Do You Leave Black and Gray Tanks Open or Closed?

So you are RV camping in a campground with full hook-ups. Do you leave your black and gray tanks open or do you leave them closed?

Find out why I leave BOTH my black and gray tanks closed when I’m hooked up to sewer in a campground. After all, leaving the RV waste tank valves open could lead to big problems that can be easily prevented. Don’t make these rookie mistakes.

But if you do, there is a fix! Let’s get started.

Why you should keep RV waste tanks valves closed while camping with full hookups with pictures of an RV sewer hose connection to sewer and an RV in a campground

Disclosure: This page contains affiliate links for various products below. You get the same low prices and we earn a small commission.

The RV Waste Tanks

Most RVs have two types of waste tanks – the black tank. And the gray tank.

The RV black tank gets most of the publicity. It’s where the toilet empties into. And the black tank is what so many people are afraid of, because yeah, poop.

But that doesn’t mean that you can’t have problems with the RV gray tank. The gray tank is where the sinks and the shower empty into.

Some RVs may even have more than one gray tank – the bathroom sink and shower in one gray tank and the kitchen sink in another. For our purposes today, we are going to talk about the gray tank as if there is only one. But the same thing applies when there are more than one.

See Also: A Guide to Common RV Abbreviations and Slang

Open or Closed Valves?

When we ask whether you leave the tanks open or closed, what we mean is the valve that you open to empty the tanks. When you are in storage or traveling, you’ll obviously leave this valve closed up.

But once you get to the campground spot with full hook-ups, you may be tempted to setup the sewer hose and leave these valves open, letting the tanks continually empty out as you flush, take a shower, or wash dishes or clothes.

After all, why would you want to let the tanks get full, just to have to go empty them?

What Happens When You Leave the Tanks Open

The easiest way to explain what happens is to describe what happens in the black tank when you leave the sewer connection open.

So you poop, wipe, and flush. You’ll have three elements – the solid human waste, the toilet paper, and the liquid (water and urine).

If you leave the valve open, the liquid will drain out, leaving the heavier solids of human waste and toilet paper. Over time, these solids will build up, literally forming a poop pyramid.

For the record, poop pyramids = bad.

See, over time the poop pyramids get to be rock solid. Like you can’t just stick a broom handle down into the black tank, stir it up and dislodge it to go down the sewer hose.

If it is only a moderate pyramid, you are going to spend a lot of time and water letting it soak and re-hydrate and then dissolve the contents so that it can all go down the drain.

That’s assuming you catch the problem before it gets worse. If the pyramid is bad enough, you may have to replace the entire black tank. And that costs lots of money.

What Happens When You Leave the RV Waste Tanks Closed

On the other hand, if you leave the tanks closed up, then all the liquid stays in the black tank with the solids.

The water works on the toilet paper and solids to dissolve them into a liquefied form. Not to put a too fine of point to it, but it is basically melted ice cream in there. A smelly melted ice cream, but you get the idea of consistency.

Nonetheless, this liquefied waste is a lot easier to get rid of than a bunch of solid waste. You can empty the valve and send the waste down the RV sewer hose.

The Importance of the Black Tank Flush

RV sewer hoses with a clear connector with brown water going throughWhen you empty the black tank, there might be a little bit of toilet paper still in a more solid state. Especially the bits that were last put in there that haven’t had time to dissolve and are floating towards the top of the tank.

These scraps have a tendency to stick to the side walls of the tank as you empty it all out. Think about when you had a sticks and bricks home and you empty the bath tub and how those soap bubbles or bath oils cling to the side walls of the tub. Same thing happens in your black tank.

That’s why it is important to periodically back flush your black tank. To get rid of the little bit that is left.

PRO RV TIP: Before you empty your tanks, unlevel your RV, so that it is tilting towards the sewer connection. Even just a little tilt will help the tanks empty better, leaving less crud behind.

Built-in Black Tank Flush Systems

Many newer RVs have black tank flush systems built in. Really high end systems may even have a similar system in the gray tank(s).

To use, you’ll hook up a non-potable water hose to the black tank flush input.

Why non-potable water hose? You don’t want to use the fresh water hose that you use for your drinking, showering, and washing water because there is a chance that you could cross-contaminate the hoses, especially at the connection. You are hooking it up directly to the black tank after all.

After you are hooked up to the back flush, you’ll want to close the valve on the black tank. Turn on the water and let it fill up awhile. Be very careful that you don’t fill it all the way to overflowing! You’ll have a mess INSIDE your bathroom!

The back flush systems have a whirly spray that hits all, or at least most, of the corners and sides of the black tank, getting all the scrapes of toilet paper and left over poo that may be sticking to the tank. After you run it for awhile, open the valve back up and check to see what is coming out.

Is the waste water running clear yet? If not, run the back flush a little while longer.

Then empty the gray tank to flush out the sewer hose.

Now you are ready to go with empty and as clean as you can get waste tanks.

See Also: Essential Gear for RV Waste Tanks

No Built-In System? Here’s How to Back Flush Your Black Tank

If you have an older RV (or even a newer RV) without a built-in black tank flush system, don’t despair. There are a couple of alternatives that you can use, depending on your setup and what you are more comfortable with.

The first option is the RV sewer hose connector with a back flush rinser. With this, you’ll hook up the water hose to the sewer connector and it will shoot water up into the black tank.

The second option involves running a hose through the RV and into the bathroom. This option puts an extension from your water hose down the toilet to get the same benefits of the built-in tank flush. You could go with a simple tank rinser, but I recommend you spend a few extra dollars to get the swivel versions for better cleaning.

The Straight Swivel Stick is great for those that have toilets directly over the black tank and it is a straight drop into the tank from the RV toilet. The swivel part at the bottom does the same whirly action of the built in black tank flush systems, aiming water all over the place. You’ll be wise though to move the stick around to help get the tank at different heights and angles.

The Swivel Flexible Stick Black Tank Rinser is great for those that don’t have a straight drop from the toilet into the black tank. The flexible hose lets you maneuver the rinser down into the black tank and then the swivel takes over, washing the black tank. Even those that have a straight drop benefit from the flexible hose – it just adds to the whirly cleaning action.

No matter which product you use, the method is still very similar to the built-in process. Fill up the tank (but not overfill!) and then empty it. Let the fresh water run until the stuff coming out of the sewer hose is clear. That means you got all the gunk.

How Do You Know You Got All The Gunk?

Clear RV sewer hose elbowSeveral times I’ve mentioned that you have to run the black tank rinse until the water runs clear. But how do you know?

That’s where a clear sewer hose adapter comes in.

Whether you are using the Hydroflush adapter above or a clear sewer extender, you’ll want to be able to see what comes out of the tanks.

I know, gross. But it just looks like muddy river water. And you’ll want to run the back flush until it looks like clear pool water. See, a much better way to think about what is coming out, right?

What About the Gray Tank? No Poop Pyramids Here!

So now you are wondering about the gray tank. Why should you leave it closed when you don’t have to worry about poop pyramids in the gray tank, right?

I mean, you are really good about making sure that you wipe off dishes and have drain traps or strainers to keep food particles from going down the sink, right?

No matter, there’s always some little particles that get by the strainers. It doesn’t seem like a big deal, but it happens. By filling the tank up and then letting it drain at once, you are more likely to get a whoosh! of draining water that will pick up the particles and empty the tank better. The extra water also helps to dissolve them.

A full gray tank is also better to rinse out the sewer hose after you empty the black tank.

See Also: RV Gray Tanks: The Other RV Waste Tanks

Critters and Smells: Another Reason to Leave the Tanks Closed

As if poop pyramids weren’t enough, there’s another reason that you’ll want to leave BOTH the black and gray tanks closed while you are hooked up to a campground sewer connection: critters and smells.

Both critters and smells can come up out of the sewer and into your RV.

The most common scenario is what we call sewer gnats or sewer flies. These sewer gnats can be a pain to get rid of since they lay eggs and hatch faster than you can get rid of them.

But those aren’t the only critters that you have to worry about – roaches and even rats have been known to come up the sewer hoses. And snakes!

Forget snakes on a plane! You could have snakes in your RV!

No thanks, I’ll leave the valves closed!

What About the P-Trap in the Sewer Hose?

Some people argue that a P-trap in the sewer hose is enough to keep the critters and the smells out of your RV.

Similar to how a P-trap works in a bricks and sticks home, the P-trap is intended to keep some water in the hose to prevent critters or smells from coming up.

I don’t think a small P-trap with just a little bit of water is going to work to stop a snake or rat that’s been living in the sewer system from coming up. But that’s just my opinion.

Personally, I’m not willing to risk the chance that something crawls its way up the sewer hose and into my RV. I’d much rather take the couple of minutes each week it takes to go outside and open the valves until they are empty and then close them back up again. I can often do this while walking the dog – open the gray tank valve on the way out and close it on the way in.

What If I Left The RV Tanks Open and Have Problems?

Uh oh. You made the rookie mistake and you left your tanks open while you were parked and using the RV. Or maybe you had a slow leak at the valve and the water leaked out, leaving you with a dry poop pyramid despite your best intentions.

What now?

First, if it is a problem with the valve, you are going to need to replace it ASAP, before you do anything else. The good news is that if you already have the poop pyramid, you may not have much leak out while you are changing out the valve. If it is only a partial blockage, well, it’s going to be fun – drain what you can out of the sewer hose and make sure you wear gloves. Don’t forget to clean the ground around you when you are done.

Second, you’ll need to attack the poop pyramid. Water is your best friend here!

Fill the tank with hot water and let it sit. The water will soak in and loosen up the poop pyramid, literally re-hyrdrating it and then dissolving it. Start with letting it soak for four plus hours or even overnight, then drain. Try the back flush (carefully so you don’t overfill) and see if it is dislodged and you can clean out the tank.

It might take several rounds of soaking, emptying and soaking again to get rid of the entire poop pyramid.

For really stubborn poop pyramids, my go to recommendation is Bio Clean. Bio Clean is a combination of bacteria and enzymes that “digest” organic material (read: poop, hair, gnat eggs). You mix some of the formula with water and then let it soak in your black tank.

Bio Clean also works great in your gray tank and the pipes to get rid of hair and other stuff that may build up in there. It’s a great thing to run through your pipes on a regular basis to prevent build-up and just general maintenance.

I Heard Ice Cubes Help

There is the pervasive rumor circulating around that ice cubes and a drive around a curvy road is going to help break up a poop pyramid.

I call BS on this.

See, I don’t know about where you are, but by the time I empty a 10 pound bag of ice into the black tank and then get in the drivers seat and pull out of the campground, all that ice is melted. It’s not going to stay frozen long enough to help dislodge anything in the black tank.

That’s not to say I’ve never used ice in the black tank. After a long weekend of tailgating or boondocking, I’ll often dump whatever ice is left into the black tank.

Because what the ice does do is melt. And leaves you water. And water sloshing around in the black tank is a good thing. Not the fact that the water started off as ice.

What About Critters?

So your most common critter problem from leaving the gray tank valve open is likely going to be sewer gnats.

The good news is that Bio Clean can also help clean up the eggs and help get rid of the problem.

For more information on how to get rid of sewer gnats in your RV, check out this article.

For other critters, you’ll have to take some action, like using roach spray or catching the darn things.

I’ve also had good luck with Ultrasonic Pest Repellers to drive and keep pests away from the RV. These do not tend to bother me or my dog at all, making them a safe, non-toxic alternative to using sprays or poisons.

Leave the RV Waste Tanks Closed

Why you should keep RV waste tanks valves closed while camping with full hookupsNow you see why I recommend that you always leave the RV waste tanks, both the black tank and the gray tanks, closed while you are hooked up to RV sewer connections in a campground.

You’ll be way more likely to avoid the dreaded poop pyramid, creepy crawlers, and odors, while also providing enough gray water to clear out the sewer hose.

You also know why it is important to use the back flush on the black tank.

Like these tips? Pin for later!

More resources on your RV black and gray tanks:


Leave a Reply

Your email address will not be published. Required fields are marked *

I accept the Privacy Policy

This site uses Akismet to reduce spam. Learn how your comment data is processed.