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Old 12-12-2016, 06:41   #1
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Waste Tank Vent

What I have currently is a 5/8" vent line that is maybe 10' long and snakes around to the vent fitting that is right under the hull to deck joint. Its small and has a convoluted run and while it does of course keep the tank from building pressure, I don't think there is any ventilation, no air getting into the tank.
I'm having triple M build me a new waste tank, I can run a 1" vent hose slightly up hill and have it exit the side of the hull a couple of feet above water line, this would give me a line length of maybe four ft with one very gentle curve in it and a straight shot to the hull, I'd use a 1" SS thru hull fitting.

Any reason not to do this?
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Old 12-12-2016, 07:11   #2
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Re: Waste Tank Vent

Excellent...with one caveat: make sure the vent thru-hull is high enough to stay out of the water at maximum heel. IPs don't heel much, but just a couple of feet above the waterline may not be quite high enough.

Put the vent fitting ON the top of the tank next to the centerline side of it...this will prevent waste from overflowing out the vent when the boat heels.
If you want to discuss fittings placement in more detail than is practical in a forum send me an email...I'll be glad to help you.

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Old 12-12-2016, 07:27   #3
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Re: Waste Tank Vent

I think that's a good idea but I wonder why holding tanks typically have a 5/8" vent. A larger vent wouldn't cost more to manufacture and the installation (for new boats) couldn't be more than a few dollars more.
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Old 12-12-2016, 07:42   #4
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Re: Waste Tank Vent

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Originally Posted by rwidman View Post
I think that's a good idea but I wonder why holding tanks typically have a 5/8" vent. A larger vent wouldn't cost more to manufacture and the installation (for new boats) couldn't be more than a few dollars more.
I don't think there is any consideration for anything other than preventing a pressure build up or relieving the vacuum for pumping out, no consideration for air exchange, and a few bucks here and there is what the difference is in between a high end boat and a "production" one.
I even considered two vents, with scuppers to get some airflow though the tank, then decided that might be overkill.

Peggie, vent location on the tank is at stock location, the inboard side of the tank, but I doubt I'll get any spillage as when the boat would heel to Starboard to submerge the vent on the tank if the tank was completely full, that raises the vent on the hull even higher, and of course when the boat is heeled to port, the vent on the tank will be the highest portion of the tank.

I'm also adding another 1.5" bung in the bottom of the tank with the idea of putting in a macerator there to empty the tank, currently the tank can only be emptied thru the deck fitting via a pump out. I plan on having a ball valve between the macerator and the tank, will this pass the poo police?
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Old 12-12-2016, 07:55   #5
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Re: Waste Tank Vent

If you are planning on relocating the vent closer to the tank watch out for the chainplate filaments and electrical wiring that are concealed in the area above the tank.
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Old 12-12-2016, 08:03   #6
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Re: Waste Tank Vent

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If you are planning on relocating the vent closer to the tank watch out for the chainplate filaments and electrical wiring that are concealed in the area above the tank.

I will, thanks. I hadn't thought about the chain plates.
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Old 12-12-2016, 08:33   #7
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Re: Waste Tank Vent

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I think that's a good idea but I wonder why holding tanks typically have a 5/8" vent. A larger vent wouldn't cost more to manufacture and the installation (for new boats) couldn't be more than a few dollars more.
Ron, they settled on 5/8" because all the hardware manufactures make 5/8" vent fittings for use with fuel tanks. No thought was given to the need for more air movement in a black water tank.
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Old 12-12-2016, 09:04   #8
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Re: Waste Tank Vent

When I replaced my tank, I used a dip tube so the tank cant leak unless it splits. No lower connections all runs out the top of tank. Just a thought. Especially if you have an aluminum tank, I would replace it before everything leaks out.
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Old 12-12-2016, 09:10   #9
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Re: Waste Tank Vent

It's a vent. There's no reason to go bigger, particularly if you have a vent canister filter, which I would never go without.

The only reason to go bigger is if you routinely overfill your tank and the vent tube gets clogged, and if you have a filter you know how expensive that can be and why it should be avoided lol.
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Old 12-12-2016, 09:13   #10
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Re: Waste Tank Vent

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Originally Posted by a64pilot View Post
What I have currently is a 5/8" vent line that is maybe 10' long and snakes around to the vent fitting that is right under the hull to deck joint. Its small and has a convoluted run and while it does of course keep the tank from building pressure, I don't think there is any ventilation, no air getting into the tank.
I'm having triple M build me a new waste tank, I can run a 1" vent hose slightly up hill and have it exit the side of the hull a couple of feet above water line, this would give me a line length of maybe four ft with one very gentle curve in it and a straight shot to the hull, I'd use a 1" SS thru hull fitting.

Any reason not to do this?
If you mean a screened fitting, I think you would be happier with a plain white mushroom. You want to be able to flush.
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Old 12-12-2016, 09:20   #11
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Re: Waste Tank Vent

It will be a plastic tank 3/8" poly I believe, it is replacing a 15 yr old aluminum tank.
Vent will not be screened, I prefer steel thru hulls, but concede that plastic ones work fine.
Purpose of it being larger is not to prevent clogging, but to let oxygen into the tank to encourage aerobic bacteria I believe and cut down on smell.
I would not put a filter on the vent, prevents air getting into the tank, a filter treats the symptom and encourages spread of the disease, you need to treat the disease, not the symptom.

Peggie can explain better I'm sure
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Old 12-12-2016, 12:19   #12
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Re: Waste Tank Vent

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I think that's a good idea but I wonder why holding tanks typically have a 5/8" vent. A larger vent wouldn't cost more to manufacture and the installation (for new boats) couldn't be more than a few dollars more.
Holding tank vents are 5/8" because it was decided that 5/8" was the right size for fuel and water tanks long before holding tanks existed. So when it became necessary to install them, as far as the industry was concerned it was just another tank. The thru-hulls for fuel and water tanks are designed to keep sea water out of the fuel and potable water supply. Not understanding--or even wanting to understand--that fuel and water don't have to be managed, but sewage does, they also use the same thru-hulls on holding tanks. Boat builders and yards are the worst sanitation system plumbers on the planet!

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Old 12-12-2016, 12:36   #13
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Re: Waste Tank Vent

It's not only the vent thru-hulls, Parks...it's also the diameter and length of the vent line that matters. I've seen vent lines as long as 30', running from a tank in the bow to the transom...vent lines with a 360 between the tank and the thru-hull...two toilets 30' apart plumbed to a single tank...one "professionally" installed system with a toilet discharge line on the aft toilet that rose 10', then ran 40' to the tank in the bow, totally oblivious that the toilet in that installation could only lift bowl contents 4'. Etc... I guess I shouldn't complain, though...industry incompetence has been very profitable for me.

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Old 12-12-2016, 13:05   #14
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Re: Waste Tank Vent

Hey A64, regarding your post about thoughts on adding a second vent line to function as an exhaust - if you don't have two lines, one a supply and the other an exhaust, you won't have any real airflow, which is what you're aiming for to keep the aerobic bacteria flourishing and eliminate the anaerobics.

I'm building a sewage tank now. It has a supply and an exhaust. The supply vent is aimed forward, the exhaust aft. Most of my time will be spent at anchor, with the boat aimed into the wind. The exhaust exits less than a foot above the water, with the line formed into a riser which runs to the top of the hull and back down to the tank. The goal is to keep the exhaust as separated as possible from the air we breathe.
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Old 12-12-2016, 16:02   #15
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Re: Waste Tank Vent

...if you don't have two lines, one a supply and the other an exhaust, you won't have any real airflow,

Not necessarily so...a single vent CAN keep a tank aerobic. When air is forced into the tank via a a forward facing thru-hull when the boat has its bow into the wind--and even the slightest bit of air movement qualifies as "wind"--it eventually has to escape. The only place it has to go is back out the same thru-hull from whence it came. And as long as the vent line is straight, doesn't rise sharper than about 45 degrees, and short-5' is the optimal max length, but get as close to that as you can, it works.
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