black tank not venting

[jbrosecity]

We have a 2020 high end 5th wheel and also own a 2019 THOR Miramar motorhome, and have owned many other coaches. The problem is actually on my 5th wheel but black tanks are pretty much the same everywhere and I appreciate the expertise on this forum so I thought I would post here. This is the first time we have had this problem and I suspect a black tank venting issue but I welcome input from other members.

When the black tank just reaches about 2/3 full it really begins to stink when we flush, especially if the roof vent is open. We use chemicals and have tried many different ones. This is not a chemical issue. We have ZERO problem in our motorhome. Let me elaborate... What immediately suggested a venting issue was this...

I ALWAYS saniflush, often twice and each time I close the dump valve and carefully and watchfully let about 20 gallons in with the saniflush attachment before I open the dump valve again and let it drain again. I always empty it the last time, then add a gallon or two of water, add chemical down the toilet and then wait until it starts to stink again, always about 2/3 full on the digital gauge.

But this was the big clue. As we were leaving for the long weekend, I added my 20 gallons one last time (3rd filling), then stopped and then watched the digital gauge until it showed FULL. Then the plan was to fill it up the rest of the way using the toilet flush valve watching from the inside with the toilet valve open until it was completely full and then add some RidEx to attempt to dissolve any accumulated solids anywhere in the tank.

OOPS - HERE'S WHAT HAPPENED. My wife stepped on the toilet flusher just slightly and said it was like a geyser of water water under pressure. NOTE, I had already turned off the saniflush attachment, planning to fill the tank the rest of the way from inside. So WHY would the tank be pressurized unless the vent were not venting and filling the empty tank with the saniflush created a lot of air pressure inside which immediately rushed to escape when my wife cracked the flush valve? Again we NEVER experienced this geyser effect before because we never do this. I normally always empty the tank again from the dump valve and just put a gallon or so of fresh water inside. I only filled it up this time to add the RidEx.

So doesn't this sound like a blocked vent? And that would explain why the stinky gas builds up inside without venting? If you agree then what is my best solution? Suspecting the smell might be a blocked vent, I had asked the last guy who was on my roof to run a hose down the the roof vent tube. He said he did, but maybe he didn't let it run long enough to either clear the tube or back up and overflow onto the roof? I am thinking to try this again and watch inside the bathroom with flush valve open and see that the water is coming down thru the vent tube and filling the black tank. Make sense? Any other ideas? What else could be the problem if the hose filling from the roof vent flows freely?

HELP!

[16ACE27]

Originally Posted by jbrosecity

We have a 2020 high end 5th wheel and also own a 2019 THOR Miramar motorhome, and have owned many other coaches. The problem is actually on my 5th wheel but black tanks are pretty much the same everywhere and I appreciate the expertise on this forum so I thought I would post here. This is the first time we have had this problem and I suspect a black tank venting issue but I welcome input from other members.

When the black tank just reaches about 2/3 full it really begins to stink when we flush, especially if the roof vent is open. We use chemicals and have tried many different ones. This is not a chemical issue. We have ZERO problem in our motorhome. Let me elaborate... What immediately suggested a venting issue was this...

I ALWAYS saniflush, often twice and each time I close the dump valve and carefully and watchfully let about 20 gallons in with the saniflush attachment before I open the dump valve again and let it drain again. I always empty it the last time, then add a gallon or two of water, add chemical down the toilet and then wait until it starts to stink again, always about 2/3 full on the digital gauge.

But this was the big clue. As we were leaving for the long weekend, I added my 20 gallons one last time (3rd filling), then stopped and then watched the digital gauge until it showed FULL. Then the plan was to fill it up the rest of the way using the toilet flush valve watching from the inside with the toilet valve open until it was completely full and then add some RidEx to attempt to dissolve any accumulated solids anywhere in the tank.

OOPS - HERE'S WHAT HAPPENED. My wife stepped on the toilet flusher just slightly and said it was like a geyser of water water under pressure. NOTE, I had already turned off the saniflush attachment, planning to fill the tank the rest of the way from inside. So WHY would the tank be pressurized unless the vent were not venting and filling the empty tank with the saniflush created a lot of air pressure inside which immediately rushed to escape when my wife cracked the flush valve? Again we NEVER experienced this geyser effect before because we never do this. I normally always empty the tank again from the dump valve and just put a gallon or so of fresh water inside. I only filled it up this time to add the RidEx.

So doesn't this sound like a blocked vent? And that would explain why the stinky gas builds up inside without venting? If you agree then what is my best solution? Suspecting the smell might be a blocked vent, I had asked the last guy who was on my roof to run a hose down the the roof vent tube. He said he did, but maybe he didn't let it run long enough to either clear the tube or back up and overflow onto the roof? I am thinking to try this again and watch inside the bathroom with flush valve open and see that the water is coming down thru the vent tube and filling the black tank. Make sense? Any other ideas? What else could be the problem if the hose filling from the roof vent flows freely?

HELP!

Hard to say. If you filled it with the saniflush until it read "full", with the accuracy of these level gages it could have been overfull and half way up the vent pipe. Then opening the toilet ball valve would let the water it the vent pipe push water up and out of the toilet. It's really difficult for a tank vent to get clogged if it was installed properly from the factory.

[Chateau_Nomad]

I agree with ACE on the cause of the toilet geyser.

Like you, I really flush the heck out of my black tank. I have the 360 roof vents. Even so, during my incessant flushing sessions (at home using well water and my own sewer connection), I will occasionally get an "off color" whiff of stank in the RV bathroom. And that's the ONLY time I've ever smelled anything.

It goes away and never happens while in actual use. I chalk mine up to air pressure and drafts. ???

[chunker21]

The vent tube is supposed to be glued into the black tank top. Just a small bit extending down into the tank. The vent tune may not be installed correctly and slid dwell down inside the tank when it was glued or it may just be in a friction fit and not glued. As such when fluids get to the tube sticking down, the tank quits venting up and takes the path of least resistance, the toilet.

[16ACE27]

Originally Posted by chunker21

The vent tube is supposed to be glued into the black tank top. Just a small bit extending down into the tank. The vent tune may not be installed correctly and slid dwell down inside the tank when it was glued or it may just be in a friction fit and not glued. As such when fluids get to the tube sticking down, the tank quits venting up and takes the path of least resistance, the toilet.

Well it doesn't stop venting in that case, it vents the same way as if the tank was full, by venting water up the vent tube.

[chunker21]

Originally Posted by 16ACE27

Well it doesn't stop venting in that case, it vents the same way as if the tank was full, by venting water up the vent tube.

But the head pressure up the vent tube is higher than the toilet so that's the path of least resistance.. Y'all figure it out. Done with this.

[16ACE27]

Originally Posted by chunker21

But the head pressure up the vent tube is higher than the toilet so that's the path of least resistance.. Y'all figure it out. Done with this.

Did you read the thread? I addressed the water up the vent tube back in post #2.

[Lt Keefer]

I fifn't see this in the comments, but how about climbing on the roof and removing the cap off the vent pipe. You could shine a light down and look for a blockage or drop a plumb bob on a string and take a 'rough' measurement to see if it goes all the way to the tank. Maybe crawl under the RV and try to get a look at the vent pipe's entry into the tank. Anyway, some kind of physical inspection would be better than the conjecture form internet forums.

[JimOIB]

Does your bathroom sink drain into the same black tank? If so, does it drain normally or slowly? If normal then it’s probably not a blocked vent stack.

I’m with 16ACE27, you probably overfilled to black tank up through the vent but not high enough for the sink to overflow. That would have made for a bad day.

From my own experience, if my black tank get 2/3 or better especially on a hot day, I get tank fumes coming in from the roof vent. With a slight breeze even more so. Open the roof vent, and any window for cross flow with the bathroom door closed and pee-yew. My black tank vent is 6 inches from the roof vent - good engineering. My solution - first smell of pee-yew and the tank gets dumped. My wife has a good pee-yew nose.

[dkoldman]

Major Capt Obvious here, but did you say what type of vent is on the trailer? We use to get whiff similar to the way you described, even when driving if toilet was used the odors would back into RV. I replaced with the Syphon 360 and we have never had any whiffs. It is almost as of the factory vents didn't work. I recall the 1st day I put the new 360 vents on, within an hour we saw flies around the black tank vent. I had never seen that before and was a little concern but after about a week, we have never seen flies again, I think it may have fully vented the tank to reasonable levels?

But what I would do in your situation is to get a plumbing drawing for the trailer's black tank. That geyser thing is odd, you may have a weird shaped black tank that may becomes pressurized due to the gases, which is indirectly what the ACE is alluding to which I agree.