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Most modern motorhomes come with an RV black tank flush. This feature helps clean your RV’s black water tank after emptying its contents. If this feature is new to you, let’s talk about how to use an RV black tank flush.
Why Use an RV Black Tank Flush?
Most RV old-timers probably remember the days before this feature was commonplace. Without it, the best way to flush your black tank is to go inside and flush the toilet a bunch of times—filling your tank with several gallons of water—before emptying it a second time.
Or, you open your bathroom window, run a hose inside, stick it down the toilet, and try to flush your tank out that way. DON’T TRY THAT! I’ve been there and done that—unsuccessfully. As a result, I think I’m blacklisted from a certain Southern California RV park. But that’s a story for another time.
Here are the benefits of an RV black tank flush:
- Eliminates the need to go inside to flush water down your toilet.
- Dislodges stuck toilet paper and solid waste.
- Clears holding tank sensors to provide more accurate level readings.
- Removes “leftovers” that can cause irritating odors.
- As a side benefit, it cleans out your sewer hose.
Where Can You Use an RV Black Tank Flush?
Campgrounds with full hookups and RV dump stations are the best places to utilize your black tank flush. They offer the two main things you need to get your black water tank clean: a place to dispose of your waste and a source of clean water to clean out your tank.
If you happen to have a sewer clean-out on your property that’s accessible with your RV sewer hose, you might be able to do this at home. But doing it at a dedicated facility makes it much easier and eliminates the potential for making a mess in your backyard.
How Often Should You Use an RV Black Tank Flush?
You can use your RV black tank flush every time you empty your RV holding tanks. But using it that frequently isn’t required. If you’re looking to save a little time, make it an RV maintenance task you do at the conclusion of each camping trip.
When you know it’s the last time you’ll be emptying your holding tanks before heading home and unloading, take the extra time to deep clean your tank with a full flush. That way, you won’t leave waste residue in the tank while your RV is sitting idle between trips.
How To Use RV Black Tank Flush
Although you’ve already emptied your holding tanks, it’s a good idea to keep wearing disposable or reusable rubber gloves for this procedure.
Step 1: Empty Your Holding Tanks
It’s important that you dump your black tank before utilizing this flush mechanism. So you’ll want to connect your sewer hose and empty wastewater completely before you go any further.
Here’s a quick overview of the process for emptying your tanks:
[vc_video link=https://www.youtube.com/watch?v=Y9UORkO3Vrk]
If you prefer written instructions, check out our step-by-step guide to emptying RV holding tanks.
Step 2: Locate The Inlet
Once you empty your holding tanks, leave your sewer connection attached and move on to operating your black tank flush valve.
The inlet for your RV’s black tank flush is located on your RV’s exterior wall. Typically, it’ll be next to your city water and cable connections. If you’re having trouble finding it, consult your owner’s manual.
Step 3: Connect a Water Hose
You can use a standard garden hose, but make sure it’s a different hose than the one you utilize for connecting to city water or adding water to your RV’s freshwater holding tank.
Connect one end of the hose to the black tank flush inlet and the other end to a nearby water source.
Step 4: Open Your Black Tank Handle
This handle should already be open if you just finished emptying your holding tanks, but if you’re running a black tank flush on a tank that’s been sitting empty for a while, be sure to open the handle completely.
If it isn’t, water will fill your black tank quickly and could overflow into the RV or cause other issues with the tank itself. Leave the handle for your gray water tank closed during this flush process.
Step 5: Run Water Through The Tank
Open the handle on the spigot for your clean water source. Water will run into the flush mechanism, through the black water tank, and out your sewer hose. Open the spigot partially at first to check that everything is working as expected before opening it completely.
Flush your tank with water for 2-3 minutes, or until the water running through the sewer hose is clear. Having a clear 90-degree connector at the end of your hose is handy for visually checking when the water is running clear.
Step 6: Turn Off Water and Disconnect the Water Hose
After flushing, turn off the water at the spigot and disconnect the hose from your flush inlet. Water will continue to drain out of your sewer hose for a short period after you’ve disconnected. When you no longer hear anything running through your sewer hose, close the handle for your black water tank.
Step 7: Empty the Sewer Hose
Leave your sewer hose connected for this step. Starting at the end closest to your RV, lift the sewer line to empty any remaining contents towards the sewer hookup. Work your way towards the outlet, using gravity to your advantage. You should hear any water remaining in the line emptying out.
Be gentle to avoid damaging the sewer hose, which would require a replacement. But you may need to do this two or three times, depending on the slope and how much water is left in the hose. The line should be light when it’s empty and you shouldn’t hear any water jostling around when you shake prior to disconnecting.
Step 8: Disconnect Your Sewer Hose
Now you’re safe to disconnect your sewer connection and replace the cap on your holding tank outlet. Remove the end closest to your RV first and keep it high to avoid spills and drain any remaining water or waste.
Follow these tips for caring for your sewer hose when cleaning and storing your hose after dumping.
That’s all there is to it! Using your RV black tank flush regularly will help you avoid clogs and other not-so-rosy plumbing issues. It’s a super easy procedure that keeps RV toilets and septic systems flowing smoothly.
Do you have any questions or ideas to share about using an RV black tank flush? Let us know in the comments below!
If you’re still learning the ins and outs of RV maintenance, check out our downloadable RV ownership and maintenance booklet!
We have a 2021 Rockwood Ultralight. We hooked up the blackwater flush connection to city water. Turn it on and no flow of water. Is there a valve I am supposed to open up beforehand? Blackwater drain is hooked up and big valve is open and blackwater tank drained before we try this. Just can’t figure out how to get the spray inside the tank to work. Thanks for the help.
Hi Bob,
Sorry for the delay. I reached out to our technical service team to get their thoughts. Here’s their reply:
There is a vacuum breaker valve that prevents the back flow of waste water from the black tank into the city water system. In this unit, it should be located under the bathroom sink and will be black or brown plastic and should have a flow direction arrow on it. These can fail or may have been installed backwards from the beginning. Locate the valve and identify the water supply line(from the city water inlet) and discharge line(to the tank) with masking tape and remove the valve. Test by blowing through it, air should flow freely in the direction the arrow points and not at all in the opposite direction. If it doesn’t flow at all in either direction, or flows freely in both directions, replace it with a new one. If air flows properly, reinstall it so the water will flow from the city water inlet in the direction of the arrow to the tank.
Let us know if this helps or if you have any follow up questions!
Just to double check on part of the instructions above. When using my black water tank flush system, I DO NOT have the grey water valve open? I’ve often wondered about this because the sticker says to ensure that the valve is open in order to avoid any damage, as stated above. Have I contaminated my grey water tank by having that valve open also? Thanks for the reply?
That’s right Don. The gray tank valve should be closed before running your black tank flush system.
You haven’t contaminated your tank. It’s just another waste water tank. But you should be regularly using tank treatments to help clean and sanitize them anyway!
I have s 2019 sonic venture 190vrb. When I first bought it the tank flush worked and you could hear it spraying back and forth in the tank. Now all you hear is the water just running. I believe the arm inside has either broken off or split. Can this be repaired and if so does the tank have to be removed? I see 3 screws holding the water inlet in, so if I take them out will the whole unit come out?
Hi Doug,
My apologies for the delay, but I wanted to reach out to our technical service team for their thoughts. Here’s their reply:
There are a number of different tank flush models out there, so you will want to obtain the OEM direct replacement. This will ensure that all of the necessary safety and performance features are where they belong. The method is easy, so long as you can reach the location. Remove the 3 stainless screws, carefully cut the black silicone sealant with a thin blade. Extract the original component and compare to the new one to verify they match. Carefully remove all of the old silicone from the tank. Apply a bead of black silicone to the new component flange and insert into the tank. Align the screw holes to the existing tank screw holes and secure hand tight. Allow to cure 24-48 hours before use. Operate the flush to confirm proper operation. Fill tank and let stand 1 hour to check for leaks.
Let us know if you have any follow up questions!
I use a splitter at the water source. one goes to fresh water inlet the other I attach to the sewer cleanout. I open the sewer cleanout only when ready to dump sewer water. I do not believe any contaniment can make its way to the fresh water. I understand the sewer cleanout line is slightly angled down. your thoughts?
Hi Francis,
I didn’t see any issue with this, so long as your sewer hose is connected and the valve for your black tank is open before you supply water to the black tank flush inlet. But I wanted to be sure, so I reached out to our technical service team and here’s their reply:
As long as the flush system was installed and maintained properly, a vacuum breaker/check valve will open once the pressure has been released preventing the backflow of contaminated water.
Hope that helps!
If you use a dump station that provides a cut off rubber hose for flush water, what is the best way to connect?
Hi Jim,
From my experience, those cut off hoses are meant only for washing out your sewer hose or cleaning up spillage around the dump station’s waste inlet. You’ll need a water spigot where you can connect a standard garden hose to utilize your RV’s black tank flush.
Hope that helps!
I’ve read where the black tank should be filled with 5 gallons of water after the final flush. Is that correct? If so, what’s the best way to do that – use gallon jugs/containers to pour down the toilets along with the chemical?
Hi Brad,
The idea here is to never leave a black tank empty. This will prevent caking and drying of leftover solids, drying and cracking of seals, and the ability to trap odors in the tank. Running about 5 gallons of water and the recommended amount of toilet chemical after flushing puts a skim coat of water and toilet chemical across the bottom of the black tank so from the next use of the toilet, the solids hit with water/chemical mixture. This breaks down the solids and locks the odors in the tank. Starting with an empty tank means the solids hit and stick to the bare plastic tank. Most toilets in RVs run about 3-5 gallons per minute, so a minute to a minute and a half will generally prepare the black tank for use.
Hope that helps!
When I hookup my water hose water won’t go into my tank. Any ideas? I have the drain open when I try.
I have the same problem and I replaced the valve under the sink. Still no water flow. Some say it was put on backward but I can’t see how that could happen. The flush where you hook up hose is blue. The blue hooks to valve and the black hooks to where arrow show flow. Any idea?
Hi Daniel,
I reached out to our technical service team to get their thoughts, so I apologize for my delayed reply. Here’s their input:
You can remove the valve and test the two lines separately. Blow air through the flush inlet on the side of the RV and determine which of the two comes from there and install it to the inlet side of the valve. Apply air again to make sure it flows through the valve. If no air flows and you are sure you haven’t installed the valve backwards, the valve is bad. Blow air through the remaining line to ensure it is not blocked. If there is no flow, they will have to follow it to the tank looking for kinks or other damage.
If nothing appears wrong externally, remove the line from the tank and inspect for internal obstructions in the tubing or the tank spray head. If the air flows fine, however, simply connect the second line to the outlet from the valve and test the system with water.
In their experience, this problem is almost always caused by a backwards valve install or by using the wrong inlet on the side of the RV.
If you’re still having issues, or you want to leave what can be a messy job to a certified technician, here’s where you can find the RV service center nearest you: https://rv.campingworld.com/rv-service-maintenance
Hi Greg!
Can you detail all of the steps in your process?
I’d love to know a bit more so provide a more accurate recommendation.
Thanks!
First we just purchased a keystone Montana high country 331rl and know our black and grey valves are backwards and the tanks won’t mt all the way even after tank flush
Hi John!
I’d highly recommend bringing your RV into a Camping World Service Center to have your waste system inspected. Here’s a link to help you book a service appointment near you: https://rv.campingworld.com/service