One Way to Clear a Vent Line

To keep a head system doing its job, you need the toilet to get the effluent to the tank and you need to be able to pump it out. The toilets need to have working pumps to get effluent to the tank. Knowing how important it is to keep the system flowing, we have always kept pump parts aboard and spare seals/parts for our toilets (VacuFlush) and the pumpout system, to keep them working. So you might think we were set. Wrong!

As boaters know, it is not good to overfill the boat’s holding tanks, because the effluent can go out through the vent line, and if the effluent dries in the line, the vent usually clogs. If there is no way for air to replace the effluent, a vacuum will form and nothing will go anywhere, which happened to us when we were at Roche Harbor Marina. The Phecal Phreak pumpout service couldn’t get anything out of the tank. Prior to that, the overboard discharge was not working in Canada, either. I thought I had a clever solution: go down in the bilge and open the overboard discharge through-hull while they are pumping, which should get rid of the vacuum.

It actually worked to get the tank pumped out. However, because I had left the overboard through-hull open and the hose loop above sea level was full of water, the tank just kept refilling because of the physics of siphoning. A scary thought in itself.

So what was John Temple-MacGyver’s next move? I tried sticking wires up and down the vent hose, which did not work. I felt I had taxed the Phecal Phreak’s patience enough, so I visited the yacht repair folks at Roche. We devised a plan to use their air compressor to push the clog out by connecting it at the holding tank connection. As the tech started the pump, I went outside to monitor the progress at the vent and hold a rag over the vent. I heard some gurgling, so I went back down to the bilge to tell the tech. Just as I got back up to the vent, it cleared. And when it cleared, crap shot out and splattered all over the boat next to us. Oops. I had to go over and visit our neighbor. As I explained what happened and offered my help for cleanup, the skipper said, “No problem, we were the boat that banged into your bow when we were docking last year. We are now even.”


  • We installed tank-full indicators. Here are some other things you can do:
  • Plan where and when you will pump out.
  • Keep the tank sensors clean of debris.
  • Double-check the level in the tanks by a visual inspection, both before and right after pumpouts.
  • When you pump out, if you do not have an odor filter, spray water on the vent and it will suck the water through the filter line and clear it out.
  • Keep parts for pumps and seals for the toilet and pumps on board.
  • If the toilets don’t work, pumping out does not matter.
  • Some boaters keep a portable toilet on board, just in case.
  • If you’re using a compressor to clean out the vent, keep it covered!


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