If you’ve spent more than five minutes around RVers, you’ve heard the argument. One camper swears, “Full hookups = leave it open.” Another camper clutches their sewer hose like a safety blanket and says, “Absolutely not—poop pyramid!”
At Diamond Lure, we’re not here to start a campground civil war… but we are here to help you avoid a weekend ruined by a prehistoric mound of regret.
Quick refresher: What’s actually happening in the black tank?
Your black tank is not a magical portal where things disappear. It’s a holding tank that needs
liquid + volume + gravity to carry solids out when you dump.
When the valve is open, liquids can run out continuously while solids and paper can stay behind and pile up right under the toilet drop—hello,
“poop pyramid” / pyramid plug. Thetford (a major RV sanitation manufacturer) is blunt about it: don’t leave the black tank valve open during use, even with a sewer hookup, because liquids drain and solids pile up.
Thetford
Keystone RV’s own owner guidance also says to
keep termination valves closed at full hookups to prevent solids sticking/clogs/odor issues.
Keystone RV
So why is it controversial? Because “open” can feel convenient… until it isn’t.
Option A: Leaving the BLACK valve OPEN at full hookups
Pros
- Convenience: It feels like having a tiny septic system on-demand. No tank-watching.
- Less “capacity anxiety”: In theory, you’re less likely to overfill the black tank.
- Works “okay” for some scenarios: Some folks claim success when they use lots of water and minimal paper—especially if they dump frequently (though it’s still not the recommended approach).
RVshare
Cons (the big ones)
- Poop pyramid / pyramid plug risk: Liquids escape, solids don’t. That’s the classic recipe.
Thetford+1
- Odor pathway: An open valve can turn your RV into an extra vent stack for the campground sewer line (and nobody wants Eau de Sewer Breeze drifting into their rig).
Unique Camping + Marine+1
- Sensor issues & sludge buildup: Stuff dries, sticks, and makes your tank sensors lie like a politician in an election year.
happycampers.store
- Harder cleanups: If a plug forms, fixes range from annoying to “why is my weekend like this?”
Verdict on OPEN: It’s the “easy button”… right up until it becomes the “emergency plumber” button.
Option B: Keeping the BLACK valve CLOSED until it’s time to dump (the “hold & dump” method)
Pros
- Best protection against the poop pyramid: Keeping liquid in the tank helps solids stay suspended and move out when you dump.
Thetford+1
- Stronger dump flow: When the tank is 2/3–3/4 full, you get a solid whoosh that carries more out.
National RV Training Academy
- Less odor risk: Closed valve = less sewer gas traveling back toward your RV.
Unique Camping + Marine
- Better long-term tank health: Fewer deposits, fewer clogs, less “mystery smell” haunting your bathroom.
Cons
- You have to monitor tank level: Not obsessively, but at least enough to avoid overfilling.
- If you “under-water” your flushes, you can still create problems: The closed valve method works best when you use
plenty of water.
National RV Training Academy
- More steps: Dumping is a routine, not a passive lifestyle.
Verdict on CLOSED: Slightly less “lazy,” massively more “successful.”
The Diamond Lure “Do This, Not That” Routine (simple + effective)
The approach most manufacturers and RV pros push.
- Keep the BLACK valve closed while you’re camping.
Thetford+1
- Use plenty of water every flush. (Dry tank = sad tank.)
National RV Training Academy
- Dump when the black tank is about 2/3 to 3/4 full (or before you leave).
National RV Training Academy
- If you have a built-in flush system, rinse per manufacturer instructions.
RVshare
- Dump the GRAY tank after the black to help rinse the hose with soapy water. (A classic move.)
- Close everything, cap everything, and go enjoy the campground like a champion.
“Okay, but what about the GRAY valve?”
Many RVers leave the
gray valve open at full hookups—and that’s common. Even Thetford says you can leave the gray valve open when hooked up.
Thetford
That said, some experienced folks still keep gray closed sometimes to avoid odors traveling back up from the sewer line (and to get a better rinse when dumping). If your site’s sewer line is extra “fragrant,” keeping gray closed can help.
Common myths (and the reality)
- Myth: “If I leave it closed, I’ll clog my RV.”
Reality: Closed is fine—just don’t ignore tank levels and don’t skimp on water. - Myth: “Open valve means cleaner tank.”
Reality: Open valve can leave more solids behind.
Thetford+1 - Myth: “Poop pyramid only happens to newbies.”
Reality: Poop pyramids are equal-opportunity chaos.
Final take: What should you do at full hookups?
If you want the least drama and the best odds of a smooth trip:
✅
Leave the BLACK valve CLOSED until it’s time to dump (ideally near 2/3 full), then dump black and follow with gray.
Thetford+1
Because nothing says “relaxing mountain getaway” like… not having to Google “how to defeat a pyramid plug” from your RV bathroom.
