Nice article, Terry!
Holding tank vent lines 101:
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.
Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.
Do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank...although not impossible, blockages in the vent LINE are rare and back flushing through a "vent" thru-hull rarely works. So start by cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line.
If there's a filter in the vent line, remove it...once a filter gets wet--which can happen when a tank is overfilled or even due to heeling--the charcoal swells and blocks the vent...one of the reasons why vent filters aren't recommended for use on most sailboats.
The best way to PREVENT vent line blockages: Replace the "vent" thru-hull with an open bulkhead, aka "mushroom" fitting that you CAN put a hose nozzle against to back flush the line. Don't wait till you have a blocked vent to do that ...do it every time you wash the boat and/or pump out as PREVENTIVE maintenance.. Anything that's tried to set up housekeeping in the vent line will just get a water slide ride into the tank.
--Peggie
