Black Tank Pressurizes During Backflush

[Snakey Jake]

We recently had a vent problem with our black tank vent. The pipe had dropped into the tank. Got that fixed, cleaned out the vent pipe and we still have pressure in the black tank. When I go to back flush the tank (52 gallon capacity) around 28 gallons it starts pushing pressure through the toilet. The gray tanks are no problem, just the black. Any ideas?

 

[Mikendebbie]

Just a guess…52 gal minus 28 gal equals 24 gals of stuff. Your rig is a 2016 (I believe) - have you ever had the tanks cleaned out professionally? There may be 2” to 3” of concreted stuff that accumulated over the years. That much would account for half of the tank capacity. Another possibility is maybe the top of the tank has collapsed?

My rig has been sitting since July not used. I have not yet tried it, but my front bath black tank (I suspect) has a pyramid chunk in it. I plan to fill the tank with water, then add Liquified additive and let it sit for couple of weeks to see if it will dissolve the pyramid.

 

[Rene T]

You have a couple of problems. That tank should never get pressurized to a point you’d see it in your toilet. That vent pipe may be plugged at the bottom. If you do have a pile of crap in the bottom of the tank and the vent fell in the tank, it may have plugged up the pipe. I would go down the pipe with a rotoruter and see what comes out. Or add water in the pipe from the top with a garden hose. See if it fills up and keeps that level of water
At the same time, go down through the toilet with a camera.

 

[Daryles]

Endoscope down the vent pipe.

Endoscope Camera with Light,1920P HD Borescope Tools with 8 Adjustable LED Lights, Endoscope with Semi-Rigid Snake Camera, IP67 Waterproof Inspection Borescope for iOS and Android-16.4ft Dual-Lens Amazon.com

 

[Snakey Jake]

Thanks to everyone for the replies. Every suggestion, has already been done. Tank was cleaned last year by Kleen Tank, and we used Unique on it this spring. Back flushed until water ran clear. If I have my wife hold the flush valve open with no water to flush it, I can get 50 gallons in. Ran a roto rotor down the vent pipe and then ran water down the pipe. Flowed freely. Tank is not collapsed, when the fixed the vent pipe they checked that. We tried everything we could think of. That is what is so puzzling. Oh well, I keep looking and maybe someone else will come up with something. lol

 

[RMcNeal]

If you get the tank about half full, 28 gallons, then it starts to pressurize, I think the vent pipe may be too low inside the tank. Once the water level gets to the bottom of the vent, air cannot escape through the vent any longer. The toilet becomes the path of least resistance.
The pipe should be just inside the tank. The tank is likely only 8-10 inches high, so if the pipe is 4 inches inside the tank, that would keep half the tank from filling.

 

[DutchmenSport]

OK, I'm just a little bit confused here....

When you "back flush" what are you doing? Are you...
1) using your black-tank, built in, tank rinser with the garden hose attached, or...

2) Are you doing an actual "back flush" from the grey tank bathroom shower/ sink water with a Flush King (attached 3rd valve) between the black and grey tank, or ....

3) Are you using the actual "Flush King" to back flush the black tank via a garden hose attached.

If using method 1, you should NOT close off the dump valve on the black tank when the black tank riser is running. The valve should remain open. Other wise, 2 things (could) happen. Pressurization like you're indicating, and / or not shutting the water off in time and having water come up through the toilet because the tank itself overflows. If you need to fill the tank, it's safer to do it through the toilet. Purchase a sleave that goes inside the toilet bowl hole to keep the water running and the slide valve on the bottom of the toilet bowl open. Fill the tank this way. When water approaches the neck in the toilet, you know it's full. When using the black tank flusher, keep the dump valve open.

If method 2, the Flush King is attached and the initial contents in the black tank are completely dumped. Flush King is closed and the grey tank is opened which will back flush into the grey tank. Even if the grey tank is completely full, at most, only half the contents will back flush into the black tank. This should not cause any pressurization because of the vent pipe. Now, close off the grey tank an open the black again and dump the contents (2nd dump now). Repeat again for a 3rd dump. After 3 dumps, close off the black and open the grey to flush out the sewer hose. This process should never cause water to pressurize, let alone ever put enough back into the black tank to overflow back into the toilet.

Third, Flush King backflush with a garden hose attached. Here I can understand where a black tank might get too full and overflow up the toilet IF the water is not turned off. When using this method, water is added to the tank only a couple minutes. The tank should not be completely filled because of possibility of overflow back-up through the toilet.

So, the question remains .... which method are you using to back-flush. There might not be anything wrong with the tank, it might be simply in the way you are doing it.

Again, if you need to actually fill the entire tank ... do it through the toilet. The tank can never pressurize that way because the toilet bowl hole itself is always open.

Edit:
You say your tank is 52 gallons and after filling 28 it begins to pressurize? Are you using a water meter to verify the gallons? If so... you can try this. (I'm assuming you are not running water through your on-board fresh water tank and water pump).

Attach the garden hose directly to the black tank flusher with the water meter attached. Fill the tank. How many gallons? Before dumping, go inside the camper with a flash light, look down the toilet hole and see where the water level is. Is it truly only half filled or did the water come up the neck of the toilet? Also it's under "pressure" when you hit that foot peddle on the toilet the pressure will instantly release. You should here a pop of some kind. Dump it now.

This time, attach the garden hose to camper fresh water system, reset the counter on the meter back to zero. Now.... fill the black tank using the toilet.

Does it STILL only take 28 gallons to fill the tank. Its a test worthy of trying.

If both fill-ups agree on the same number of gallons, then either your black tank is smaller than advertised, or you have a lot of "stuff" still stuck "in there." Doing this will either validate it's a vent problem or stuff stuck in the tank problem. Good luck!