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!

 

[Snakey Jake]

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!

Just saw your message. Most of the things you have mentioned have been tried. If we hold the foot valve down while using the Flush King, we can get 50 gallons of water into the tank. That is the confusion. We had the vent pipe checked and it was to low, but even after that we still get pressure. I have talked to multiple techs and all the things you mentioned have been discussed. lol That is where the crazy comes in. I'll keep playing an see what happens. Thanks for the reply.

 

[Mikendebbie]

Snakey Jake said “If we hold the foot valve down while using the Flush King, we can get 50 gallons of water into the tank”

“Water in” rate has to equal the “air out” rate. Sounds like your “air out” rate is hampered or blocked.

So…as you fill the tank with water from the outlet/low point using the flush king…the tank fills because air is allowed to escape thru the open foot valve. For me this reinforces the idea that the vent pipe lower end is stuck down into the tank by 3”…4”…5” ???. When filling water level reaches the bottom of the pipe - no more air can escape to allow more water to come into the tank. The tank is probably not perfectly sealed…i suspect there may be a “slow rate” of water injection/filling that would equal the rate that air can escape and the tank would accept more water to a FULL point…but that rate is not useful to you. You need full free flow going both ways.

You said the vent pipe had been checked… but i dont see any other way the “air out” (air escaping the tank) volume could be affected. That vent pipe is either clogged or poking into the tank too much.

 

[Mikendebbie]

This sketch illustrates my theory...

 

[mlh]

It’s impossible to get pressure in a tank with a 2 inch opening in the top of it.
You say you get pressure in the tank. How much pressure a half pound 20 PSI. Just saying pressure means almost nothing. Explain how much pressure.
Lynwood

 

[Snakey Jake]

I have no idea if it is 1 pound or 20. It will spray water all over the underside of the seat. Don't tell me it is impossible to get pressure in a black tank. I know better.

 

[ChuckS]

Don't understand why you would want to fill the black tank full or close to full using the King Flush... and I am surprised that much water can actually be back pushed thru the black tank drain line with a water hose to the King Flush..

I use the King Flush "Only" for my black tank cleaning... never had connected a hose to the built in tank sprayer.... I run the water for about 5 minutes max with the King flush valve closed, turn water off and open king flush valve and drain.. I repeat this process about 2 more times and I am done... black tank is clean and I also back flush both gray tanks...

I never get any back pressure in the black tank with king flush valve closed during back flush fill from the hose... tank vent does its job....

 

[mlh]

I’m sorry I didn’t say it was impossible to get pressure in the black tank. It’s impossible with a 2 inch hole in it. If that pipe isn’t allowing air to escape then you don’t have a 2 inch hole in the tank.
Lynwood

 

[Mikendebbie]

When I connect the water hose to the flush “fitting”, the pressure from the hose will start filling the tank - until it gets to a point that the weight of the water in the tank (tank is higher than the hose connection) overcomes the hose pressure and the water just sort of “spins” (I can see it churning thru the clear elbow) and the tank can not be filled anymore. That is the point where I know to pull the valve and let the water out/drain the tank. The water hose can only push “so much” into the tank due to gravity, or water column (I think it is called).

I doubt very seriously that the tank is full of water at the point where a hose just can defeat the water weight in the tank above. I’m thinking this is the “back pressure” you are experiencing with your flush king - not some pressure pushing down from inside the tank due to a plumbing/piping mistake. I figure - that’s all the hose can do…let them ponies out!

 

[Snakey Jake]

Don't understand why you would want to fill the black tank full or close to full using the King Flush... and I am surprised that much water can actually be back pushed thru the black tank drain line with a water hose to the King Flush..

I use the King Flush "Only" for my black tank cleaning... never had connected a hose to the built in tank sprayer.... I run the water for about 5 minutes max with the King flush valve closed, turn water off and open king flush valve and drain.. I repeat this process about 2 more times and I am done... black tank is clean and I also back flush both gray tanks...

I never get any back pressure in the black tank with king flush valve closed during back flush fill from the hose... tank vent does its job....

I am just a kid, 73 years old. Been camping since I was 8. Have had RVs for 20 years and running the tank up to 80% is what I was shown. It gets the crap off of the sensors and off the side walls of the take. We wanted to see how much the tank would hold so we disconnected the water from the trailer and filled the tank with the toilet valve open, using the flush king.

 

[Snakey Jake]

I’m sorry I didn’t say it was impossible to get pressure in the black tank. It’s impossible with a 2 inch hole in it. If that pipe isn’t allowing air to escape then you don’t have a 2 inch hole in the tank.
Lynwood

I agree with your statement about you can't get pressure with a 2 inch hole. That is what makes this so puzzling. We ran water down the vent an it never filled up the vent pipe. Filled the tank but not the pipe.

 

[twindman]

I agree with your statement about you can't get pressure with a 2 inch hole. That is what makes this so puzzling. We ran water down the vent an it never filled up the vent pipe. Filled the tank but not the pipe.

Running water down the pipe will have water pressure in the pipe (due to the weight of the water) and push the water into the tank, I think. And perhaps air can escape up into the toilet then. You would have to completely fill the tank and then it would back up in the vent. Just seems strange tho!!!

 

[RMcNeal]

It still sounds like the vent pipe is protruding into the tank a few inches. Once the water level reaches the vent pipe, it's sealed and air can no longer escape. I have an endoscope I bought on Amazon that I could run down the toilet and look inside. It might be worth looking. Only paid about $20 and it hooks up to my phone for the display. I would bet a whole cup of coffee that the vent pipe is several inches inside the tank.

 

[Snakey Jake]

When I connect the water hose to the flush “fitting”, the pressure from the hose will start filling the tank - until it gets to a point that the weight of the water in the tank (tank is higher than the hose connection) overcomes the hose pressure and the water just sort of “spins” (I can see it churning thru the clear elbow) and the tank can not be filled anymore. That is the point where I know to pull the valve and let the water out/drain the tank. The water hose can only push “so much” into the tank due to gravity, or water column (I think it is called).

I doubt very seriously that the tank is full of water at the point where a hose just can defeat the water weight in the tank above. I’m thinking this is the “back pressure” you are experiencing with your flush king - not some pressure pushing down from inside the tank due to a plumbing/piping mistake. I figure - that’s all the hose can do…let them ponies

It still sounds like the vent pipe is protruding into the tank a few inches. Once the water level reaches the vent pipe, it's sealed and air can no longer escape. I have an endoscope I bought on Amazon that I could run down the toilet and look inside. It might be worth looking. Only paid about $20 and it hooks up to my phone for the display. I would bet a whole cup of coffee that the vent pipe is several inches inside the tank.