Thursday, March 23, 2023


Ask Dave: Gross! Why did the black water flush valve erupt?

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.

Dear Dave,
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

Dear 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.

Click to enlarge.

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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Dick and Sandy near Buffalo, NY
1 year ago

What Jeremy did does make sense, but unfortunately not in the RV world. If you are not familiar with how things work in your RV things like this are bound to happen. In our homes (not RV’s) our toilets need lots of water to flush. SO if you just see a sign on your RV that says “BLACK TANK WATER FLUSH” (or something to that effect) that could (but does not) be thought to help flush out the Black Tank when dumping. We had our coach built with a bath and a half with Marine style toilets that require very little water as they are vacuum operated and macerate any waste before it gets to the Black tank. In fact our Gray tank is larger than our Black tank to accommodate our stacked washer/dryer. Of course he should have dumped his Black tank before using the Black tank flush system to clean out the tank. But if you are not familiar with how things work and just read the writing on the wall, well, stuff happens. YOUTUBE has almost anything about everything RV related. Use it.

1 year ago

Almost sounds like he pulled the grey tank valve and not the black tank. The black tank should have been empty before using the tank sprayer.

1 year ago

He should have opened the black dump valve FIRST before turning on the back flush sprayer.

Bob p
1 year ago
Reply to  Dr4Film


Jesse Crouse
1 year ago
Reply to  Dr4Film

“A man has got’s to know his limitations” as Strother Martin said in “Cool Hand Luke”. Read all instruction manuals FIRST and then ask an old timer SECOND. Before touching anything RV related. It does not work like at home.

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