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Old 07-02-2019, 10:04   #1
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Holding tank venting

Hi, I am going to be increasing the size of the holding tank capacity on our boat. It is a Nordic Tug 32.

The existing tank is glassed into the keel (about 30 gallons). As there is no easy way of getting to this and adding more capacity we plan to add another tank above it of about 25-30 gallons that will sit about 12 inches above the top of the existing one and off to the side on a shelf.

This new tank will be connected via hose to the lower tank so the idea is once the existing tank fills up the new tank will be the overflow for the additional 30 gallons. The head is a Vacuflush and the pump unit will be roughly at the height of the top of the new tank.

So, 2 questions

1) Does anyone see any issues with this set up.

2) I'm trying to aerate the new set up as much as possible. I was thinking about installing 2 vents, hopefully cross vented but it may be one side vent and one roof vent and probably 1 inch vents to maximise the air flow. My concern is with having a lower tank in the keel that is plumbed into the top tank and only the top tank is vented. Will having large cross vents in the top tank be enough to aerate the lower tank?

Many thanks!
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Old 07-02-2019, 10:56   #2
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Re: Holding tank venting

Hi Nick...welcome aboard cruisers.com!

I definitely have issues with that setup if I understand you correctly. You're saying that your toilet will continue to flush into the existing LOWER tank and waste is supposed to rise into the upper tank when that one is full. Bad idea for a bunch of reasons, not the least of which is, liquids run DOWNhill, which is what any contents in the upper tank will be determined to do...and at 8 lbs/gal, even 2 gals would be more downward pressure against the contents pushing up than they could overcome. But it's a problem easily solved/prevented: route the line from the vaccum pump to the UPPER tank and let waste flow into the lower one from it.

Will having large cross vents in the top tank be enough to aerate the lower tank?

No. It will be necessary to vent both tanks. And you definitely want to go with 1" vents on both. A li'l gizmo called the Uniseal UNISEAL makes it pretty easy to add/relocate fittings on a tank. Whether to cross vent or not depends on the tank(s) location in the boat and how long the vent lines will have to be.

--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein
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Old 07-02-2019, 11:54   #3
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Re: Holding tank venting

Hi Peggie, thanks so much for the quick reply (I bought your book a couple of weeks ago by the way, good reading!).

Sorry I may not have worded it correctly in my previous post but I do intend to connect the new higher tank as the first entry point. So yes gravity will move contents from the top new tank into the old lower tank.

For the lower existing tank venting, presumably as long as I have the vents for that lower tank exiting above the new tank height I should be ok right? Or is there something else that will dictate the height for the lower tank vents?

Would you also go for 1 1/2 inch vents over 1 inch vents? Is there a downside to making the holes larger?

Also, would you make the connection between the 2 tanks via hose or rigid plastic pipe?

Many thanks! Nick
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Old 08-02-2019, 04:20   #4
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Re: Holding tank venting

Greetings and welcome aboard the CF, Nick.
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Old 08-02-2019, 07:10   #5
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Re: Holding tank venting

For the lower existing tank venting, presumably as long as I have the vents for that lower tank exiting above the new tank height I should be ok right? Or is there something else that will dictate the height for the lower tank vents?

No need for the vent thru-hull to be higher than the new tank. As far as venting is concerned, each tank is a separate stand-alone installation. If you have my book, you should know that ideally, both vent lines just need to be as short, straight and as close to horizontal as possible.


Would you also go for 1 1/2 inch vents over 1 inch vents? Is there a downside to making the holes larger?

In my seminars I'm fond of saying that IMO all vent lines should be at least 3", but I wouldn't want a hole that big in the side of my boat either. So whether you should go with a 1" vent or 1.5" really depends on whether a 1.5" thru-hull is cosmetically too big or not.


Also, would you make the connection between the 2 tanks via hose or rigid plastic pipe?

If you only need fittings at both ends of the pipe, you can use sched 80 PVC. If you'd have to insert any fittings in that line go with top quality rubber hose...my choice would be Raritan SaniFlex RaritanSaniFlex hose 'cuz it's proven to be totally odor permeation resistant and is so flexible that it can be bent like a hairpin without kinking.



You might want to consider emailing me a drawing of your proposed installation. To email, click on my photo at the upper left of this post...you'll be presented with a choice of PM or email...choose email. It's possible that, as a brand new member you won't have that choice yet. If so, send me a PM and I'll reply with my email address.



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Old 08-02-2019, 08:09   #6
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Re: Holding tank venting

Quote:
Originally Posted by peghall View Post
...
No need for the vent thru-hull to be higher than the new tank. As far as venting is concerned, each tank is a separate stand-alone installation. If you have my book, you should know that ideally, both vent lines just need to be as short, straight and as close to horizontal as possible...
... You might want to consider emailing me a drawing of your proposed installation...
--Peggie
If the 2 tanks are interconnected (gravity feed), then, I think, they are as one, not 2 separate.
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Old 08-02-2019, 09:55   #7
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Re: Holding tank venting

Nope, both tanks must be vented. Among other reasons, USCG regs require that ALL black and gray water tanks must be vented to the outside of the boat. There are other reasons, but that one over-rides any other pros and cons re venting only one tank.



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Old 08-02-2019, 11:10   #8
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Re: Holding tank venting

Many thanks everyone for all your advice so far! Peggie, I did read the part in your book about the vents being short, straight and horizontal and that is what we are planning to do where we can. But, I'm a bit confused over the physics of the suggestion of having the lower tank vent hull exit hole lower than the high tank?

I can certainly email over a diagram, I'm just traveling at the moment so can't do that right now.

So both tanks will be connected, imagine 2 identical tanks, one straight above the other one. They are connected by a short hose. Surely if there are enough contents that the top tank is full and the lower tank vent hull exit hole is below the top tank won't the pressure of fluid in the top tank force out liquid in the bottom tank vent? It's been a while since I was in a physics class so I might be missing something

Cheers, Nick
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Old 08-02-2019, 15:35   #9
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Re: Holding tank venting

Why would the upper tank be full unless the lower tank is also full?

imagine 2 identical tanks, one straight above the other one. They are connected by a short hose.

Unrestricted pass through...no shutoff valve? Without a shutoff valve, how CAN the upper tank fill up faster than the lower tank?

Surely if there are enough contents that the top tank is full and the lower tank vent hull exit hole is below the top tank won't the pressure of fluid in the top tank force out liquid in the bottom tank vent?

The only thing that can force liquid out either vent is contents in that tank that have risen to the level of the vent fitting in that tank...iow, an overfull tank. Whether either vent line goes up, down or sideways, or one exits the hull higher than the other is irrelevant.

I have an idea: When you get home and have the time to do it, buy two plastic boxes--the kind with snap on lids that Walmart sells in their housewares dept, a piece short piece of PVC that can be the conecting pipe and some 1/4" clear plastic tubing to approximate your vent lines. Drill holes in both that approximate where the connecting pipe and vent fittings will be in your proposed installation and run the vent lines where you think they should be relative to the tanks. Then try to create a situation in which pressure from the top tank can force liquid out the bottom tank vent line. Try to create ANY situation in either tank that can force anything in the other tank out the vent.

Should be a fun project!

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Old 08-02-2019, 16:22   #10
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Re: Holding tank venting

Thanks Peggie. Yes the top tank will only be full when the bottom tank is full. So liquid enters the top tank, then flows into the lower tank which fills up first and then the upper tank fills up via the hose connecting the 2 tanks.

The situation I'm trying to fathom is where both tanks are full and your suggestion of having a thru hull vent for the lower tank below the upper tank. My reasoning is telling me it will be venting out liquid as there will be contents higher up above it. Does that make sense? Thanks. nick
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Old 08-02-2019, 18:22   #11
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Re: Holding tank venting

Re the shut off valve between tanks, I was thinking of just having them connected with an open hose, no valves to keep it simple. So it is essentially one tank, sort of.
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Old 08-02-2019, 19:32   #12
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Re: Holding tank venting

Ok. I finally get what you're asking: So yes...if the lower tank is so full that it's already overflowing out its vent and the upper tank continues discharging into the lower tank, waste will continue to flow out the lower tank's vent thru-hull for as long as that keeps happening--not because of any pressure from above, but because the upper tank is replacing what's going out the lower tank's vent and will continue to do so as long as there's anything in the upper tank. That will happen no matter how high the vent thru-hull is on the hull. But the length of the vent line(s) and degree of rise does matter when it comes to preventing odor out the vent by keeping the tanks aerobic.



The solution: a tank level indicator on the lower tank and an owner with enough of an attention span to monitor it and make sure to pump or dump the tanks before that can happen. 'Cuz if you treat either of the vent lines as an "overflow," waste WILL build up in them and block the vents. Putting all fittings on the top of the tank increases its capacity by at least 5% and delays waste overflowing into the vent until it's filled the very last cubic millimeter in the tank...at which point waste from the upper tank will begin fill the inlet hose too.



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Old 09-02-2019, 23:08   #13
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Re: Holding tank venting

Thanks Peggie for your insights. Are you saying that 2 tanks stacked one above the other with just a hose connecting them are not going to work? It sounds like you are saying that the lower tank will always overflow out the thru hull vent before the upper tank can fill up? Regardless of how high up the lower tank thru hull vent is?

Just to clarify, the contents would flow from the head, into the top tank and down through a hose into the bottom tank. When the bottom tank is full the contents would then start filling up the top tank. It's effectively one tank but with a real skinny waist.

My plan was to install the lower thru hull vent above the level of the top tank to avoid it overflowing as the top tank filled up.
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Old 10-02-2019, 05:13   #14
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Re: Holding tank venting

As a consequence of Pascal's principle, wherein a change in the pressure, at any point, in an enclosed fluid that, is at rest, is transmitted, undiminished, to all points in the fluid and in all directions.
"Water seeks its own level"; hence, the water levels in the holding tank, and the vent pipe must be the same.

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Old 10-02-2019, 10:34   #15
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Re: Holding tank venting

Hi Gord, thanks for the diagram. That is how I was thinking it works too.

So in your diagram 1 illustration, if the vent was lower than the water level then the contents would empty to the height of the vent.

In my set up, my first tank, the funnel one, would be above the cylindrical one. If the vent to the lower cylindrical one is below the funnel tank then the funnel tank would not hold any liquid. Do my physics make sense to you?
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