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Old 02-11-2018, 10:02   #1
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Holding tank vents

Having issues last year with one of my small 15 gal. Head holding tanks not wanting to gravity flush without blowing pressurized air back through vent. 16 year old boat, not sure the size of vent lines but think maybe only 5/8". Boats in Guatemala I'm headed back soon. I want to redo vents is 3/4" big enough or up to 1 1/8"- 1 1/4". Defender jumps to that size for plastic above water line through hulls. Also what is the best sealant to use around those fitting especially in the tank. Tank sits above waterline, vent hose is maybe 4' long. Messy job not looking forward to it.
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Old 02-11-2018, 10:11   #2
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Re: Holding tank vents

You know, the problem may be with the tank or the drain. Gravity isn't much. Do you have trouble pumping in to the tank? You only need a tiny hole to admit enough air to drain, and it sounds like you have that. Think of how small the vent is in a jerry can. Maybe not.
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Old 02-11-2018, 10:24   #3
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Re: Holding tank vents

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Originally Posted by thinwater View Post
You know, the problem may be with the tank or the drain. Gravity isn't much. Do you have trouble pumping in to the tank? You only need a tiny hole to admit enough air to drain, and it sounds like you have that. Think of how small the vent is in a jerry can. Maybe not.
Like thinwater says:

If the vent was really plugged, you would not be able to pump INTO the tank because the air needs a way to escape.

Putting air pressure into a holding tank vent is truly a potential disaster. Don't try this at home, people! The square, flat sided holding tank was NOT made to contain even a tiny bit of overpressure. The consequences of a split side on a full holding tank is downright scary to even think about...

Before you go changing the vent, you will want to be SURE that the drain is not clogged.

It is easy to check the vent. Just remove the hose make sure it is clear, and the fittings at either end are clear.
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Old 02-11-2018, 10:41   #4
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Re: Holding tank vents

Bet all you have is a plugged vent line
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Old 02-11-2018, 11:08   #5
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Re: Holding tank vents

Thanks, seems to be no problem pumping into the tank. I have run wire and such up through the through hulls all clear. I'm guessing some kind of a recurring flap or? Of old sealant around the fitting on top of the tank. Once I blast air up the vent it flows out fast.
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Old 02-11-2018, 14:28   #6
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Re: Holding tank vents

Is there a sag in the vent line where liquid could pool? That blasting air through the vent line allows the tank to drain is a strong indication that's the problem.


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Old 02-11-2018, 17:02   #7
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Re: Holding tank vents

Not that I can picture in my mind but I'll be back to the boat in a couple weeks and definitely check it out. That would make sense to me. Thanks
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Old 10-11-2018, 07:13   #8
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Re: Holding tank vents

i have dealt with a manufacturer that ran a vent line that dipped around 12" before going up to the thru hull. definitely a bad situation. but a 4' vent hose should be easy to remove or replace.
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Old 10-12-2018, 07:12   #9
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Re: Holding tank vents

what type of 1 1/2" fitting would screw in or glue in (epoxy?) in a black holding tank? Don't know the type of plastic? My water tank is white, my holding tank is black.
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Old 10-12-2018, 08:20   #10
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Re: Holding tank vents

If you're relocating or adding tank fitting to the tank, you'd create a "fitting" using a Uniseal UNISEAL and a short piece of PVC pipe as the hose fitting. The OD (outer diameter) of the pipe needs to be the same as the inner diameter of the hose....which doesn't need to be any any larger than 1" on a 27' boat. I don't think you'd need more than one. Use an open 'bulkhead' or "mushroom" thru-hull.

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