Help needed: "How do I fix my plugged black water tank?

A question to the RV Doctor, Gary Bunzer

Dear RV Doc,
Our black tank in the RV is plugged up by the toilet. We bought one of those Flush Kings and back-washed the tank several times. We took a snake and ran it as far as it would go from the toilet side and still it did not drain. The snake seemed to just coil up. Is there something else we could try? --Randall Peters

Dear Randall,
The subject of those dreaded holding tank blockages surfaces from time to time. It's further compounded when the holding tank is not positioned directly under the toilet.

The optimum set-up finds the toilet drainpipe going straight down and into the holding tank without the use of elbows. Unfortunately some manufacturers, because of the floor plan design, are forced to use elbows to connect the toilet to the black water holding tank, which I'm guessing is the situation on your coach. Additionally, toilet tank blockages often occur when tank valves are left in the open position while in the campground allowing all the liquid to drain out leaving the solids behind to dry out and stick in the drain piping or coagulate at the tank outlet.

Always keep the tank termination valves fully closed until each tank is above 3/4 full before evacuating. In most cases, an almost full tank will provide enough force to completely drain all the solids along with the liquids. This, of course, is assuming the slope of the drain piping, including the sewer hose, is adequate and the tank vents are fully functioning. You might want to inspect your entire waste system design and look for deficiencies. For severe blockages, it may be necessary to use a powered snake down through the toilet; a simple, manual twist snake may not suffice.

ANOTHER OPTION is to disassemble the termination fittings on the outlet of the tank. The termination valve is bolted in place using four bolts. After all the liquid stops flowing, remove the bolts and the valve and see if you can gain access with a snake from that end. It may be messy and it certainly is undesirable, but a handy do-it-yourselfer can easily accomplish this task. Make a funnel out of a large plastic water jug -- you see them upside down on common water coolers. Cut off the neck including down about four inches of the widest circumference. Position the coach with the termination valve directly over a sewer dump site. Insert the new funnel into the sewer inlet and then take off the valve. Be sure to wear rubber gloves and long sleeves! This will allow you access at both ends of the system.

If you still cannot remove the blockage, perhaps it's time to contact All-Pro Water Flow (www.allprowaterflow.com). Their specialty is hydro-cleaning holding tanks and drain assemblies by using very high water pressure and a special nozzle that allows cleaning of the entire tank interior. I recommend this procedure at least once a year anyway to keep the tank probes clean and to eliminate false readings on the monitor panel. Back-flushing with typical city water pressure will not adequately clean or clear stubborn blockages.

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