Why You Shouldn’t Leave Your Tank Valves Open

Written by Lee who takes our tank management VERY Seriously

Before I even get started, I want to point out that this is intended to help people, and the opinions are mine, based on my experiences. Your mileage may vary. I am aware that some people feel differently and do things differently. I wrote this post because I am constantly seeing online discussions about  all the reasons why and why not, to leave either a gray tank, or a black tank, or both (?!?!?!?!?!?!?) open when parked somewhere and hooked up to a sewer line. Most of the time these discussions are about people being parked for a long time, sometimes with a semi-permanent “hard” sewer line from their outlet to the sewer hookup. But sometimes people talk about doing it when they’re somewhere for a weekend. I have seen this discussion literally hundreds of times and I have spent so much time trying to help people, but I am tired of typing out the same things over and over, and arguing with experts who have been RVing for XXX years and know everything, but are still wrong. So now I can just put this link in and walk away. Here’s a spoiler: if you trust me and want to know what to do, and don’t want to read the entire post. Ready?

Do NOT, ever, under any circumstances, for any reason, leave your gray tank valve open, for any length of time other than to empty or “dump” and/or rinse the tank.

Do NOT, ever, under any circumstances, for any reason, leave your black tank valve open, for any length of time other than to empty or “dump” and/or rinse the tank.

Thanks for stopping by!

Here’s the long version….

Since the first day we owned our rig I have been obsessed with our gray and black holding tanks. (Initially I found that kind of weird but later I was grateful for it – Tracy) There’s a lot of reasons for my obsession, but the number one reason is that I can’t see them. They are hidden above the “belly pan”, (which is the plastic sheeting that protects everything on the bottom of your RV. It’s what you see when you get under your rig and look up) and below the floor of the “basement” storage area. It really stresses me out that I can’t see these things, and inspect them periodically, and even check out the inside of them. As full timers, I am acutely aware of how important these tanks are, and so from day one I have considered taking good care of them to be as important as keeping rain out of the rig. They are also HUGE. In most rigs they are 8-12″ tall and generally 4-5′ long and 2 1/2-3′ wide. Really big. More importantly, they are generally difficult (if not impossible) to remove without doing some major tearing apart of a rig. So you don’t want to have to replace these things, or have to get to them to do any repairs unless you absolutely have to. For me,  I have been trying to make sure that I do everything I can to keep the insides of my tanks as clean and trouble free as is reasonably possible.

Here’s the bottom of your rig. See the tanks? Exactly.


The good news is, they are incredibly uncomplicated, and generally give people no trouble. There’s not much to them. Usually, RV holding tanks are made of ABS plastic or polyethylene. They will have an inlet, and an outlet. Some black tanks might also have another inlet for rinsing. On a black tank, they are usually directly below the toilet, and the toilet drains directly into them. That’s a great setup, because you can open the flush valve and look directly down with a flashlight when the water is turned off, and actually see inside the tank. A small portion, at least. In some cases, like ours, the pipe is offset a little between the toilet drain and the tank inlet. For gray tanks, it matters less, but they take drain water from usually at least two sinks and the shower. All of those drains come together into one pipe that then goes into the tank. In some cases there is more than one gray tank.


This is what most gray and black holding tanks look like. Not much to them.



Each tank is emptied or dumped through the use of a gate/blade valve. The valve is a very simple door that is a thin piece of plastic sandwiched between two plastic flanges and rubber gaskets. The rubber gaskets are what make the gate waterproof, but the flanges are what makes it possible for the gate to close.



For most rigs, there’s a handle for each tank and it’s usually 5 or more feet from the valve.


This is the important part! Molded into the flange is a very shallow (1/4″) indentation of the plastic, all the way around the bottom of the opening. This creates a void that is the same thickness as the gate, and the gate slides into and seats in this void. Remember this, because it’s going to come back to haunt you. You can see the void in the picture below, with the arrow pointing to it. Just above it is the edge of the blade that slides down into the void.

When you open your gray valve, liquids from the kitchen, bathroom sink, and shower will flow into the tank.  Then they flow right out through the gate into to your outlet hose and finally into the sewer. Except what goes down the drain isn’t just liquids. In the shower, for example, there’s the water, of course, but there’s also dirt, and oil, and hair, and soap (which is a dissolved solid) and tiny little pieces of you. I know, it’s gross, but it means that at best, the shower water is a solution of various things dissolved in water, and at worst a liquid carrying small solids.

The bathroom sink is not quite as bad, but again, soap, dirt, dust, hair and toothpaste, all going down the drain. If you have a washing machine, again, soap is in that water, the dirt from your clothes, and of course, whatever little schnibblies you had in your pocket, all going down that drain into your gray tank.

The worst culprit is the kitchen sink. Now I know lots of people are reading this and thinking to themselves, “I use a sink strainer religiously, no food EVER gets into my gray tank!” Good for you.  You have successfully kept ALL food particles out of your gray tank, except for the particles small enough to fit through the holes, and the ones that are around the edges of the strainer that fall into the drain when you pull out the strainer and they slip right down into the drain into the gray tank. And of course all the other stuff that sneaks by that you didn’t even notice. Trust me, there’s a LOT more going down that drain than you think.

Depending on your lifestyle and your tank capacity, you are going to fill your gray tank anywhere from every two days to every week. And it’s full of water, hair, soap scum, grease scum, particles of food floating and settled, dirt, all manner of nasty stuff, almost all of which is organic, which is why it smells soooo bad. It’s rotting biological material, which creates it’s own gunk. It’s not so much a tank full of water as a tank full of a solution, which includes a fair amount of sediment, or sludge, and some solids.

To illustrate this, I recently extracted the solids from the water during one of our gray tank dumps. I know everyone is a little different, so this is just us, but I also think we’re all mostly in the center of the spectrum. Here are our details, as pertains to this topic: we haven’t been using the washing machine for months because where we are has a washer and dryer that we can use. And I aggressively try to avoid food getting down the drain. We typically fill our gray tank up every two and a half days. We both shower every day. So let’s see what was in our tank besides water!

When you dump your tank, if there’s nothing blocking it, there’s a WHOOSH as the weight of all that gray water is pushed out of the outlet hole in the tank and out through the gate valve, through your hose and into the sewer pipe. Water weighs eight pounds per gallon, and the average gray tank is 50-60 gallons, so you’ve got nearly 500 lbs trying to push that water through the hole. Water doesn’t compress, so there’s a fair amount of pressure, especially at the very beginning. If you have any kind of a transparent add-on gate between your rig and your sewer hose, or a section of clear pipe as part of your sewer hose setup, you can sometimes see glimpses of white chunks flying past in the water, that’s soap scum and/or grease that you’re seeing. It tends to clump together and since it’s white it’s easier to see.

I decided to use nylon hose, because it’s strong and flexible, and stretchy, but has a very tight weave, letting water out, but no solids or even grease or soap scum. I put it between the sewer outlet on the rig, and the sewer hose. It’s important to note that when I did this, it significantly slowed down the draining process, because the water had to go through the hose, and everything that was caught in the hose. So that “whoooosh” of force that pushes gunk out was much, much less than it usually is, so I don’t think this is everything. Using this less than ideal method, what you see below is what got trapped in the “filter”. That’s about 2 tablespoons of gunk. Plus there was a fair amount of grease coating the inside of the pantyhose, that couldn’t remove from the hose. So that should answer any questions people have about what’s in their gray water besides gray water! (On a side note I had no idea he was doing this and I walked by our outside table and thought the dog had thrown up.  It was super disgusting and I was shocked when he told me it had come from our gray tank. – Tracy)



So, you’ve opened your valve and dumped your tank. You should close it before it’s completely empty. Once the water flow slows down, close your valve before it trickles or stops. The bottom of your tank includes quite a lot of sludge/sediment, and you don’t want that to get to the gate if you can avoid it. Close the valve and go on about your life. When you travel, be sure to travel with at least 1/2 of a tank of water, or if you’ve been sitting for a long time, try to have a full gray tank on your first travel day. It will slosh around and loosen up the sediment, and when you stop for the night, dump the gray tank as soon as possible. You will be amazed at how much solids are in that water. It will be almost black and be more like a thick milkshake than water. (I know many people will fuss about the extra gas this will cost, but we have done this for years and I am always amazed by how dirty that first flush is after a travel day. – Tracy)

So here’s the part that matters and why you shouldn’t leave that gray valve open. When the valve is open, and water hits the tank from the shower or sink drains, the water will flow right out, but it will leave behind the solids and the sediment. But, the sediment and solids such as are pictured above will slowly flow towards that gate, and eventually, it will slide through, and the smallest bits will settle in that 1/4″ molded recess. That’s bad, but not as bad as what happens next. When you finally close the valve, it comes down like a hydraulic press and compresses that sediment. Then it all dries. And now you have a thin layer of dried “sediment cement” that’s pushed and hardened into that tiny void. Do this enough times and that recess will be completely filled. And then the gate will no longer seat properly, and eventually it won’t even close all the way. The gate is curved, so there will be a LOT of open space, and it will leak gray water into the outlet pipe and every time you take off the cap, you will get a half gallon or so of that water on your hands and shoes. Unfortunately, there really isn’t a way to remedy the problem of that void being gunked up short of removing the gate and scraping out the gunk, or replacing the valve. Usually those valves are hard to get to, and if you’re paying someone else to do it, that means expensive.

In summary Lee’s recommendations:

Do what you can to avoid stuff getting down that drain!
But remember that some will always sneak past.
Whenever possible, dump the tank only when it’s completely full, to get the benefit of the water pressure.
Close it before it’s empty.
When you travel after sitting for a while, try to drive the first day with a full tank and dump as soon as you can after stopping for the night.

If anyone has any questions about this or anything else, please feel free to ask them. I’ll do my best to answer them or get an answer, or make something up that sounds plausible.

Coming up next I will talk about holding tank treatments, whether or not they do anything, and if they’re worth the money.


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15 thoughts on “Why You Shouldn’t Leave Your Tank Valves Open

  1. LOVE this post…I have sent it to my hubby to read, hoping that after reading it he might finally stop putting his coffee grinds down the sink…it DRIVES ME CRAZY!!! I knew I could count on a good friend to set the record straight!

    • Likewise with my wife 🙂 though I think she gets it these days. I will have to try traveling with gray tank 1/2 full. some suggest laundry detergent in with the water. Thank you for the info! and best wishes, Mac

  2. I’m with Kelly……Great Post!!! My Bill is ALSO ALMOST as anal as Lee. Guess that means the Lees are more anal than Bills – LOL!! All’s well with us down here in South Florida…hope the same for you two:o))

  3. Great post Lee! Now I have a perfect link to share with “those people” that insist that it’s okay to leave the gray tank open!

  4. Nice detailed write up. I agree and follow the same steps and process. But like you I have been asked questions about takes and the proper procedure, and your post hits all the key points. Good job Trace! Hope you both are doing well!

  5. I have to replace my gate valves. I am buying the single piece collector that includes new gate valves. The whole piece is 1.5 or 1″ Grey water and a 3″ black water this is the collector piece that flows out of a 3″opening to the ABS hard lined to sewer dump hole in ground. Aside from making 100% sure that the black water tank is completely free of debris and is cleaned to include tightening the unions and ABS cement or glue very well, do you have any suggestions.. Am I going about this wrong. Any help or suggestions are greatly appreciated. My trailer sits year round, I live in it for work purposes. Thank you.

    • Scott, I’ve never replaced what you’re describing, so I can’t advise. Since the tanks are a big deal, I highly recommend speaking to an RV tech for advice, or do some reading up on forums. YouTube can also be a HUGE help. – Lee

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