Should I Leave My Black Tank Open or Closed? Waste Tank Tips

Full hookup RV parks and campsites are the creme-de-la-creme of campsites, providing all the convenience of a sticks-and-bricks house in nature, with an ever-changing backyard. If you’re on full hookups or planning to be, you might be wondering: should I leave my black tank open or closed? 

You should always leave your black tank closed, even when on full hookups, because leaving it open can cause solid waste buildup and black tank and toilet clogs that are extremely difficult to remove. 

While it might seem like leaving your black tank open with full hook-ups is a no-brainer, it’s actually the other way around! In this guide, we’re talking about why you should never leave your black tank closed, what to do about your gray tank on full hookups, and we’ll go into some troubleshooting about what to do if you encounter issues from leaving it open in the past. 

Should I Leave My Black Tank Open or Closed with Full Hookups? (CLOSED, Here’s why)

As we just discussed, you should always leave your black tank valve closed until you’re ready to dump the black water. 

When you are on full hook ups and you leave your black tank valve open, every time you use your toilet, the water and liquid waste will drain right into the sewer hookup, just like it’s supposed to. But you know what doesn’t drain out? The solid waste and the toilet paper. 

An RV toilet works with a gravity flush, so everything flushed basically drops directly into the black tank through a straight pipe. The solid waste and toilet paper will stay right where they land in the black water tank. And, over time, that waste will dry out, harden, and compound on top of itself. If you leave your black tank open every time you camp on full hookups, eventually the waste will build up all the way to your toilet, and this is what’s known as an RV poop pyramid. 

Sounds gross, right?! Well, it is gross. And it’s extremely hard to get rid of.

What To Do if You Have a Clog from Leaving Your Black Tank Open

There are a couple ways a clog can manifest from leaving your black tank open. First, the RV poop pyramid can build up to your toilet, clogging the toilet and making it impossible to flush. Or, the solidified waste in your black tank may break loose and block the drain hole leading to the hose. 

Either way, this is a crappy situation that no one wants to be in. 

If you get a clog in your black tank from leaving your valve open, you need to prepare to work on eliminating this clog, and it can take up to 72 hours or longer. 

The process involves breaking up the clog if necessary, using digestive enzymes to break down the waste, and more. Learn how to get dried waste out of your RV tank to take care of this issue, and then keep your black tank closed to make sure it never happens again!

An Overview Of Your RV Holding Tanks

Your RV has three types of water holding tanks: 

  • Fresh water
  • Gray Water
  • Black Water

Your fresh water tank holds fresh potable water, your gray water tank holds waste water from your kitchen sink, bathroom sinks, showers, dishwashers, and washing machines, and your black tank holds waste water from your toilet only. 

Some RVs have the bathroom sink set up to run into the black tank, but this isn’t very common. 

Your black tank and gray tank each have a pipe that runs to your sanitation center where you dump your tanks. Each tank has its own waste gate valve that you pull when it’s time to dump the tank, and they should be labeled or colored according to which tank they belong to. Learn more about RV plumbing systems.

Should You Leave Your Gray Tank Open or Closed with Full Hookups? (OPEN, Unless…)

If you’re at a full hook up campsite, you’re free to leave your gray tank valve open all you want! Gray waste water tanks tend to fill up pretty quickly, in a matter of just a few days. Because of this, it can be more convenient to leave your gray tank gate valve open while you’re on full hookups. 

Most experienced RVers recommend leaving your gray tank open until your black tank is ⅔ full, and then closing your gray tank. Allow your gray tank to fill up for a day so you can use your gray water to rinse out your sewer hose after emptying your black tank.

Reasons You May Want to Keep Your Gray Tank Closed

Some RVers choose to leave their gray tanks closed, even on full hookups. When you are hooked up to a campground sewer outlet, there is a risk of having some unwanted visitors and unwanted smells entering your RV if you leave your tank open. 

The most common unwanted visitors you’ll get in your RV if you leave the tank open are sewer gnats. These sewer flies multiply quickly and are very hard to get rid of. However, rats, bugs, and even snakes have been known to crawl up out of sewer hookups. 

And if it’s not unwanted guests, sewer gas can come up through your sewer hose into your RV. This can create a bad odor in your RV, which is why some RVers choose to leave the gray tank closed until it’s full. Most of the time, the P-trap in your RV sink will prevent any sewer gas from coming up your drain. And You’ll never deal with a pyramid plug in your gray tank, because the only solid waste that goes into your gray tank is the occasional food particles from your sink!

How Long Does it Take for Your Black Tank to Fill?

should i leave my black tank open or closed

Depending on the size of your black tank and how often it is used, it can take anywhere from a few days to a week or longer for it to fill up to the point of needing to be dumped. 

Just as you should never leave your black tank open, you also shouldn’t dump it before it’s at least ⅔ full. This is because it’s generally believed that a black tank needs to be at least ⅔ full to make sure there is enough fluid in the tank to help flush out the solid waste. It doesn’t have to be a completely full tank, but it needs to be more than half full.

Dumping a mostly-empty tank runs the risk of leaving remnants behind like solid human waste and paper, and this can also gunk up your tank sensors, causing them to read inaccurately.

How Often Should You Clean and Flush Your RV Black Tank?

Ideally, you should clean and flush your black tank every time you dump it. However, this isn’t always possible due to time constraints, etc. Here’s when to flush your black tank:

  • After Every Trip: It’s a good practice to empty the black tank after every trip. Before storing your RV, make sure the black tank is empty and flushed out to prevent any residues from hardening or causing odors.
  • Deep Cleaning: At least once or twice a season (depending on usage), it’s beneficial to do a deep clean. This might involve filling the tank with water and a special cleaning solution or tank cleaner, then driving around to agitate the solution inside the tank before you empty the tank.
  • Before Long-Term Storage: If you’re putting your RV into storage for an extended period, especially over winter, it’s crucial to thoroughly empty and clean the black tank to prevent any issues when you start using the RV again.
  • Monitor Sensors: If your RV has sensors that indicate the level of waste in the black tank, keep an eye on them. If they start showing inaccurate readings (e.g., indicating the tank is full when you know it’s empty), it might be a sign that waste or toilet paper is adhered to the walls or the sensors, indicating a need for a thorough cleaning.
  • Usage-Based: If you’re living in your RV full-time or using it frequently, you’ll naturally need to empty and clean the black tank more often than someone who only takes their RV out a few times a year.

How to Avoid and Eliminate Black Tank Odors in Your RV

If odors are making you want to leave your black tank open when hooked up at a campsite, there are ways you can deal with them to eliminate them and prevent them altogether. 

There are a wide variety of RV black tank treatment options to help cut down on odor, and many of them are enzymatic additives that use enzymes to help break down the bacteria in the waste that causes the odors. 

You should aim to keep a good balance of liquid in your black tank in order to give the enzymes enough water to work properly and also to help your black tank fully empty when you dump. Always use plenty of water when you flush your RV toilet by holding down on the pedal for a few seconds to allow water to flow into the tank. 

If you experience odors from your toilet, ensure that the rubber gasket seal on your toilet flap is in good working condition. This seal can sometimes fail, leading to toilet odors escaping into your RV. 

In Conclusion, Keep Your Black Tank Closed and Gray Tank Open

To wrap it up, it’s totally okay to leave your gray tank open when you have full hookups, but you should always leave your black tank closed to avoid clogs. Flush your black tank as often as possible, and make sure you do a deep cleaning a few times a year to keep your tank clean and your holding sensors functioning properly. And remember to always flush with plenty of water and use holding tank treatments to combat odor! Following these tips should help ensure you never have any major issues with your RV black tank.

RVing Headquarters Team

RVing Headquarters is operated by long-time RV enthusiasts with over 8 years of RV industry expertise and full-time and part-time RV ownership experience. From motorhomes to trailers and campervans, we've done it all. Our Mission: to inspire, educate, and entertain anyone who is interested in the RV lifestyle.

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