It’s not a question we get asked a lot, but some RVers wonder whether they can — and should — leave their black tank’s valve open. Everyone’s gut reaction is “yuck,” and here’s a tale of what happens when you do it.

To be direct, no. Absolutely not. We were called the other day by an RV owner at a local RV resort. The customer couldn’t get their toilet to flush. “The stuff won’t go down,” she said. We knew exactly what the problem was.

What did we find?

We got the RV and took a look. All three gate valves were open and we knew exactly what to expect. The owner said she had her son “go and flush some water up into the tanks” but all that happened was the toilet overflowed and there was a gurgling sound from the shower and galley drains.

Long story short, the customer had a clog in the waste drain pipes from the holding tank to the flexible sewer hose. A big one. A monumental one. One that required we suit up in Tyvek. You get the picture.

So what happened?

Think about it. By leaving the black tank’s valve open, water and liquid waste seeps out. What’s left is a concentrated mix of solid waste and toilet paper. The more liquid that seeps out, the more compacted and dried out the solids become.

It took us nearly an hour just to clear the clog, plus another half hour to clean the insides of the tank with our hydrojetter. Eleven back flushes (one away from our record!) and the tank was finally clean.

The final word.

Many RVers leave their gray tank valves open. We’re not big fans of that either. Your RV’s holding tanks work best when there’s at least a 50:50 ratio of waste to water. Leaving the release valves open removes the water from equation. It’s not so bad in gray tanks, but those too can have solids drop out and accumulate in tank corners and other areas that don’t benefit from the rush of water when emptying the tanks.

Keeping the black tank valve open is just asking for trouble. Don’t do it and remember that your RV’s holding tanks are not a septic system, they are a holding system. Waste and water is intended to be retained in the tanks (essentially, large plastic boxes) until they are full and ready to be empties periodically.

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