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

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.

Our RV black tank flush system connection.

Our RV’s black tank flush connection, marked “sewage tank rinse,” is located in the water bay of our rig.

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!

Sale
Camco Rhino 25-Ft Clean-Out Camper/RV Black Water Hose | Features a Heavy-Duty PVC Design & Bright Orange Color | Clean-Out Camper Black Water, Grey Water or Tote Tanks | 5/8” Inside Diameter (22990)
  • DESIGNED FOR RV SANITATION: Keep your RV waste system clean & odor-free with Camco's reliable clean-out water hose. It can be used as an RV grey water...
  • HEAVY-DUTY CONSTRUCTION: Built to last, this Camco RhinoFLEX 5/8″ internal diameter RV hose is constructed of durable PVC.

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.

VALTERRA Products, Inc. A01-0141VP Plastic Carded Removable Anti-Siphon Valve
  • Package Dimensions: 4.5 L x 1.5 H x 3.75 W (inches)
  • Package Weight: 0.1 pounds

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:

RecPro RV Vacuum Breaker Check Valve | Protect Fresh Water From Pollutants 571-VAC-CHK-A | Made in America
  • Although this piece is small and simple-looking there are several points that need to be addressed when it comes to proper water safety and...
  • Second, this vacuum breaker is a high-hazard protectant. Because of this, it can be used in a low-hazard application as well.

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.

Sale
Camco Clear 45 Degree RV Sewer Hose Adapter Fitting | Features 4 Bayonet Prongs, a Heavy-Duty Polycarbonate Construction, and is Compatible with 3-Inch Diameter Sewer Hoses (39432), Clear
  • Clear RV Sewer Hose Adapter: See-through hose adapter allows you to see when your RV sewer system is clean
  • Compatibility: Fits 3-inch diameter RV sewer hoses

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.

Camco Camper/RV Holding Tank Swivel Stik Rinser | Features Powerful Rotary Cleaning Action & 34-Inches of Flexible Reach | Equipped with 1/4 Turn Shutoff Valve & Ergonomic Easy Grip Handle (40074)
  • POWERFUL ROTARY CLEANING ACTION: Enjoy superior cleaning power with this RV tank cleaner. Its powerful rotary cleaning action shifts even the toughest...
  • FLEXIBLE REACH: The black tank rinser has a 34" flexible section that is perfect for hard-to-reach RV tanks. Get into those offset basement tanks with...

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.

Sale
Camco Tornado Rotary Tank Rinser Features a 6-foot Hose, 360-degree Rotating Head and High-Pressure Nozzles (40126)
  • Easy Cleaning with Rotary Power: Helps you reach every area of your RV holding tank; Creates the easiest, most sanitary way to clean your holding tank
  • Quick and Easy Holding Tank Cleaning: Dislodges and flushes stubborn waste deposits and odor-causing particles left after holding tank is emptied

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:

Valterra A70 No-Fuss Flush with Check Valve , Yellow
  • Product Type :Valve
  • Package Dimensions :11.4 Cm L X15.2 Cm W X25.1 Cm H

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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Morgan55

Thursday 4th of April 2024

You should not close your black tank while flushing to fill the tank. This pressurizes the tank and can damage the seals. Always leave the valve open. The flush system has a nozzle designed to wash the inner walls of the tank.

TheRVgeeks

Friday 5th of April 2024

Actually, your black tank has a vent pipe (usually at least 1.5" diameter) that goes to the roof, allowing any air pressure to exit as the tank fills. If it didn't, you'd have problems when you flushed the toilet! The warnings about black tank flush systems not being run when the black valve is closed are mostly driven by the flush system manufacturer's lawyers. The biggest issue you'd encounter would be if you forgot you were running the flush system with the black valve closed... you could overfill the tank (which could allow black tank contents to escape in places/ways you wouldn't want). Which is why you should never leave it running without staying nearby (and keeping an eye on the time to make sure you don't run it too long).

TheRVgeeks

Thursday 4th of April 2024

It’s a myth that closing the black valve while flushing is harmful. The tank cannot be “pressurized” because it’s vented (on the roof). We’ve used this method every time we dump during 20+ years of full-time RVing, with zero problem, including no damage to the seals.

JohnS

Monday 22nd of January 2024

Question for you guys, and the group: if you don't have a flush/back flush system or valve built in, how do you connect your flush hose and wand to flush down through the toilet? Do you feed the flush hose around the RV, in through the door-kitchen-bath entry to reach the toilet? Currently that's the only way I could connect a flush wand. Thanks!

TheRVgeeks

Monday 22nd of January 2024

Hi John. Yes... if you're using the through-the-toilet flush wand (which is, honestly, the most effective option), you just have to feed the hose through somewhere (door-kitchen-bath entry like you said) so you can use it.

The alternative is to use one of the tank flushing systems that attaches to your RV's sewer outlet and allows you to connect the hose right there (like this one on Amazon: https://amzn.to/3U6e2D3). When you turn it on, it sprays a stream of water up into the tank to help flush it out. But its effectiveness depends a lot on the configuration of your black tank & dump plumbing as well as the size & shape of your tank. For some RVs, it's great and very effective. For others, it's (effectively) worthless. But it does have the advantage of allowing you to connect the fresh flush water right there on the same side, instead of having to snake the hose around and into the RV.

Lam0

Monday 13th of February 2023

We have a 2012 Keystone Cougar XLite. We also have a factory installed flush system. When we flush after emptying the black tank - using the 2 grey water tanks, it seems as though the grey water doesn’t completely empty.

On occasion we get a stagnant odor inside the camper. We’ve attempted to solve this on our own but are baffled. Once we moved the trailer after all was dumped and water poured out of a mysterious location. Another time, while allowing grey water to put out in a boon-docking situation a smell was detected & water was coming out the grey water hose even though it should have been empty. Please help if you can because the manual offers nothing!

The water draining is not black tank. It’s grey water.

Thank you in advance for your help

Lam0

Steve O

Tuesday 3rd of January 2023

I have a couple of Geek questions...

I use my black tank rinse religiously at every tank dumping, if I can. My Lance truck camper came with a tank rinse system, and I modified my outside shower hose with a hose adapter that allows me to hook the shower hose to the black tank water inlet, and then use my water pump and onboard water to rinse the tank. As an extra benefit, I can flush with hot or warm water. As another benefit, when I winterize, I can winterize the black tank flush line. I found this system really handy, since most dump stations don't have hose ends available, as you pointed out. However.... I'm getting a new truck camper, and it does NOT have a tank rinse system. Since it's brand new and the tank hasn't been used, I've already purchased the Valterra No Fuss Flush sprayer, and plan to drill into my tank. I also bought a vacuum breaker to install in a cabinet or behind the shower wall (the instructions say the vacuum breaker has to be at least 6" higher than the liquid level, but I'll probably put it a few feet higher, just to be safe. But now I've got 2 question before I install this...

1) The Vacuum Breaker (a Camco version) says "Do not install any other back flow devices in this system. Doing so will trap water." The Valterra sprayer has a built-in back flow preventer to ensure any black tank liquids do not return to the supply line. But why would I care if there is water trapped between the sprayer and the vacuum breaker? Doesn't seem to me like this would be an issue, and your article says you put a secondary backflow preventer in your system as well - so I'm not understanding why Camco has this note in their instructions... Is this a health or safety concern? If so, I don't get it.

2) Why can't I just plumb the tank rinse right into my water supply system, ASSUMING adequate backflow safeguards? In my current camper, I use the outside shower hose, but it would be a lot more convenient if I just put in a T in the water line, ran it to a stop valve (1st line of protection), then to the Vacuum breaker (2nd line of protection), and then to the tank sprayer with integral backflow preventer (3rd line of protection). With 3 devices all intending to keep flow from going backwards into the water supply, why do factory tank flush systems all get built with a separate hose connection? Is this an overabundance of caution? I'd really like to be able to put this together such that I can rinse at any dump station by simply turning on my water pump and turning a valve. This doesn't seem like risky behavior to me - the shower hose version on my lance worked well, but it was still a nuisance pulling the hose and connections out, when it would be easier to just have the system hard plumbed together... I would think the chance of backflow would be near zero, and wonder how much of the normal design is for real health concern, and how much is for the mental hurdle of having any connection at all between fresh water and poop. Is this really a risk? Have you heard of others doing this? Your thoughts on this?

TheRVgeeks

Wednesday 4th of January 2023

Hi Steve. You pose an interesting problem, solution, and questions. Honestly, we can't think of a specific reason why what you're proposing to do would be a problem... other than a potential code violation (and maybe an issue and/or confusion for a future owner: "Hmmmm... what does THIS valve do?!?!"). While we certainly sympathize with your desire to always use your tank flush (in 20 years of full-time RVing, we've NEVER dumped our black tank without rinsing it), we couldn't imagine connecting our fresh water supply system (that we drink out of) directly to anything that connects to our black tank. We deal with this issue in three ways: (1) staying a night at a full hook-up RV park in between stretches of dry camping, (2) using dump stations with threaded connections, or (3) using our Water Thief for dump stations without threaded connections. We know there are dump stations that even a Water Thief won't work for (those odd little brass tubes with a right angle bend near the end, etc), but we've managed to make this work. One advantage we likely have is that our 45-gallon black tank lasts us a solid 2 weeks+, making black tank dumping much less urgent/regular as a rule.

Sorry we don't have more thoughts on this one... it's just not a modification we've ever needed/considered.

Michael Suchinski

Wednesday 12th of October 2022

Hi Geeks, I have a 2021 coleman travel trailer. I was confronted with water leaking out from the trailer while trying to flush the black tank. found out that the anti siphon valve was poring out water at the top of it. I find the design and location is really bad, a failure like mine could do a lot of water damage. I plan to remove the device and put a by pass line in and use an external anti siphon device. this is much safer less damaging. my problem is which device failed, the flushing device or the siphon. once I replace the anti siphon device with a by pass hose I will see if the flusher still works. If its bad, what flushing device is a good replacement? I feel they used a real cheep product and the bearings on the device are garbage.

thanks

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