Black tank flush

[brclark82]

Above my black tank flush valve is a sticker that says do not use the black tank flush unless the black tank drain is open. Is the only reason for this to keep the tank from overflowing into the rv?

I assumed so and flushed for a few minutes with the valve closed to get some water built up in the tank to help flush it out better with the rush of water and it seemed to work fine, but thought I would ask and make sure it isn’t going to hurt anything.

Also, I was monitoring the tank level as the black tank flush was running just to make sure I wouldn’t overflow and I ran the flush for at least 5 mins and it never even registered 1/3 full and there really wasn’t a ton of water that flushed out when I opened the drain. Is that normal?

 

[SeilerBird]

Tank sensors are rarely accurate.

 

[brclark82]

Mine are currently accurate, at least in every other scenario I have encountered

 

[dcrbtt]

If the blank tank Drain valve is closed and you use the black tank flush, and someone uses the Toilet when they flush they will be sprayed by the over pressured black tank. And you will never hear the end of THAT....

 

[brclark82]

So after a couple mins of having the hose turned on I did go flush the toilet and left the drain open for 15 seconds to help get some more water in the tank and nothing happened

 

[HappyWanderer]

It’s simply a warning label for stupid people, like the one that warns not to use a hair dryer in the bathtub.

 

[Rene T]

If the blank tank Drain valve is closed and you use the black tank flush, and someone uses the Toilet when they flush they will be sprayed by the over pressured black tank. And you will never hear the end of THAT....

Very unlikely that your tank would be pressurized because the tank is vented with a 1 1/4” ABS pipe or similar.

 

[Rene T]

Mine are currently accurate, at least in every other scenario I have encountered

Don’t trust them especially while filling your tank with the flusher. Yes you can add water to your tank and then open the drain valve after 3 or 4 minutes (depending on what size tank you have) just make sure you do not get sidetracked and forget that it’s filling.

 

[JudyJB]

Confession: I left the flush hose attached to my black tank for too long and the drain valve closed, and sewage blew out of the vent in the roof. Son had to climb up there and rinse off my roof. Luckily, it did not back up and explode out of the toilet, which would have made a real mess. Very embarrassing to have sewage flowing down the side of your RV.

 

[ChasA]

I had a cg neighbor do the same as Judy. Same result. His wife wasn't there. She still doesn't know about it several years later.

 

[steveblonde]

Some sensors may appear to be more accurate than others but they are notorious inaccurate. I always start flushing my tank by hooking up the flush hose turning it on. Then opening the valve , then the kitchen valve because there is always coffee grounds etc then waiting a couple minutes and opening the bathroom valve cause its just soapy shower water

 

[Gary RV_Wizard]

Is the only reason for this to keep the tank from overflowing into the rv?

Yes. Too many messy accidents from people who get distracted while flushing.

 

[2392pep]

I too left flush on too long with the drain valve closed however the vent pipe is cut just above the ceiling and not above the roof line, septic spilled into the space between the ceiling and roof and out the toilet on to the floor.
Does anyone know if RV manufacturers are required to vent above the rubber roof line? My rep says 'no, they're not'.
My wife jokingly told her brother we may not make to our 51st anniversary.
Thanks
Dave

 

[Gary RV_Wizard]

Are you talking about a backflow preventer or air admittance valve rather than the tank plumbing vent? As far as I know, there is no requirement for backflow devices to vent anywhere in particular - it's an emergency thing rather than normal operation. And AAV's always open to the inside (it's supposed to be an inlet). If you are talking about the primary waste line vent, I thought the RVIA construction standards (NFPA 1192) called for the tank vent to terminate above the roof. However, there are exceptions for things like horizontal vents. I suspect you would have to analyze the plumbing system as a whole to figure out what rules apply. Buy yourself a copy of NFPA 1192 from the NFPA documents website.

 

[2392pep]

Thanks Gary, appreciate your comments and link. Have an appointment to take it to a repair shop for an estimate next week.

 

[Ex-Calif]

I would also guess that the sensors are distributed from top to bottom on the tank. If there is standing water in the tank the bottom sensor doesn't get washed?

 

[Oldgator73]

I’ve flushed mine a few times never leaving it unattended during the process. I let the water run for awhile with the valve closed and then open it. I put a clear elbow on so I can see when the tank starts to rain clear. I have a clean out for our sewer close to my driveway so that makes the process easier. You might have to fill your black tank with several gallons of water and drive around the block to slosh that water around to break up deposits.

 

[Mark_K5LXP]

warns not to use a hair dryer in the bathtub.

Wait - it says that?

I don't have a black tank flush. I've noted that I can dump a few gallons into the black tank after dumping and that usually results in a bit more of a cleanout. But it's a PITA to do especially when there's a line at the dump station. It seems to take forever to put even a few gallons into the tank with a hose. This season something I've tried is after the black tank is done draining I'll open up the gray tank with the black valve still open, and quite a bit of gray will backflow into the black tank. Enough that it seems to wash out a bit more than without. Cycle the gray a few seconds 2 or 3 times letting the black drain in between, then close off the black tank, and open the gray to drain the remaining and wash out the stinky slinky per usual. Quick, easy, very little additional time and a bit of a positive effect.

Mark B.
Albuquerque, NM

 

[Rene T]

This season something I've tried is after the black tank is done draining I'll open up the gray tank with the black valve still open, and quite a bit of gray will backflow into the black tank. Enough that it seems to wash out a bit more than without. Cycle the gray a few seconds 2 or 3 times letting the black drain in between, then close off the black tank, and open the gray to drain the remaining and wash out the stinky slinky per usual.

I do that all the time except I installed a spin on valve on the end of my sewer pipe and it stays there all the time. By shutting this valve I can put about 1/2 of the water that was in the gray tank into the black tank. I have 2 gray tanks so I do the same with the other tank too.
Also that valve serves as a backup valve in case one of my drains valves are leaking by a little.

 

[Seon]

I just drained the black tank after a week's camping but the discharge fluid didn't flow out like it normally does so I placed the magic wand down the toilet to hose the sides then opened the valve that had a better drain flow. I repeated the flush and filled the tank twice thoroughly. Water was clear and flow was normal.

While at it, I noticed that the panel showed 2/3 full but the water level below the toilet was not even half way. Took a while to completely fill the tank and the panel read full.