Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses the black water flush valve.
I have a 2020 Host Everest truck camper. I connected the black tank sprayer and sewer drain, and then I turned on the water supply to the sprayer and opened the drain valve. Somehow black water erupted into the bathroom. I’m not sure where it came from. Have you ever heard of this happening? If so, how do I avoid it in the future? It is a disgusting mess! —Jeremy
My first question would be, was the black water tank full or partially full? This would create quite a bit of pressure in the tank and push sewage back up any drain pipe going into it. There is also a vent pipe going up to the roof that hopefully did not get backflow. If the tank was empty, I would suspect you have a faulty seal at the toilet to drain pipe.
There is a flange that is mounted to the floor that has a rubber cone-shaped gasket that the bottom of the toilet fits into. This should keep moisture from blowing up into the toilet. However, high pressure will make its way through this. As you can see from the diagram, there is an open cavity between the flange and the ball seat of the toilet.
If the black water tank was empty when you turned on the water to the flush valve, I would suspect the water pressure you were using was too high. You might want to put a 40 psi regulator on the hose. I would also suggest removing the toilet and inspecting the flange, then put a bead of silicone around the flange where the toilet attaches.
And, just for future reference, always drain the black water tank first, then turn on the water to the black water flush valve. Check your levels and make sure you turn it off at approximately 2/3 full and drain again.
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