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Old 27-12-2017, 10:33   #1
djs
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Internal vent for holding tank. Madness or not?

Hi All, and besr wishes for the new year.

I'm about to plumb in the holding tank. New install in an old boat. The tank isnt close to the bow and thus if the vent line us to be kept short, as recommended it needs to exit the hull at a point where immersion when heeled is inevitable.

If i can avoid drilling a(nother) hole in the hull i'd like to.

Is it at all possible to keep the vent internal? I.e. in a locker up against the deck with a big charcoal filter attached.

Or am I being totally unrealistic?

Cheers
Dave
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Old 27-12-2017, 10:37   #2
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Re: Internal vent for holding tank. Madness or not?

If you read Peggy Halls book, it tells you the lunacy of installing a vent filter to anywhere. The holding tank needs to be able to "breathe" in order for the bacteria to digest the solids. Read her book, and she does a great job of explaining it.
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Old 27-12-2017, 10:43   #3
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Re: Internal vent for holding tank. Madness or not?

Quote:
Originally Posted by djs View Post
Hi All, and besr wishes for the new year.

I'm about to plumb in the holding tank. New install in an old boat. The tank isnt close to the bow and thus if the vent line us to be kept short, as recommended it needs to exit the hull at a point where immersion when heeled is inevitable.

If i can avoid drilling a(nother) hole in the hull i'd like to.

Is it at all possible to keep the vent internal? I.e. in a locker up against the deck with a big charcoal filter attached.

Or am I being totally unrealistic?

Cheers
Dave
No! Only do this if you want sewer gases in the cabin. No charcoal filter will keep your cabin from smelling like a sewer. And think what will happen when you accidentally overfill the tank, which you will do some day.

Some alternatives that might, or might not work for you:
Our boat tanks are vented through the deck fitting. Works well. No extra holes in hull. I think Plastimo makes the vented deck fitting.
How about a riser pipe inside a stanchion?
Maybe a vent on the side of the cabin above deck level?
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Old 27-12-2017, 10:55   #4
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Re: Internal vent for holding tank. Madness or not?

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If you read Peggy Halls book, it tells you the lunacy of installing a vent filter to anywhere. The holding tank needs to be able to "breathe" in order for the bacteria to digest the solids. Read her book, and she does a great job of explaining it.
Ms Hall might do a great job explaining the theory, but there is NO way in the real world you can get enough air into a holding tank to keep it from going anaerobic. Not without actually pumping air in and stirring it.

Just think about this for a second... If you put your sewage in an open bucket, and just let it sit there, are you telling me it will not stink? Even in an open bucket the liquid at the bottom will be anaerobic in very short order unless it is rather strongly agitated.

Vent filters are bad ideas for other reasons. Not the least of which is if the tank is ever overfilled, they clog. Now you have an unvented tank and very, very bad things will happen, up to having the tank rupture from building pressure inside.
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Old 27-12-2017, 11:27   #5
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Re: Internal vent for holding tank. Madness or not?

Quote:
Originally Posted by djs View Post
Hi All, and besr wishes for the new year.

I'm about to plumb in the holding tank. New install in an old boat. The tank isnt close to the bow and thus if the vent line us to be kept short, as recommended it needs to exit the hull at a point where immersion when heeled is inevitable.

Is it at all possible to keep the vent internal? I.e. in a locker up against the deck with a big charcoal filter attached.
Nope...in fact not only is it a very bad idea, but it's illegal, at least in the US. Because methane is flammable, USCG regs REQUIRE that all waste tanks be vented to the outside of the boat. A a static electricity spark in that locker could blow up your boat!

Unless you routinely heel far enough to put your rails in the water for an extended amount of time, dunking the vent thru-hull briefly even repeatedly isn't likely take on more than a few spoonfuls--if that much--of water...and a clamshell cover over it will keep that out.

If you can keep the vent line straight and it doesn't rise sharper than 45 degrees, it can be 5-6' long.

Send me a PM if you'd like to discuss the whole installation in more specific detail than is practical in a forum.

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Old 27-12-2017, 11:32   #6
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Re: Internal vent for holding tank. Madness or not?

Ms Hall might do a great job explaining the theory, but there is NO way in the real world you can get enough air into a holding tank to keep it from going anaerobic.

It's not just theory...it CAN be done, and is done by many many people...you just have to know how to do it.

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Old 27-12-2017, 11:51   #7
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Re: Internal vent for holding tank. Madness or not?

Well, I suppose I was clutching at straws .

Thanks for the responses everyone. And Hi PegHall. Word in the uk was that you had retired, so lovely that you were both online and took the time to reply.

Now where’s that hole drill...

All the best
Dave
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Old 27-12-2017, 20:28   #8
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Re: Internal vent for holding tank. Madness or not?

"Nope...in fact not only is it a very bad idea, but it's illegal, at least in the US. Because methane is flammable, USCG regs REQUIRE that all waste tanks be vented to the outside of the boat. A a static electricity spark in that locker could blow up your boat!"

EWWWW!!!
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Old 27-12-2017, 21:13   #9
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Re: Internal vent for holding tank. Madness or not?

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Originally Posted by peghall View Post
Nope...in fact not only is it a very bad idea, but it's illegal, at least in the US. Because methane is flammable, USCG regs REQUIRE that all waste tanks be vented to the outside of the boat. A a static electricity spark in that locker could blow up your boat!

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Old 28-12-2017, 08:36   #10
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Re: Internal vent for holding tank. Madness or not?

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Originally Posted by peghall View Post
Nope...in fact not only is it a very bad idea, but it's illegal, at least in the US. Because methane is flammable, USCG regs REQUIRE that all waste tanks be vented to the outside of the boat. A a static electricity spark in that locker could blow up your boat!
You need 5% Methane in Air to create an explosive atmosphere which is actually a lot of Methane inside a boat. 5% is 1 in 20 volumes. Highly unlikely a "bad" black water tank would be able to produce that much especially without you being able to smell it first.

Correct that Methane is flammable or we'd not be able to cook our dinner with it but unless you reach that LEL (Lower Explosive Limit) you will NOT be able to blow up your boat with it. Same goes for Hydrogen from battery charging which is 4% LEL

Venting inside is just plain wrong on so many other levels.
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Old 28-12-2017, 08:49   #11
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Re: Internal vent for holding tank. Madness or not?

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You need 5% Methane in Air to create an explosive atmosphere which is actually a lot of Methane inside a boat. 5% is 1 in 20 volumes. Highly unlikely a "bad" black water tank would be able to produce that much especially without you being able to smell it first.

Venting inside is just plain wrong on so many other levels.
But if you really had aerobic bacteria in your head as some people think you can have, there would be ZERO methane...

Methane only comes from anaerobic digestion, which is pretty much the only thing that happens in a holding tank.
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Old 28-12-2017, 09:19   #12
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Re: Internal vent for holding tank. Madness or not?

You're kidding, right?
If you can't find a way to vent it outside, hire someone who can.
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Old 28-12-2017, 09:31   #13
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Re: Internal vent for holding tank. Madness or not?

Every time you pump a new load into the tank, it compresses what is already in there, it has to go somewhere. I can tell when someone is pumping from the cockpit, on a still day at anchor. Not venting is just not possible...you soon wouldn't able to pump your head against the pressure. And as said earlier, it is methane, don't want it in an enclosed space to possibly reach explosive limits, let alone the stink. Mine is right under the cap rail on the port side midships, where the tank is. It's an Island Packet so I never submerge it for any length of time. It also has a very fine screen on the hole that is pointed aft, very little if any water could get in there if you are moving forward. BTW, if memory serves, Peggy recommends (or might be Calder) two vents, one on each side for cross ventilation if possible.
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Old 28-12-2017, 10:07   #14
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Re: Internal vent for holding tank. Madness or not?

I'm kinna surprised (or I missed it) that no one mentioned overfills.

Although I love reading outside-the-box thinking, all vents go outside, even freshwater. I've seen a boat nearly sink due to a freshwater overfill. Many thousands in damage.

The sole exception is batteries. Even then it would be nice, but practicality rules. Hydrogen generation is slow, much lighter than air, and dissipate quickly. Acid gases don't make it out of the box before condensing.
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Old 28-12-2017, 10:31   #15
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Re: Internal vent for holding tank. Madness or not?

Vents for the holding tank and the battery box must go outside. The LEL for methane is 5% which is surprisingly low, and for hydrogen is even lower (2%).
Hydrogen from batteries is a different matter. When charging, the hydrogen escapes from the fillers and somewhere across the top of the batteries will be a 4% explosive mixture. When you apply a spark in that area you will definitely get an explosion. This is why you must wave your hand across the top of the batteries to disperse any gases, and always connect the positive first and connect the negative last, and when disconnecting battery terminals, disconnect the negative first and the positive second.
As I adjust my hearing aid it reminds me of the time that I did not do that and the battery top exploded covering me with acid and the 'pop' destroying my hearing in one ear!
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