldnt want them coming out on the transom which is right where I sit most of the time in my aft cockpit boat.
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Old 06-07-2012, 17:22   #13
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Re: Holding Tank Venting

Quote:
Originally Posted by Lagoon4us View Post
Ah theres always more to things than first appears especially on boats.

Brass vent gauze prevents flashback explosions from fume/methane build up. Ignition can be from a lamp flame match cigarette anything. You may not have realised this.

A good vent loops and goes down to just above the waterline. No gauze needed then but a seacock is good practice... Cheers Frank
----

Do you have some sort of code reference? Anecdotal information regarding and explosion? I don't believe I've heard this or seen it in the USCG code, so I think you need some back-up for such bold statements. The code does prohibit nonexplosion-proof electrical devises in sewage spaces, suggesting we do all realize this.

While sewer gases certainly can reach explosive limits, with a 1 1/2" vent the likelihood is effectively zero. I say this based upon having used a gas sniffer on numerous holding tanks as part of a testing program. In fact, I never saw a tank over 3% LEL, the the H2S in many were lethal enough. I'm betting the CG has done this as well. Comparing a sewer system, where gases can flow and pool to a single tank is incorrect.

Running a loop does insure that the vent will not function as designed, allowing oxygen in and CO2 and methane out.

Adding a screen does greatly increase the chance that sewage with clog the screen, leading to a collapsed tank.

Having a valve on a tank that can change temperature and that will release gasses (lots) when agitated is asking for a ruptured tank.

I think we need to see the back-up.
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Old 06-07-2012, 17:33   #14
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Re: Holding Tank Venting

A scupper valve or clamshell are going to make it difficult to clear the vent if it does plug, though I think that is improbable with such a large vent. Yes, that's big, but it will reduce odors considerably.

The location? On the hook it's good. In a marina, it's right next to the cockpit, which is why vents are generally forward... though that can be a problem on the hook. Water flow into the vent? If it is up high, I can't imagine that being a common problem.
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Old 06-07-2012, 17:38   #15
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Re: Holding Tank Venting

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----

Do you have some sort of code reference? Anecdotal information regarding and explosion? I don't believe I've heard this or seen it in the USCG code, so I think you need some back-up for such bold statements. The code does prohibit nonexplosion-proof electrical devises in sewage spaces, suggesting we do all realize this.

While sewer gases certainly can reach explosive limits, with a 1 1/2" vent the likelihood is effectively zero. I say this based upon having used a gas sniffer on numerous holding tanks as part of a testing program. In fact, I never saw a tank over 3% LEL, the the H2S in many were lethal enough. I'm betting the CG has done this as well. Comparing a sewer system, where gases can flow and pool to a single tank is incorrect.

Running a loop does insure that the vent will not function as designed, allowing oxygen in and CO2 and methane out.

Adding a screen does greatly increase the chance that sewage with clog the screen, leading to a collapsed tank.

Having a valve on a tank that can change temperature and that will release gasses (lots) when agitated is asking for a ruptured tank.

I think we need to see the back-up.
Quite honestly i couldn't care what back up you need? Under the USL Code that AMSA here in Australia adopt/use as their guideline, require all waste tanks, fuel tanks to be gauzed to prevent flash back.

Comparing a sewer system, where gases can flow and pool to a single tank is incorrect.??? Not sure i went down that line in my comment??

As you say a tank "that can release lots of gases when agitated" Can't have it both ways...

Valves on a fuel tank are no different it comes under your management plan, it's a boat you deal with valves.....

Running a loop etc? Even our lagoon vents to the water line from the high point down to the WL ...Even the French do it....They vent to WL so it is an overflow as well to avoid pressurising, in this form they don't gauze which is fair enough.

Cheers
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