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Old 17-03-2011, 21:29   #31
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Re: Holding tank vent

Stick around...I'm typing as fast as I can!

My book (See my signature) is a lot more up to date than that d'd article, but so many toilets have been discontinued and/or new models/technology introduced that now it's no longer as up to date as it needs to be. I'm working on the "2nd edition, revised and expanded" now and hope to have it out in a couple of months.

Meanwhile, the online store at sailboatowners.com is the best source for the current edition. Amazon carries it, but seem to be out of stock more than they have it...I dunno what's going on between them and my publisher.

Meanwhile, I'm glad to answer questions and try to troubleshoot problems. I've never claimed to have ALL the answers...I've learned lot of 'em and I know where to find the rest of 'em.
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Old 17-03-2011, 21:35   #32
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Re: Holding tank vent

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Originally Posted by Therapy View Post
What diameter vent line would be proper for an 18 gal holding tank?
It's not the tank size that determines the size of the vent line...it's how much air can travel in and out of it and how big a hole you're willing have in the side of your boat. 1" should be the minimum... 1.5" is about the max size hole anyone would consider acceptable even on 40' + boat.
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Old 18-03-2011, 14:24   #33
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Re: Holding tank vent

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Originally Posted by peghall View Post
It's not the tank size that determines the size of the vent line...it's how much air can travel in and out of it and how big a hole you're willing have in the side of your boat. 1" should be the minimum... 1.5" is about the max size hole anyone would consider acceptable even on 40' + boat.

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Old 18-03-2011, 14:32   #34
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Re: Holding tank vent

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Not according to the very brief description per the Dometic/Sealand link.

"Protects holding tank from excessive suction by dockside pumps."
The fallacy here is that if the carbon gets wet or exposed to holding tank waste liquids (long phrase for crap) the resistance goes WAY up. In part, this is why the sea land tanks come with vacuum breaks. A safe installation would also require pressure relief and would get complicated.
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Old 18-03-2011, 21:30   #35
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Re: Holding tank vent

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A safe installation would also require pressure relief and would get complicated.
I think you're confusing "safe" with "fool proof" (synonymous with "requires -0- maintenance or attention span"). However no installation could ever be 100% safe/fool proof 'cuz fools are so d'd creative!
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Old 18-03-2011, 22:16   #36
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Re: Holding tank vent

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However no installation could ever be 100% safe/fool proof 'cuz fools are so d'd creative!
Too true!

Since you mentioned 45 degrees may be impossible on many sailboats for the vent, what can we get away with? I know there's no through-hull for my holding tank vent, nor my fuel or water tank vents. I'm guessing they vent inside the boat somewhere, which may explain the smell.
For instance, the holding tank might vent inside my lazarette, but that would involve a 15 foot run.
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Old 18-03-2011, 23:04   #37
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Re: Holding tank vent

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Stick around...I'm typing as fast as I can!


Meanwhile, I'm glad to answer questions and try to troubleshoot problems. I've never claimed to have ALL the answers...I've learned lot of 'em and I know where to find the rest of 'em.
Peg am I right in assuming that the vent line should be as short as possible and as close as possible to the top of the tank (not greater than 45 degrees) or am I missing something?
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Old 19-03-2011, 07:00   #38
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Re: Holding tank vent

Could you also explain the reasoning of not going greater than 45 deg? I'm thinking air doesn't flow vertically as well as horizontally, but would like a confirmation on that. So if I have to go greater than 45, in my case almost vertically, I must go up in size on the vent I take it.
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Old 19-03-2011, 07:12   #39
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Re: Holding tank vent

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Originally Posted by peghall View Post
I think you're confusing "safe" with "fool proof" (synonymous with "requires -0- maintenance or attention span"). However no installation could ever be 100% safe/fool proof 'cuz fools are so d'd creative!


I design equipment for refineries; fool proof and safe are related. Zero maintenance requirement is generally a step in the right direction.
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Old 19-03-2011, 08:50   #40
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Re: Holding tank vent

You have it right...the gasses in the tank are heavier than ambient air, and ambient air--unless it's cold--doesn't fall...so a rise in the line that's steeper than 45 degrees doesn't allow very much exchange of air with gasses.

It's not too difficult to have a horizontal vent line on a powerboat, but the thru-hull has to be pretty close to the toe rail on a sailboat. A larger vent line definitely helps...and STRAIGHT is essential. But sometimes on larger boats a second vent and some mechanical intervention (exhaust fan in one of 'em) is needed. But on most boats, a 1" vent line that isn't longer than 5' and the right tank product works very well. Odorlos is a Norwegian product that I really like because it doesn't use any toxic chemicals. The active ingredient is nitrates, which promote oxygen release from organic material (the waste in the tank). When used as directed, it can completely elimnate odor out the vent.
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Old 19-03-2011, 08:51   #41
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Re: Holding tank vent

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Peg am I right in assuming that the vent line should be as short as possible and as close as possible to the top of the tank (not greater than 45 degrees) or am I missing something?
You got it right.
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Old 19-03-2011, 08:59   #42
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Re: Holding tank vent

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Originally Posted by Zednotzee View Post
Since you mentioned 45 degrees may be impossible on many sailboats for the vent, what can we get away with? I know there's no through-hull for my holding tank vent, nor my fuel or water tank vents. I'm guessing they vent inside the boat somewhere, which may explain the smell. For instance, the holding tank might vent inside my lazarette, but that would involve a 15 foot run.
Oh dear...That's NOT good! CG regs require all fuel and waste tanks to be vented to the outside of the boat because fuel is flammable and waste can produce methane which is also flammable (but odorless). Water tanks can be vented inside the boat, but I wouldn't want to drink water that's had air from most bilges pulled into the tank by the water pump. So I think you need to reroute ALL your vent lines to the outside of the boat somewhere. Btw...you canNOT manifold 'em into a single vent thru-hull. I'll be glad to help you if you'd like to email me.
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Old 19-03-2011, 09:52   #43
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Re: Holding tank vent

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Oh dear...That's NOT good!
Well, it could run to a cowl vent in the lazarette- I haven't managed to trace it all the way. I can say for sure it doesn't go out throught the hull, & it certainly isn't 3/4 to 1 inch.

I suppose once it's warm enough I'll have another crawl throught the boat to track it all down.
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Old 23-03-2011, 11:40   #44
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Re: Holding tank vent

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Well I've heard it all now, so where is the pressure created supposed to be relieved while doing this? as the deck suction should be going to the bottom of the tank below the liquid level.
You need to be doing this while pumping out.
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Old 23-03-2011, 11:45   #45
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Re: Holding tank vent

I've been tempted to modify the vent system on my tanks to have two vents. 1 with a cheap low wattage air pump to keep it ventilated so we don't have to evacuate the boat everytime someone flushes. A good question is why don't they come that way???
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