How to Check a Holding Tank Vent and Replace the Vent Filter

Look on the top of the holding tank for the smallest hose; that's the vent. Follow it to the hull. Note whether there's a filter (a canister or tube, often mounted on a bulkhead or the tank) and where the vent fitting is outside. Write down the filter brand and model so you can order the right cartridge.

BoatUS says the usual cause of a blocked vent is a clogged line or a clogged fitting where the vent leaves the hull, and to check the vent hole first. From the dock, a dinghy or a ladder on the hard, look into the fitting with a flashlight. Insect nests, salt crust and a corroded screen are common. Clear it gently with a bottle brush or pipe cleaner, wearing gloves and eye protection.

If you can see the tank, have someone watch it while you pump out. If the sides pull in, BoatUS says the tank is under vacuum, which means not enough air is getting in through the vent (or the vent is too small). Stop the pump immediately and clear the vent before trying again.

Reach the vent hose where it connects to the through-hull or the top of the tank. Loosen the clamp, work the hose off the fitting into a bucket, and push a round plastic snake down the hose until you reach the clog, as BoatUS suggests. If the hose won't clear or is packed with dried sewage, replace it. If a carbon filter is in the line, take it out of the circuit first: Practical Sailor warns that water ruins the carbon, and salt water does permanent damage.

Many owners rinse the vent line with a garden hose from the outside fitting after each pump-out, which BoatUS recommends when possible to keep it clear. With a filter installed, protect it: Practical Sailor suggests a small valve on the outboard side of the filter that you close only while back-flushing, so the water goes to the tank and not into the carbon. Do this right after a pump-out, when the tank has room.

Follow the hose from tank to hull. Look for kinks, low sags where liquid can sit, cracks, and the gummy or permeated feel of old hose. BoatUS says to use proper sanitation hose for everything, tank vent hoses included, and that sanitation hose has an expected life of about 10 years. Check that every clamp is stainless and tight.

If the vent is clear but you still smell the tank outside, or the filter is more than a year or two old, replace the filter media. Also replace it right away if it ever got soaked with water or sewage. Practical Sailor's testing found that wet carbon loses its ability to absorb odors, that salt water leaves residue that is fatal to it, and that a plugged filter can cause the same vacuum and pressure problems as a blocked vent.

With gloves on, loosen the clamps or quick-connects and remove the old filter or cartridge. Dometic's DVF uses a replaceable cartridge; Perko's Odor Eater uses quick disconnects so the cartridge snaps out without tools. Install the new one following its instructions, with any flow arrow pointing the way the maker shows, and secure it in its bracket. Bag the old cartridge and put it in the trash.

Practical Sailor stresses that the filter must stay dry: mount it as high as you can above the tank and well above the heeled waterline, or put a high loop in the hose between the filter and the through-hull so splash can't reach it. Filter makers recommend mounting as high as possible. On some sailboats there's no good high spot; if so, Practical Sailor suggests rethinking whether a filter suits your boat, because a poorly placed filter plugs quickly.

Overfilling is the fastest way to plug a vent and ruin a filter. BoatUS's advice is to pump out well before the tank is full. Tank gauges on boats are often unreliable, so learn roughly how many days of use your tank holds and pump out early. Many tanks have no level gauge at all; a marine surveyor writing for BoatUS (who once watched an overfilled tank blow sewage out of its vent and pop the vent hose off) thinks every holding tank should have one. Sensors that sit inside the tank, such as float switches, eventually get coated and stick; sensors that stick to the outside of a plastic or fiberglass tank can't foul but cost more and need access to the tank wall. Some monitors can light a "do not flush" warning or cut power to an electric head when the tank is full. Fitting one is a reasonable owner project if you can reach the tank; follow the maker's instructions and check what is behind any panel before cutting. The EPA's rules mean that inside 3 miles and in no-discharge zones the tank has to go to a pump-out station or pump-out boat, so plan your trips around them.

Practical Sailor notes that newer tanks often have vacuum break valves but older ones don't, and that most vent filters lack one. A vacuum relief protects the tank if the vent or filter ever plugs during a pump-out. If your boat has neither, ask a marine plumber whether one can be added.

Look at the vent fitting and filter each spring, when insects are building nests, and again before you lay up the boat. Clear the fitting, check the hose and filter age, and pump out and rinse the tank before storage.