Black Tank Flush System Diagram: How Does The Flush Work

Waste needs to go somewhere. Learning how to use and fix the black water tank is essential for RVers. Proper flushing and maintenance of the black water tank may be a dirty job, but the mess that comes when you don’t do those things is even worse.

The system may be different in different models of RVs. But the basic design is that you need to hook up a hose to the inlet valve to supply the water. Then the rinsers spray that water in different directions to clean out the black tank. The key is to have around 40 psi of water going through the hose.

To learn more about the black water system, how it works, and how to repair common problems, just continue to read our article. It has the information you need to handle those important black tank issues

How Does Black Water Tank Flush Work?

How-Does-Black-Water-Tank Flush-Work

The black tank flush has two basic purposes. The first purpose is to help you clean out the tank when you go to the dump station and empty it. The spray is supposed to help push out anything that was left behind during the dumping process.

The second purpose is to clean the sensors. Keeping the sensors clean is important to the function of the black tank and its alert system that tells you when it is full. Contaminated sensors will malfunction when they are blocked by black water residue.

To operate this system, and this information comes from Airstream, you are supposed to open the black water valve and empty the tank first. Then after the tank is empty, close the valve and hook up the water hose.

Turn on the flush system and let it run for a couple of minutes before opening the valve again. You should do this a few times to make sure the black water tank is completely clean and free from any leftover residue.

No matter which brand of RV you own, this system should be simple to operate. There may be slight differences in operation but the basics are the same. You need the water hose for the sprayers to work.

Some owners have their own preferred method of operation which will differ a bit from the manufacturer’s instructions.

Common Black Tank Flush System Diagram

This is a diagram of the basic common black water tank flush system. As you can see it is a very simple system. Nothing should or could go wrong, right? Even with simple designs, something can and does go wrong.

The check valve is one of the parts that can break down and it is the most common part that will. The sprayer or rinser will be the next most common part to break down. However, there may be a problem with finding the right part when the check valve fails.

It seems that on some RV models, this part is not listed in the parts section of your owner’s manual or the parts look-up manual. Some owners have searched and searched for the part number and have not been able to find it.

One reason for this omission is that it is not a part supplied by the RV manufacturer. The check valve is from the company that made the flush system and it should not be listed with the RV manufacturer’s supplied parts.

In other words, to find the part number you need to look for the company’s name that supplied and built the flush system and look in their manual if it is given to you. or contact the company to get the part number.

Jayco Black Tank Flush System Diagram


As you can see, the Jayco design is not that much different from the common black water tank flush system used by other companies. It is a basic system that does not need a lot of complicated parts or angles to work.

Also, notice that the check valve is in the same position as the valve on the common design. It is not sure if you will have the same parts location problem that the grand Design RV owners have in that you need to go to the company that actually built the system to get the parts number when this valve breaks down.

You would have to talk to Jayco directly to see what the situation is if you cannot find the part number in the manual or the part look-up sheet. That look-up sheet may be with the company, it may be on the Jayco discussion forum or some other location.

If you are having problems locating where this valve is when it does fail, sometimes it is located right behind your shower. That hose will be right next to the hot and cold water hose going to your shower faucets.

But you never know. Jayco may do something different with its hose layout so check any diagrams that come with your manual to see the location of that check valve.

Black Tank Flush System Diagram Forest River

We checked the owner’s manual for the diagram of this system but the only photo or image that was there was for the location of the inlet valve where you hook up your water hose.

The diagram above is said to be for the Forest River RVs but that has not been verified yet. However, with the lack of space that RV makers have to work with, it is highly likely that the Forest River design is similar if not the same as all other RV models.

We suggest that you talk to your Forest River dealer and see what they say about the design of this system. They may be able to provide you with an image of the diagram so you can find all the parts yourself.

While you are at it, make sure to get the parts number for the different components. That is if you cannot find it anywhere else. In looking at different diagrams of this system, it seems that the RV makers will take the shortest path possible to get the hose and other components to the black water tank.

One way to find where the check valve is to go to your black water tank and trace the hose back from the sprayer to the inlet valve. That way you will have a clear idea of where all the parts are.

RV Black Tank Flush Check Valve Not Working


This is a common problem as this valve is known to fail from time to time. However, sometimes it may not be possible to know if the valve has failed or not. The reason we say that is because if the valve is stuck in the closed or shut position, it works like normal.

The problem will come in when it is stuck in the open position. This is a situation where you will see water leaking onto the floor of your RV. That is not a bad mess but leaving the water on the floor can cause other damage that can be expensive to repair.

Also, you may find that the seals on the check valve are not working as they should. This failure can be for any number of reasons including hard water deposits. The result will be the same as you will see water dripping in your bathroom cabinets or onto the floor.

Another problem is if you do not have the right water pressure running through the flush system. Some people have said that 40 psi is needed for some models but one company stated that for their flush system, you only need 7 psi for the seals to seat properly.

Watch your water pressure or you may contribute to the failure of the seals.

Black Tank Flush System Clogged

There are a couple of sources for this problem. The most common one would be the check valve and many people have had the problem of this part clogging up. But there are those other parts that may also clog up on you and if you go to a dealer to fix it, it could cost you over $400 for parts and labor.

The sprayer or rinser will be the part that is clogged when it is not the check valve’s fault. This will happen through black water residue or possibly hard water deposits. The source doesn’t really matter as you have limited options in fixing this problem.

One is the dealer route and that is costly. The other is to use a drop-in cleanser that is supposed to eat away at the residue or deposits. All you have to do with those drop-in solutions is put a few ounces into the black water tank and fill it up. Then wait for 24 hours.

When that time frame is up, go to your nearest black water dump station and dump the tank. Then hook up your flush system and turn it on. If the drop-in solutions worked, everything should work normally.

If these drop-in solutions fail, then you are left with the expensive repair option.

Black Tank Flush Leaking


Normally, this problem’s source is the check water or vacuum valve that is located behind your shower in most cases. If you see a little dripping from the valve, that is called normal. by little dripping it is meant only a drop or two at a time.

When you see standing water on your bathroom floor, then you know your check valve has failed and you need to repair it. The first thing you do is to gain access to the valve and check to see if it is tight and secure.

Usually, this valve is only hand-tightened so you need to see if it is loose. If it is, tighten it back up again and see if that solves the problem. Check all the connections to make sure none are loose.

If the seals are not working right, check your water pressure first. Make sure it is at or above the manufacturer’s suggested psi. The seals won’t sit properly if the water pressure is too low and that malfunction will cause the leak. Adjust your water pressure if that is the problem.

In case of the check valve being stuck in the shut or open position, you would have to replace the valve.

Where Is The Black Tank Flush Valve Located?

Generally, this valve is located in your bathroom, behind the shower area. One image that we saw was that the check valve was located right where the hot and cold hoses connected to your shower faucet.

It should only take a screwdriver and the removal of four screws approx., to gain access to this area. This part is one of the easier ones to find on an RV even though it is found in a very small room.

Some of these valves may be plastic as RV makers are known for using the cheapest parts they can. However, there is no problem replacing the plastic version with a metal one that should last longer.

Some owners have removed the vacuum check valve and replaced it with a normal check valve. They say that the only concern is to not let the black water level rise up high enough to contaminate your water supply.

The location of the nozzles is a different story as some owners have found theirs on the outside on the passenger side of their RV. it was labeled as No Fuss Flush and it was held in place by 3 screws.

How Long Do You Run Black Tank Flush?


This does not have to take a long time. All you are doing with the flush system is cleaning the sensors and making sure all deposits have been cleaned off the sides, etc. generally, all you have to do is run the flush system for up to 3 minutes and the black water tank should be rinsed.

Some owners start the system first, then open the dump valve and get their tank cleaned that way. It is not the generally accepted method as Airstream has said that the dump valve should be closed after the initial tank dump.

Then run the flush system for a few minutes before emptying the tank again. You do have a little leeway in how you operate your system as long as the sprayer, which sprays in different directions when turned on.

The key is to make sure you have a strong enough spray for the system to do its job properly. Check your owner’s manual or e-mail the company to see what is the minimum amount of water pressure you need.

This information is important as if you go too low, then you could be damaging your check valve and causing a leak in your RV.

Do I Need a Pressure Regulator For Black Tank Flush?

The short and quick answer is ‘no’. There is really no need for a pressure regulator to be installed on your black water flush system. While the higher water pressure may put your plastic hose lines at risk, that risk is minimal.

The reason you do not need a pressure regulator is that you need a lot of water pressure for the sprayer or rinser to do its job. The lower the water pressure the harder it is to clean the black water tank. Also, the low water pressure will do some damage to your check valve.

If your owner’s manual does not provide the minimum amount of psi you need for your system to work properly, then contact your dealer to find out what that is. Double-check that information as well.

Even if you add on a pressure regulator, you must make sure that the line has a backflow prevention component. This is not something that is a suggestion it is mandated by law. if a previous owner has done some DIY in the area, double-check to make sure the system complies with the law.

The last thing you want is contamination to spread through your water supply lines. When you read the different RV forums on this topic, you will find that there will be differing opinions on whether to use a pressure regulator or not. You really do not want one no matter what others say.

Some Final Words

The black water tank flush system is there to help you out. It makes sure your tank is clean and there is no leftover residue once the dump is finished. Also, it keeps your sensors clean so you get accurate readings of the level of black water in the tank.

The key to the system will be maintaining the right water pressure to avoid damaging other components.

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