Should I Leave My RV’s Black Tank Open Or Closed?

“Should I leave my RV’s black tank open or closed?” is a common question for RVers who often stay at campgrounds and RV parks with full hook-ups, which of course, includes a sewer connection. The question is understandable, but the answer is clear, and we’re here to tell you why.

In today’s post, we’re looking at whether you can choose to leave your RV black tank open or closed when hooked up to a sewer connection.

Should I Leave My Black Tank Open or Closed?

As we’ve talked about on this blog in the past, choosing to leave your RV’s gray tank valve open when your rig is connected to a sewer hookup can be a reasonable thing to consider. The gray tank holds water from the shower and kitchen & bathroom sinks and doesn’t pose the same issues as the contents of the black tank.

Leaving the gray water tank valve open is very different than leaving the black valve open, and many RVers do indeed leave their gray tanks open at full hook-ups. When you leave the gray valve open at a full hook-up campground, you can enjoy unlimited water usage without worrying about the gray tank filling up. For more about that, check out our post: Is It OK to Leave the Gray Tank Valve Open On My RV?

We know that many RVers choose to keep both of their tank valves closed at all times. But as 20-year full-time RVers, we’ve generally tended to leave our gray valve open, and haven’t had any problem doing that. We boondock so much, that it’s a real luxury to have a day or two of free-flowing water to take long showers and get multiple loads of laundry done in our on-board washer and dryer.

Anyone who always keeps their gray valve closed may have ended up standing in a puddle of murky water in the shower, which is the first place a full tank of gray water will back up. If you’re not comfortable with the idea of leaving your gray valve open, you might consider a compromise, as we did in the following video.

It might seem to follow that leaving both the gray and black tank valves open would provide additional convenience. However, leaving the black tank open is not recommended, for several important reasons. It can actually lead to one of the most dreaded (and unnecessary) self-inflicted wounds in RVing!

Leaving your black tank valve open allows the liquids to drain out, but leaves solid waste behind. Over time, this will cause something called “pyramiding.” That’s what happens when “solids” (which are, of course, 💩 and toilet paper) pile up into a pyramid-shaped tower, which will dry out and harden in place over time. Related: Don’t waste your money on “RV Toilet Paper!”

Keeping the black tank closed allows for the accumulation of enough liquid to facilitate effective waste removal during the dumping process. Leaving the black tank valve open is likely to cause several serious issues.

Reduced Tank Capacity

Black tank poop pyramids that sit and dry out reduce the black tank’s capacity. When you empty the black tank in the future, you may think that any accumulated dried-out waste will be flushed away. But once it’s hardened into place, that’s usually not the case, even with an effective tank flush. So solid waste remains in the tank and takes up space, reducing the usable capacity of your black tank.

Clogs and Dumping Issues

Black tank pyramids and the drying out of waste can result in clogs and difficulties when you go to empty your tanks, and no amount of simple tank cleaning is likely to be very effective if you’ve let it build up over a long period of time.

Ineffective Tank Sensors

When waste is allowed to build up on the interior walls of an RV black tank, it can render your tank sensors ineffective, because they’re unable to read the tank level properly. Generally, if you’re having trouble with your tank sensors, it’s often because the holding tank isn’t as clean as it should be.

Sewer Flies

RV sewers can house flies and eggs that are ready to hatch into flies, and they can travel up your sewer hose and right on into your RV. The good news is that if you keep your tank valves closed when your rig is hooked up to a sewer, you probably won’t have an issue with sewer flies.

That said, as we mentioned above, we’ve kept our gray valve open many times over the years and never had this problem (watch our video embedded above to see our sewer hose tip). If you do get sewer drain flies, you’ll want to have a look at our post on how to get rid of drain flies in an RV.

Horrible Smell

If your black tank valve is open, it leaves an open path between your RV’s toilet and the sewer. That means you may be letting odor from the park’s sewer system into your RV every time you flush. When you open your RV toilet bowl’s valve to allow waste to drain, that odor can enter the living space. Again, check out the video above for a little odor-busting sewer hose trick that can eliminate this particular issue.

So, the answer to the question, “Should I keep my black tank open or closed?” is clear. Your RV’s black tank valve should remain closed until you’re ready to dump your black tank.

PRO TIP: Always use a good amount of water when going #2. This will help ensure there’s enough liquid in the tank to help flush both the toilet, as well as the tank itself when dumping. A good practice is to always pre-fill the bowl about ⅓ or ½ way before sitting down.

How Do I Clean My RV Black Tank to Get Rid of Poop Pyramids?

Well, of course, the best way to avoid having to deal with poop pyramids and solid waste messing with your tanks, dumping, and tank sensors, is to be sure to keep plenty of liquid in your RV black tank. The best ways to do this are to use plenty of water when using the toilet for #2, and to always keep your black tank valve closed until it’s time to dump your tanks. Also, letting the black tank fill to at least 2/3 to 3/4 full before dumping also helps create good flushing action.

Getting poop pyramids and solid waste left behind on the tank walls can be a real challenge. In the case of dried-out solids, putting ice in the tank and driving around is unlikely to be very helpful. That works really well for small amounts of waste dried to the bottom of the tank (we’ve seen that work first-hand), but not for pyramiding.

If necessary, you can check out our post and video all about doing a thorough RV black tank flush and cleaning. But once a pyramid has started to grow, that won’t likely be enough to help. Keep reading for the ultimate solution to the worst black tank nightmares.

As we noted in our guide to dumping tanks on an RV, you should always use a clear sewer elbow to see just how clean you’re getting your tanks at every dump. We know that means actually seeing the brown color inside the clear elbow. Our attitude is that if you can’t handle that, maybe you should consider a different pastime… or travel with someone who always handles the dumping. 😉

The latter has been John’s solution for over 20 years. He’s only dumped our tanks once (when Peter was really ill). It’s not that he “can’t handle” seeing the contents of the black tank go past. It’s just the typical division of labor that most RVers have, and Peter takes care of tank dumping. Simple as that!

You can also try using a tank cleaning wand with hot water. If the solids haven’t been sitting and drying out in the tank for too long, you may be able to beat the pyramid back a little.

Soaking the tank overnight with a solution of serious tank treatment may help, too.

But honestly, if you’ve been keeping your black tank valve open while you’re connected to a sewer, you’re likely to have bigger problems than the rinsing wand or any ordinary chemical can handle. In this case, your tanks likely need what we call a professional “RV colonoscopy.”

Even when you do everything correctly, a substance called struvite tends to build up inside RV holding tanks over time. As thorough as we’ve always been with managing our holding tanks, we had some struvite buildup after many years of RVing. And we had the cleanest tanks around… or so we thought!

But if you’ve left your black valve open for any length of time, you’ll almost certainly need professional help to clean that tank with a high-pressure cleaning system. This includes the bonus of cleaning the walls of your tank and cleaning your RV tank sensors so they’ll read more accurately.

If you’re dealing with a clogged RV black tank, the first thing to do is check out our post on how to unclog an RV black tank. If that isn’t enough to do the trick, an RV Colonoscopy may be in your rig’s future!

And another issue that could plague your nose with awful smells is if your black (or gray) tank valve is leaking. Be sure you know how to replace it: How to Replace an RV Waste Valve: Either Black OR Gray!

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