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Old 03-05-2013, 05:54   #1
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Vent Tube Size?

I was curious if there is a minimum ID (inside diameter) of a vent tube from the waste tank to the outside of the boat?
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Old 03-05-2013, 06:33   #2
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Re: Vent Tube Size?

Generally speaking, the bigger the better. Poor air circulation, because of a too-small vent line, is one of the leading causes of bad head odors. I've even considered multiple vent lines, to get some cross-flow ventilation.
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Old 03-05-2013, 07:01   #3
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Re: Vent Tube Size?

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Originally Posted by denverd0n View Post
Generally speaking, the bigger the better. Poor air circulation, because of a too-small vent line, is one of the leading causes of bad head odors. I've even considered multiple vent lines, to get some cross-flow ventilation.
Indeed.

See also ➥ http://www.practical-sailor.com/issu...s_10949-1.html
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Old 03-05-2013, 07:18   #4
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Re: Vent Tube Size?

I don't know if there's a volume to I.D. ratio, but multiple ones make sense in some ways. Two 5/8" vent lines going through a Y-connector to a 3/4 well above the waterline and with a gooseneck from (say) a 50 gallon waste tank sounds about right, and would be helpful, too, at pump outs.

Should I invoke Saint Peggie Hall here?
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Old 03-05-2013, 13:31   #5
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Re: Vent Tube Size?

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Originally Posted by BlackOak View Post
I was curious if there is a minimum ID (inside diameter) of a vent tube from the waste tank to the outside of the boat?
Why? There's no law on a minimum size, but as others have posted, the bigger the better. Pretty much you would normally match the hose size to the outlet on the holding tank.

If you are building a boat and having a custom holding tank built, one or even two one inch vents would be a lot better than what comes on most production boats. have one vent fitting put on each side of the tank and run vent hoses from these to each side of the boat.
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Old 04-05-2013, 00:09   #6
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Re: Vent Tube Size?

Thanks! The vent fitting built into the waste tank is 1/2" od. Guess I need to get a different waste tank, with a bigger vent fitting.
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Old 04-05-2013, 00:37   #7
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Re: Vent Tube Size?

There are water proof fittings that can be easely fitted to your tank in most any size you might want.
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Old 04-05-2013, 00:48   #8
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Re: Vent Tube Size?

If they are real small or plugged up one can have quite the explosion. You can ask my ex in-laws about that!
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Old 04-05-2013, 06:04   #9
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Re: Vent Tube Size?

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Thanks! The vent fitting built into the waste tank is 1/2" od. Guess I need to get a different waste tank, with a bigger vent fitting.
That's not necessary. A "fitting relocating kit" is available from West Marine. Check to see what sizes are in the kit.
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Old 04-05-2013, 06:21   #10
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Re: Vent Tube Size?

trying to wrap my head (no pun intended) around this. My plastic waste tank has 3 fittings. The discharge fitting @ 2.5", the input fitting about 2", and the vent fitting in 1/2".

So...do I cap off and seal my present vent fitting, then cut a larger hole into my waste tank and install a new larger fitting? Then bore a hole through the deck or hull of me boat to make a larger venting system?

Or

Use the present 1/2" vent fitting, then use a larger adapter screwed/mounted/attached onto the 1/2" vent fitting to increase it to 3/4 of an inch or larger, then bore a hole into me boat to accomodate the larger vent line?

The bore-a-hole is a little unsettling for me.
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Old 04-05-2013, 06:59   #11
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Re: Vent Tube Size?

Quote:
Originally Posted by BlackOak View Post
trying to wrap my head (no pun intended) around this. My plastic waste tank has 3 fittings. The discharge fitting @ 2.5", the input fitting about 2", and the vent fitting in 1/2".

So...do I cap off and seal my present vent fitting, then cut a larger hole into my waste tank and install a new larger fitting? Then bore a hole through the deck or hull of me boat to make a larger venting system?

Or

Use the present 1/2" vent fitting, then use a larger adapter screwed/mounted/attached onto the 1/2" vent fitting to increase it to 3/4 of an inch or larger, then bore a hole into me boat to accomodate the larger vent line?

The bore-a-hole is a little unsettling for me.
Look at the fitting relocation kit. I have never used it or inspected it but I believe it has several fittings and plugs for unused fittings and instructions for installing the new outlets.

You have a really good option here. Use the existing 1/2" outlet and hose, then add a larger outlet and hose to the other side of the tank and install a matching thruhull on the other side of the boat. This will provide cross ventilation.

Adapting the existing 1/2" outlet to a larger hose will not be an improvement.

If you're afraid to bore a hole in your boat for a vent, have a pro do it. It's not that hard.
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Old 04-05-2013, 21:11   #12
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Re: Vent Tube Size?

Me I would use a 1/2 by 5/8 increaser and just use the 1/2" port. It will act as a slight restrictive orifice but the pressure drop across it will be minimal compared to a 1" port at low flow velocities.

Then use 5/8" hose or even 3/4" hose (ID). For a short run say under 8-10 feet that will be fine. If you have a long run or electric toilets use one size larger hose. The electric toilets have a higher flow velocity so move more air per second. Also longer runs have a higher pressure drop then short runs, so a larger size vent hose would be better.
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Old 05-05-2013, 05:55   #13
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Re: Vent Tube Size?

Thanks all for the advice on this topic. It has been a great help to improve my venting system.
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