The RV Black Tank Flush System – All You Need to Know

Every RVer knows that of all the duties an RV owner has to tend to, dumping the black tank is the least enjoyable. That said, there are ways to make tasks related to the black tank considerably easier to endure. A prime example is the RV black tank flush.

Not every RV has a black tank flush system, but those that do provide a beneficial service to their owners from a couple of perspectives. But we’ve actually met RVers who didn’t realize that their RV had a black tank flush and others who weren’t really sure how to use it or whether it was all that useful.

So, in today’s post, we’re talking about RV black tank flushing systems – what they are, what they accomplish, and how to use them!

What is an RV Black Water Tank Flushing System?

An RV black tank flush is a cleaning system that works in tandem with your regular RV tank dumping procedure. For example, you generally dump your black tank and then flush out the sewer hose by dumping the contents of your gray tank immediately thereafter. (You do dump the black tank first and the gray tank second, right? If that’s not what you’ve been doing, start doing that the very next time you dump!)

This removes the contents from the black tank and then, as the gray tank empties through the system, it flushes things out the sewer hose relatively well. The operative word there, though, is “relatively.” And what about the inside of the black tank itself?

This is where the RV black tank flush comes into play. Essentially, it clears out what’s left behind from the initial dump-and-flush by providing a secondary flush that cleans the hose out more thoroughly, as well as the inside of the black tank, too.

If you’re not sure whether your RV has a black tank flush system, have a look around. Usually (but not always), a black tank flush system is located in the water bay, somewhere in the general vicinity of the black tank dump valve.

How Does an RV Holding Tank Flush Work?

The black tank flush system requires a separate garden hose that connects to the flush inlet valve. We highly recommend using a dedicated hose for this purpose, perhaps of a unique color, but certainly not your drinking water hose!

A nozzle (or several) inside the tank supplies a spray of water that rinses down the tank walls and helps to flush out more waste that might otherwise have been left behind.

Most RV black tank systems also have an anti-siphon valve that serves as a back-flow preventer. These are often installed inside the RV in a cabinet located above where the tank flush inlet is installed.

The purpose of the anti-siphon valve is to ensure that waste water in the tank can’t be siphoned back into the freshwater system. We always use an additional anti-siphon valve like the one below in line with the supply hose as a fail-safe secondary backup to protect the freshwater system we’ve connected our flush hose to. We simply screw it onto our RV black tank flush connection and then connect our hose to it.

Note that if the anti-siphon valve in your RV fails, it can allow odor from the black tank into the RV, so if you’ve got a mysterious odor in your RV, that’s one of several possible sources you’ll want to check. This would be a replacement for that valve:

An RV black tank flush system comes with an anti-siphon valve/vacuum breaker like the one above that is installed inline with the incoming water supply. It’s typically installed inside the RV in a cabinet located somewhere right above where you connect an external hose to the black tank flush connection. (Ours is inside our bathroom cabinet, right under the sink, and it’s mounted up high – so the plumbing for it goes up high under the cabinet with the anti-siphon valve/vacuum breaker installed at the top of the water lines.)

Again, its purpose is to ensure that water in the black tank can’t get siphoned back out through the fresh water connection when flushing your black tank. The inline anti-siphon valve/vacuum breaker lets air into the line, breaking the suction, which prevents siphoning.

How Long Should I Flush My RV Black Tank?

We suggest flushing for as long as it takes for the water to run clear. How do you know when it’s running clear if you can’t see through the sewer hose? We use, and highly recommend, a clear elbow for this exact reason. It may not be pretty to look at, but knowing what’s happening inside your sewer hose as you’re dumping the black water is key to properly flushing the black tank.

As you begin dumping your black tank, you’ll begin rinsing with your black tank flush system. Close the black tank valve and allow the tank to fill a little, then open the valve again and watch your clear elbow. If the water isn’t running clear yet, repeat the process — close the black tank valve, partially fill the tank, then pull the black valve again. Once clear water is running through the elbow, your black tank is well rinsed and you’re done! Now you can turn off the black tank flush, close the black valve, and dump the gray tank.

We then disconnect the sewer hose, and rinse everything off (including the inside of the sewer hose) with our dedicated tank flushing hose, and we’re done.

How Often Should I Flush My Tank?

To facilitate good flushing action, it’s typically recommended to wait until the black tank is at least 1/2 to 3/4 full before dumping it. This varies, of course, depending on how many people are traveling on your RV, and how large your black tank is. Dumping frequency can also be reduced by taking advantage of other facilities when available, such as campground bathrooms.

That said, we (the two of us) can go a comfortable 2 weeks on our 45-gallon black tank, with room to spare. That’s when we’re boondocking with no other facilities available except our own bathroom.

Rather than utilize a dump station, we always try to dump the black tank when we’re at a full hook-up (electric, water & sewer) campsite. That’s because dumping and flushing properly can take a little extra time, and you’ll want to be cognizant of others who may be waiting to dump their tanks.

Another reason we try to avoid dumping (especially the black tank) at a dump station (vs a full hook-up site) is that you can’t always be certain that they’ll have a threaded spigot available to attach your flush system hose to.

Lastly, dump stations don’t generally provide the opportunity to hard-connect your sewer hose to the sewer opening the way you can at a campsite (which we do using this adapter). That means you’re a lot more likely to catch a nose-full when you pull that black valve for the initial dump. We also use a sewer hose support at full hook-up campsites, which isn’t practical at a dump station.

What Can I Do If My RV Doesn’t Have a Tank Flush Built In?

If your RV doesn’t have a black tank flush system, you can use an RV tank rinsing wand like this one to rinse your tank in a similar way, which you stick down into the tank through the toilet.

These have a standard ¾” female garden hose connection to connect to your dedicated tank flushing hose, and have rotary spraying ability to flush the remaining waste from the sides of the black tank. The solid section of this wand is 24” long, and the flexible section is another 34” that allows the nozzle to reach holding tanks that are offset from the toilet.

Can I Install an RV Holding Tank Flush Myself?

In addition to wands like the one shown above, there are various aftermarket tank flush systems available. Most require drilling a hole into your black tank. You can choose to install it yourself or ask an RV dealer or technician to do it for you.

This is a very inexpensive and simple rinse system, but the installation still requires drilling into your black tank. We’ve never used this product ourselves so we can’t vouch for it – but this is one example of an inexpensive aftermarket black tank flush system.

Another option for an aftermarket black tank flush system is this one from Valterra. Again, we’ve never used this system ourselves – be sure to read reviews and also remember that installation of this product involves drilling into your black tank:

Remember also that a rinsing wand such as the one shown in the previous section is popular for a reason. If you don’t want to drill a hole in your RV black tank, look into the rinsing wands as a simple option, with no installation required.

Why Should I Flush My RV Black Tank After Dumping?

Keeping your black water waste holding tank well rinsed not only helps to reduce odors, but also helps to keep the dump valve in good working order. It also prevents solid matter from hardening down onto the bottom of the tank, making the entire dumping system work more effectively.

Also, as “stuff” starts to build up on the walls of your tanks, the ability of your holding tank sensors to provide information can be significantly reduced. Keeping your tank sensors clean is very important if you want to be able to keep an eye on how full your tanks are. And flushing your tanks well – particularly your black tank – is one way to keep your holding tank sensors clean.

So, whether you have a built-in RV black tank flush system (some RVs even have gray tank flush systems!), use a wand, or pour a few extra gallons of water down the toilet to flush the tank until the water runs clear, flushing your RV’s black tank is a good idea for lots of reasons.

And speaking of dumping, be sure you know the rules when it comes to releasing gray water on the ground!

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