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groundhog 06-17-2007 03:09 AM

Holding tank vent..
Looking for a holding tank vent through-hull or through-deck piece.

I have searched and the only thing out there is fuel tank vents and they seem to have very small openings. Will that be ok?

When the holding tank sucking machine starts up, won't the vent be the only place that the air can come into the system? Creating a strong vacuum?

What part did others use?

Thanks... (sleepy),

PBzeer 06-17-2007 03:18 AM

Best I can tell you is that my fuel vent and holding tank and water tank vents all look the same.

Currently at 34 44 23 N 76 40 45 W

dsmylie 06-17-2007 07:52 AM

I have searched and the only thing out there is fuel tank vents and they seem to have very small openings. Will that be ok?

That will work. Just make sure its kept clean and open.

David :)

ollie820 06-17-2007 08:23 AM

I would suggest following the line from the tank. If you don't have a vent line creating suction is the least of your worries. You will have a geiser when you open the cap due to pressure build up. I was talking to a dock guy and he said a power boat had his vent plugged with wax. When he took the cap off to pump out the contents shot up in the air almost 10 feet.:eek:


SailorMitch 06-17-2007 09:45 AM

I just replaced my holding tank and lines and am wrestling with the same issue. The vent on my tank is 5/8 inch hose that goes up to one of those little silver vents on the topsides just below the hull/deck joint. No way that allows the air to flow correctly in a 5/8 inch hose. What I plan to do is replace that vent fitting with a regular thru-hull so that the maximum amount of air can move freely in and out.

sailingdog 06-17-2007 10:19 PM

The holding tank is usually connected to the head and can draw air through the pump and water out of the bowl of the head on most boats. The vent is necessary to keep the holding tank from building up pressure, but isn't the only source of "air" into the holding tank when pumping it out.

In fact, I generally fill the head bowl with water just before I pumpout, and you can hear often hear the bowl being drained, and will know that the entire tank has been evacuated when the head bowl is empty. The water in the bowl also helps "rinse" the lines and the holding tank, which can help prevent odor problems from beginning.

morganmike 06-18-2007 06:16 AM

Actually, one article I read (I think here on Sailnet) recommend two vent lines - the size is less important than the fact that you will get some flow with two opposite facing vents (i.e. one facing fwd, one facing aft) so that the breakdown of waste in the tank is aerobic vs nonaerobic (not stinky vs. stinky, respectively.) I have a single vent to a small deck fitting on my tank now, presumably similar to what's described above, but when I put the tank back in (after rebuilding my head this year and replacing the Groco with a Lavac) I will add a second vent line.

sailingdog 06-18-2007 07:52 AM


It really depends on the size of the holding tank. Larger holding tanks will often require two vents, but a smaller one may be just fine with one.

jackytdunaway 06-18-2007 08:21 AM

i made it a point yesterday to have water in the toilet to see if it would empty and it wouldn't. Are some different or is there a problem with mine?

sailingdog 06-18-2007 07:37 PM


It would help if you said what kind of marine head you have. Also, did you leave the pump lever in the flush dry position.

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