Holding Tank Vent Blocked

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Jeff Brinker

I recently had to replace my macerator pump. *yks Really turned out to fairly easy once I found a working pump out station. One remaining problem: my holding tank vent appears to be blocked. Any suggestions on how it can most easily be cleared? Thanks Jeff Brinker SV "Moondance"
 
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Peggie Hall/HeadMistress

Two most common places for vent blockages:

1. The thru-hull...you may need to scrape it out with a screwdriver. If there's a screen on it, don't worry about destroying it...screens are just dust and dirt magnets, plus they corrode..you're better off without it. 2. The connection on the tank...even more likely if you allow your tank to overflow out the vent--hard to prevent when you're heeled if the vent fitting is in the wrong place (which it is on most boats), but if waste out the vent is your method of determing when the tank is full, you deserve far worse punishment than just a blocked vent. If cleaning out the thru-hull doesn't clear it, check the line for a kink...if there is none, remove the vent hose from the tank and clean it out. If THAT doesn't cure it, it's prob'ly gonna be easier to replace the vent line than it will be to clear a clog somewhere in it. Don't use the toilet again or attempt to pumpout again till you have cleared the vent..flushing the toilet against a blocked vent will pressurize the tank, which can have all kinds of nasty results--from a waste explosion back through the toilet to a cracked tank...pumping out against a blocked vent can put enough suction against the tank walls to crack the tank.
 
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Terry

Jeff, Peggie offers several good suggestions...

The vent on our P42 was plugged when we purchased it from the previous owner. The first time it was the through hull that was plugged from either waste or a bug nest. To unplug it I had to disconnect it from the tank and connect city water using a special adapter that I made. Even then I had to use a small screw driver hanging over the toe rail gently scraping the screen to get it to break loose. The second time was the bronze elbow that attaches to the tank clogged with crud. I used a wire coat hanger to auger it out. Now when I return from a cruise I do the following to make sure the system is working properly: 1. Pump out, then fill with fresh water and pump it out again. Then I repeat with about five gallons of water each time until the water comes out clear, per Peggie's advice. 2. Our boat has the clam shell shaped through hull scupper. I made a water hose nozzle from one of those sprayers so that it would cover the scupper. I then squirt city water back through the vent scupper to make sure it was clear. 3.Pump out one last time and use the holding tank treatments that Peggie recommends. Our system has been trouble free for almost two years and no stink. Terry
 
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Steve Carpman

Starboard Lazarete

Jeff We did the same thing on our 37.5. The vent was clogged at the tank in the elbow that attached the vent to he tank. You can reach the elbow from above through the removable floor section in the starboard lazarete. It's a tight fit, so you need to induce someone of small stature to help you with the fix. A bent coat hanger is a great tool for this job.
 
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Jeff Brinker

Thank you

Thank you all for the advice. Tommorow I plan to replace the batteries in the starboard lazarette and will remove the access floor panel to check the elbow fitting at the tank. Peggy: I checked the aluminum holding tank and see it already has bowed outward! It did pressurize, although I think I dodged that sewer bullet (bad pun!) this time. Thanks again, Jeff Brinker
 
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frank arndorfer

A little hose pressure

I have a 426DS and on a couple of occasions I've had vent blockage problems also. I've found the simplest solution is whenever I'm pumping the tank, I take the water hose (which is usually present at most pump-out stations) and give the vent line a quick shot of water before pumping. Haven't had a problem since.
 
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Terry

Frank, I'm not sure what you mean by a...

"quick shot of water". I have found that if I just squirt water at the vent scupper I really did not know whether the line was clear. Our P42 has the clam shell shaped scupper. I have a cone shaped hose nozzle with a rubber seal around the edge that I made so that I can cover the scupper with the nozzle and actually apply city water pressure back through the vent line to make sure it is clear. Now I can tell right away whether the vent line is clear before I finish the pump out procedure. Otherwise, by just spraying the scupper (before I made the cone nozzle) I did not know that the elbow had plugged from hardened sewage. Terry
 
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Peggie Hall/HeadMistress

Vent blockages should be prevented...

Not just cured...'cuz a blockage can result in a pressurized tank from using the toilet. So don't only backflush the vent during pumpout, but every time you wash the boat too. And check it regularly before leaving the dock. If you feel any backpressure while flushing, stop pumping and don't use the toilet again OR try to pump out or dump the tank at sea till the vent has been cleared. Continuing to pump a toilet against increasing backpressure can result in anything from an eruption back through the toilet to a cracked tank. Opening the deck fitting when the tank is pressurized has given more than one person (including me!) a VERY unpleasant bath!
 
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