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Black tank won't drain

If you're having trouble emptying your black tank, e.g. only a small amount of water comes out at a time, forum staffers Gary and Steve suggest these possibilities and remedies.

  • Some object (e.g. a cloth or wad of tissue), has fallen into the tank and is blocking the drain hole in the tank, so water only seeps into the drain line.

  • Black tanks sometimes become clogged because previous owners unwittingly left the valve open all the time, rather than letting the tank fill and dumping a tank with a high concentration of water. This can be treated by filling the black tank about 3/4 full with hot water. After an hour or so, dump several pounds of ice cubes into the toilet and take the rig for a ride over rough roads. The hot water will soften the hardened solid matter and the mechanical agitation of the ice cubes will break it up.

    It is recommend that the dump valve be examined before starting the latter procedure, to ensure the valve paddles and seals are intact. These can be replaced at minimal cost, and will prevent unnecessary suprises while doing the other tests.

  • The vent to the roof is blocked so air cannot flow in as the water runs out.

    You can check for the blocked roof vent by having someone hold the toilet flush valve open while you drain the tank; This will let plenty of air in and will let the tank drain normally if venting is the problem.

    A blocked roof vent can also be checked by running water under pressure into the roof vent using a hose to dislodge possible nests formed by birds or insects.