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Old 08-21-2019, 05:40 PM   #1
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Waste water tank vent location

I am wondering where is located your waste water tank vent (well not yours but the one on your boat)?
Mine, the one on my boat, is aft on the side of the head while the tank is a third of the boat length from the bow.
I will replace the vent hose but the vent is in the worst accessible place and you need to be a contortionist to access it so I am thinking about relocating it. Two easily accessible places would be really aft, above the swim platform, accessible from a head cabinet or mid length on starboard side again accessible from a cabinet.

L
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Old 08-21-2019, 05:46 PM   #2
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Our holding tank vent is located on the stern and there is what looks like a 1" tube that runs from there to the holding tank that is located under the aft bed. We have a second holding tank forward that vents along the bow.

I'm not sure the location is critical as long as it is above the waterline.
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Old 08-21-2019, 05:49 PM   #3
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Our holding tank vent is located on the stern and there is what looks like a 1" tube that runs from there to the holding tank that is located under the aft bed. We have a second holding tank forward that vents along the bow.

I'm not sure the location is critical as long as it is above the waterline.
Well I guess it is better not to have it just below a windows that would allow some odor to come back in but well not easy to find a place with no windows above

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Old 08-21-2019, 05:52 PM   #4
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Originally Posted by Lou_tribal View Post
I am wondering where is located your waste water tank vent (well not yours but the one on your boat)?
Mine, the one on my boat, is aft on the side of the head while the tank is a third of the boat length from the bow.
I will replace the vent hose but the vent is in the worst accessible place and you need to be a contortionist to access it so I am thinking about relocating it. Two easily accessible places would be really aft, above the swim platform, accessible from a head cabinet or mid length on starboard side again accessible from a cabinet.

L

Mine is about 1/3 back from the bow on the starboard side. The holding tank is right in the center of the boat.



I see two reasons for a vent. The first is to provide a way to equalize the pressure inside and outside of the tank. This is both for when contents are added and when the tank is being pumped out.


The second purpose (one that no builder ever seems to consider) is to allow enough air exchange so your tank contents get enough oxygen to keep them from stinking to bad.


Anywhere you put the vent will likely take care of that first purpose, but not the second. The longer the vent line, the less air exchange you will get and therefore the less O2 you get mixed with the holding tank contents. So the shorter the run the better. You might consider adding another vent line instead of just replacing the one you have. If you can put another vent line on the opposite side of the boat that would be great, or aft would be fine as well. I would just make sure that wherever you run it, it doesn't have any dips in the hose.
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Old 08-21-2019, 06:11 PM   #5
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Dhays makes a good point. A loop or U in the vent line will keep water out of the tank if the vent opening gets sprayed or rained on. Boats moving around plus wind can cause water to be pushed into the vent. Loops are cheap in composite and can be had for a sum in bronze.


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Old 08-21-2019, 06:46 PM   #6
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If you consult Peggie's book the preferred arrangement for vent(s) is to have 2 - one either side as large and as straight as possible w straight bore thru hull no screens, baffles, etc.
I installed a second large straight vent on previous boat and it fixed an ongoing odor problem. Thank you Peggie
If above is not possible there are other alternatives... one large or add a bubbler to inject air.
I installed an inexpensive fish tank bubbler that cured a major odor problem on current boat. Details available if anyone interested just PM me and I can provide a link.
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Old 08-21-2019, 08:06 PM   #7
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If you consult Peggie's book the preferred arrangement for vent(s) is to have 2 - one either side as large and as straight as possible w straight bore thru hull no screens, baffles, etc.
I installed a second large straight vent on previous boat and it fixed an ongoing odor problem. Thank you Peggie
If above is not possible there are other alternatives... one large or add a bubbler to inject air.
I installed an inexpensive fish tank bubbler that cured a major odor problem on current boat. Details available if anyone interested just PM me and I can provide a link.
+1. Absolutely correct. I had an 80 gallon holding tank in a previous boat that would knock you off the dock when you flushed a head. Talked to Peggie and I didn’t have a way to run a second vent so I put in a Groco Sweettank and it fixed the problem completely. I never added any chemicals for the rest of the 8 years I owned the boat and absolutely no smells.

First try to add a second vent, if not able put a bubbler in. And don’t add a vented loop. You want to be able to flush the vent with a hose to keep it clear if some sewage gets up into the vent hose. Don’t worry about water getting into a holding tank vent.
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Old 08-21-2019, 08:11 PM   #8
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I installed an inexpensive fish tank bubbler that cured a major odor problem on current boat. Details available if anyone interested just PM me and I can provide a link.
I copied the idea from Bacchus and it works great. Easier than trying to install a second air vent in my case.
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Old 08-21-2019, 08:38 PM   #9
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Some precision, I do not have any odor issue or any other issue, just that I want to replace the aging vent hose and the vent is accessible if you are a hobbit that is gymnastics world champion lol
So the question about where your vent is so to relocate it in a more accessible place in case of maintenance.

L
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Old 08-21-2019, 08:39 PM   #10
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If you consult Peggie's book the preferred arrangement for vent(s) is to have 2 - one either side as large and as straight as possible w straight bore thru hull no screens, baffles, etc.
I installed a second large straight vent on previous boat and it fixed an ongoing odor problem. Thank you Peggie
Yes, yes, yes.

We upgraded ours to include a second vent. Now have TWO massive 1-1/2" vents, on either side of the forward quarter, one at each end of the elongated tank.

Adding an Additional Holding Tank Vent
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Old 08-21-2019, 09:06 PM   #11
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Some precision, I do not have any odor issue or any other issue, just that I want to replace the aging vent hose and the vent is accessible if you are a hobbit that is gymnastics world champion lol
So the question about where your vent is so to relocate it in a more accessible place in case of maintenance.

L
If I were going to do the work on it I would go ahead and add the second vent. If I am going to work on the holding tank it is going to be as good as possible when I am done. If you don’t add a second vent at least go to a larger vent, as big as possible up to 1.5”.
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Old 08-21-2019, 10:45 PM   #12
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If you consult Peggie's book the preferred arrangement for vent(s) is to have 2 - one either side as large and as straight as possible w straight bore thru hull no screens, baffles, etc.

You need to read it again, Don...'cuz I do NOT say that two vents are the preferred arrangement....see page 47:

"So the tank vent line should be as short, as straight, and as horizontal as possible, with no sags, no arches, and no bends. The minimum I.D of the hose (which is the “standard” size in use today, but for no reason other than being “standard” in fresh water and fuel tanks) is 5/8”. However, I recommend that it be at least 1” or even larger if the boat is large enough to let a larger vent thru-hull be cosmetically acceptable. Ideally, it should be no more than 3’ long, but no longer than 5’.

"If it has to be much longer, or if running the vent line uphill at angle sharper than 45 degrees can’t be avoided, or if it’s impossible to run a vent line that does not go around a corner, it may be necessary to install a second vent line in order to create cross ventilation, or install some mechanical means of forcing air through the tank."

I dunno how you manage to translate that into "Peggie's preferred arrangement is two vents."

You didn't get this quite right either:
I see two reasons for a vent. The first is to provide a way to equalize the pressure inside and outside of the tank. This is both for when contents are added and when the tank is being pumped out.

Tank vents don't "equalize" pressure, they simply provide an escape for air displaced by incoming contents and a source of air to replace contents as they're drawn out. This applies to all tanks--fuel, water and waste. Aerobic/anaerobic matters only in waste tanks, both sewage and gray water.

Replies to several other comments:

A loop or U in the vent line will keep water out of the tank if the vent opening gets sprayed or rained on. Boats moving around plus wind can cause water to be pushed into the vent.

Only if the vent thru-hull is at the boat's waterline, which is a very bad location for it Spray, splashing or rain couldn't put enough water to notice into any vent. If you think it can, lay a 2 liter soda bottle on its side and aim a garden hose with the nozzle set to spray at it from just a foot away and see how much water you can put the bottle.

But even if splashing, rain and spray could put water into the tank, so what? It's obviously necessary to keep sea and rain water out of f.uel and water tanks, but we're talking about sewage holding tanks here! A whole bucket full of water down the vent would only dilute it a little.

I'm not sure the location is critical as long as it is above the waterline


Location DOES matter a LOT! Your forward tank is vented correctly..in fact, ideally except for replacing the "vent" thru-hull with an open one! But as for your aft tank...an aerator is the only way to prevent odor out a vent through the transom. Boat builders vent out the transom in a misguided attempt to send odor away from the boat. It doesn't do the people downwind of your boat any favors though!



And finally...


Lou...looking at the photo of your boat and going by your description of the tank location, I'd run a new 1"--or even 1.5"--vent line off the top of the tank at the forward end of it (installing a new vent fitting in a plastic tank is actually pretty easy, thanks to a little gadget called the Uniseal UNISEAL ) running the line forward to ONE new open bulkhead or "mushroom" thru-hull. Your photo makes it appear that you can bring it out only about 3' aft of the bow, which would be ideal because air would be forced in--and thererfore also out again--to the tank when the boat is underway, on an anchor or even at the dock as long as there's even the lightest breeze. Using No-Flex Digestor Noflex Digestor in the tank will ensure you'll never have odor out the vent.

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Old 08-22-2019, 05:12 AM   #13
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Thank you Peggie!

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Old 08-22-2019, 05:58 AM   #14
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Peggie,..
My apologies for misinterpreting your advice.
It has been awhile since I pulled your book out and reread it. I just recall adding a second vent on my previous boat after receiving and reading your book and it made a world of difference (original vent was next to impossible to access to enlarge).

The pressure equalization comment wasn't mine but IMO? technically Dave is correct...
Without a pressure differential there will be no flow... flow is a function of the P differential. If one adds contents to the tank and no vent the P will increase. With the vent is increases (very little) and the vent allows the flow (escape) until the P are the same. Wind creates a pressure that can induce a flow back in. The only other drivers can be gravity due to a specific gravity difference of the contents and/or a temp difference that causes convection.
Think of a water filled hose... if both ends are the same height there's no flow. If you raise one end you create a head pressure and the water flows until there is zero differential.
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Old 08-22-2019, 08:21 AM   #15
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You're forgiven. It was late...I'd had a long frustrating day and I was tired. So I over-reacted just a bit.


As for pressure equalization...you and Dave are prob'ly correct and I may be splitting semantic hairs, but it's gonna take more conversation than is practical in a forum so I need to talk with one of my engineer friends about it.


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Old 08-22-2019, 10:52 AM   #16
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I thought I read that a loop was needed to keep water out.
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Old 08-22-2019, 12:07 PM   #17
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You need a loop on fuel vents to keep water out but on the holding tank a loop will just introduce possible clogging. It doesn’t matter if a bit of spray gets into the holding tank as long as it is not positioned so it picks up a lot of water.
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Old 08-22-2019, 05:02 PM   #18
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I thought I read that a loop was needed to keep water out.

It's very important to keep water--especially sea water, but even lake or river water too--out of fuel and water tanks...so you weren't very far off base. It's just not necessary to keep it out of waste tanks, while it's very important that there be nothing in waste tank vent lines that could clog 'em or impede the free exchange of air with gasses in the tank that are generated by organic matter as it breaks down. Which is why waste tank vents need to be as short, straight and as horizontal as possible...and also at or slightly above the level of the top of the tank.


Re-reading my reply to you last night, I realize that while I'm always pretty straightforward and "to the point" in my comments, I went a bit over the top in my reply to you too...I apologize.



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Old 08-23-2019, 09:25 AM   #19
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It's very important to keep water--especially sea water, but even lake or river water too--out of fuel and water tanks...so you weren't very far off base. It's just not necessary to keep it out of waste tanks, while it's very important that there be nothing in waste tank vent lines that could clog 'em or impede the free exchange of air with gasses in the tank that are generated by organic matter as it breaks down. Which is why waste tank vents need to be as short, straight and as horizontal as possible...and also at or slightly above the level of the top of the tank.


Re-reading my reply to you last night, I realize that while I'm always pretty straightforward and "to the point" in my comments, I went a bit over the top in my reply to you too...I apologize.



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Old 08-23-2019, 10:01 AM   #20
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I installed two 1.5” vents to my new tank and I use Noflex as per directions religiously. One vent is just forward of where the hull begins to taper to the bow and one is on the flat side and the side one is maybe a foot long. This provides a flow of air while I am running, from the front higher pressure area to the rear. The tank performed flawlessly although if you are collecting pee you still need Noflex!

Don’t put a vent above your transom as it will spoil your swimming activities. The fact is that you have a tank full of sewage and a happy sewage tank will smell like its full of sewage a bit, occasionally until the Noflex gets to it but if you have a healthy tank it won’t be anaerobic which REALLY stinks!

Lou, if you can get access to where you want your new vent, try using schedule 40 pipe. Its easy to use and will never permeate and you can use these great threaded/sealed joints so with a little planning you can unscrew and remove the entire octopus and remove it without tools, if you ever need to again. No more hose clamps or “5 year hose” ever again. The connectors attach to the through-hulls with threaded fittings directly.

All my plumbing is now either Pex/Sharkbite or schedule 40. I’ve done my last poop hose change and had my last plumbing leak. Now if my guest didn’t break the toilet seat I’d be done with sewage!

And a shout-out to Peggy, I attended a lecture of yours at Grand Banks Rendezvous when I had a GB and that has guided my efforts. Thank you.
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