black tank not venting?

[SalorJoe]

We have a 2020 high end 5th wheel and also own a 2019 motorhome, and have owned many other coaches. 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!

[dougrainer]

SJ-Chris wrote:

Boon Docker wrote:

SJ-Chris wrote:
It isn't clear to my why, even if the vent pipe is inserted 1/3 the way down into the black tank, why there would be pressure built up. It isn't a sealed system (because of the vent...even if the tube is submerged) so there shouldn't be pressure. Yes, the toilet might burp a bit, but not actual pressure.

The liquid would rise up the vent tube as the tank fills. When the toilet is flushed the liquid would rush back down the vent tube. Causing a release of pressure through the toilet.

The liquid would NOT rise UP the vent pipe beyond the liquid level (1 inch? 2 inches? 3 inches?). As more liquid is put into the tank (from the toilet), it would simply flow into the black tank. I agree with the OP that as liquid is flowing into the black waste tank, some air will release back into the toilet.

-Chris

IN NORMAL operation the liquid in the tank will not raise above the bottom of the toilet ball valve. BUT, if you connect the Black Water rinse and FORGET it, the water pressure WILL push the water UP the vent pipe and on the roof and IF the 1.5 inch vent pipe is NOT sealed correctly at the roof that black water will flow into the roof cavity. BIG MESS and smell. There can be 2 possible problems. The 3 inch toilet down pipe protrudes into the tank too far. OR the 1.5 inch Vent pipe also protrudes into the tank too far. I have seen BOTH happen but not both at once. DUMP THE TANK. Then flush the toilet a few times. IF not burping or same type problem, then the vent or 3 inch is too far down. Doug

[valhalla360]

Boon Docker wrote:

SJ-Chris wrote:
It isn't clear to my why, even if the vent pipe is inserted 1/3 the way down into the black tank, why there would be pressure built up. It isn't a sealed system (because of the vent...even if the tube is submerged) so there shouldn't be pressure. Yes, the toilet might burp a bit, but not actual pressure.

The liquid would rise up the vent tube as the tank fills. When the toilet is flushed the liquid would rush back down the vent tube. Causing a release of pressure through the toilet.

If you've ever done home brewing, they way you carbonate beer is add some extra sugar after fermentation, then seal the bottle, so pressure builds up carbonizing it.

Sewage decomposing will also off gas (different gas types and nasty odors). At 2/3 full, the air space will pressurize pushing water up the vent tube. This may have pushed water up the vent tube far more than you might expect and when you open the toilet valve all that pressure releases.

[Pipeman]

I was rinsing the black tank when I decided to go into the 5er for something. As I walked around the side of the 5er I noticed water leaking over my basement storage door. I ran back and turned off the water to the black tank and pulled the dump handle. The black water dump hose then jumped up into the air and proceeded to dump all over me. So the moral of the story is, yes pressure can build up. The water leaking over the basement storage door was coming from the toilet. Just slowly leaking over the top of the toilet bowl. Glad I didn't try to flush the toilet. I have never left the side of the trailer while flushing the black tank since then.

[SJ-Chris]

Boon Docker wrote:

SJ-Chris wrote:
It isn't clear to my why, even if the vent pipe is inserted 1/3 the way down into the black tank, why there would be pressure built up. It isn't a sealed system (because of the vent...even if the tube is submerged) so there shouldn't be pressure. Yes, the toilet might burp a bit, but not actual pressure.

The liquid would rise up the vent tube as the tank fills. When the toilet is flushed the liquid would rush back down the vent tube. Causing a release of pressure through the toilet.

The liquid would NOT rise UP the vent pipe beyond the liquid level (1 inch? 2 inches? 3 inches?). As more liquid is put into the tank (from the toilet), it would simply flow into the black tank. I agree with the OP that as liquid is flowing into the black waste tank, some air will release back into the toilet.

-Chris

[Boon_Docker]

SJ-Chris wrote:
It isn't clear to my why, even if the vent pipe is inserted 1/3 the way down into the black tank, why there would be pressure built up. It isn't a sealed system (because of the vent...even if the tube is submerged) so there shouldn't be pressure. Yes, the toilet might burp a bit, but not actual pressure.

The liquid would rise up the vent tube as the tank fills. When the toilet is flushed the liquid would rush back down the vent tube. Causing a release of pressure through the toilet.

[SalorJoe]

SJ-Chris wrote:
It just occurred to me that you could have BOTH problems....the tank vent could be 1/3rd into the tank AND you could have a small leak in the connections to the black tank (either the vent, or the toilet, or maybe your bathroom sink). If it is a small leak, then perhaps when the tank is less than 2/3rds full MOST of the smelly air vents out the vent pipe to the roof. But when it is more than 2/3rds full, maybe the vent pipe is submerged and now the stinky air cannot go up the vent pipe and finds its way through a leaky black tank connection (and then the smell drifts up into your RV).
Good luck!
Chris

Chris - no leaks at all... not ever and it never smells EXCEPT when we flush and the tanks reaches about 2/3 full. When it is 1/3 full, even if it has been sitting a while, there's no real noticeable smell. That is why I think the most likely scenario is that the factory glued the vent pipe in place poking too far into the tank as suggested by others as well. At that point, as you say there is no longer any vent tube for the stinky air to escape so it comes right out of the toilet when flushing... and much worse if the vent fan is on in the bathroom. That makes total sense to me.

As for the air pressure that built up in there, it is basically a closed space the the water level is above the bottom of the vent pipe and with the toilet valve closed and water on top of it, the pressure would then build in the open space with no way to escape and would blow out the toilet when it opens. "Geyser" was much too strong a word by wife gave to it. I think it was basically like air pressure that built up and then blew the flushing water up in a spray into the bathroom and onto my wife's shirt. The tank had been saniflushed twice with 20+ gallons in there, so the water in there wasn't particularly dirty but anyhow, I doubt there was much of the standing water that sprayed out when the air pressure escaped... just the new fresh water trickling down the bowl when she stepped on the flusher. That's how I see it. I am not much inclined to tackle repairs myself. I know some mobile techs who will come over when the black tank has been emptied and saniflushed and try to raise the pipe up so it barely enters the tank and then reseal it.... and then presumably shorten the piece of pipe coming up thru the roof.

I'll post back to the forum after a tech has tackled it. Meanwhile for now we just no we have to dump and saniflush each time the tanks reaches about 2/3.

[SJ-Chris]

It isn't clear to my why, even if the vent pipe is inserted 1/3 the way down into the black tank, why there would be pressure built up. It isn't a sealed system (because of the vent...even if the tube is submerged) so there shouldn't be pressure. Yes, the toilet might burp a bit, but not actual pressure.

Question: Does the smell start happening after it goes over 2/3rds full AND you are flushing the toilet (ie. toilet line open directly to the black tank)? Or does the smell just start showing up in the bathroom and the RV in general once your tank is 2/3rd full? If it is just starting to show up in general (not while flushing) then it seems like there is some other problem. The black tank is supposed to effectively be a closed system (except for the roof vent). That means there shouldn't be any smell while the "stuff" just sits there in the tank. Your toilet itself has a valve/seal and a water barrier so the smell shouldn't be coming from there. Shouldn't matter if it is 1/3rd full, 2/3rd full, or 100% full.

This may or may not be helpful....
Recently, I had a crack/leak on my black waste tank where the 3" pipe from the toilet connected to the top of the tank. That also meant "smell" could leak out anytime there was "stuff" in the black waste tank. Since that connection is somewhat exposed to the floor/underside/plumbing of the RV, I could smell it. ESPECIALLY when driving as the wind (from driving) would swirl around underneath the RV and push the smelly air up into the plumbing connections area/etc which happened to be underneath the kitchen sink/cabinets. The only solution for me was to drop the black waste tank, fix and reseal everything, and then install it back in place. Here's a link to that job in case it is helpful.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30326133.cfm
If you go to post #10 you will see the complete job with pictures. It Also shows you what the black vent pipe looks like (for my rig) and might give you ideas.

On my 2015 Thor Majestic 28a, my black vent pipe was installed in a way that there would be NO CHANCE to slide it in/out (seemed to almost be glued in place).

You can test to see if there is a leak at the top connections to your black tank... The first step is to EMPTY your black tank and give it a good flushing. You don't really want to be testing for leaks with poopy water. Then, simply fill it 90% of the way with water with your flush system. Then, turn off your water pump, get a 5 gallon bucket and slowly start to pour water into your black tank through your toilet. Eventually the black waste tank will fill up (you will kind of be able to hear as it is filling to the top). Then continue a bit more and you will fill the 3" waste line that is going from your toilet to your black waste tank. Then you can even fill up to your actual toilet about 5-10% full (keeping your foot on the flush pedal). If you have no leaks, the water will just sit there and not go down at all. If you have a leak, you will notice that the water level in your toilet starts to go down and the water down in your toilet waste line will go down also. Presumably, you should see a puddle of water underneath your RV if you have a leak (...the water has to go somewhere, right?). If you have a leak in your black waste tank/plumbing, that could be what is causing your smell.

Sorry you are going through this...not a fun problem. Keep us posted.

It just occurred to me that you could have BOTH problems....the tank vent could be 1/3rd into the tank AND you could have a small leak in the connections to the black tank (either the vent, or the toilet, or maybe your bathroom sink). If it is a small leak, then perhaps when the tank is less than 2/3rds full MOST of the smelly air vents out the vent pipe to the roof. But when it is more than 2/3rds full, maybe the vent pipe is submerged and now the stinky air cannot go up the vent pipe and finds its way through a leaky black tank connection (and then the smell drifts up into your RV).

Good luck!
Chris

[SalorJoe]

thanks opnspaces... good idea as well... I have a hunch the tube is just too long and submerges once the tank reaches 2/3 full... never smells before that. But I will try everything soon 🙂

[opnspaces]

Empty the tank and then get up on the roof. Remove the black tank vent cap and see if the black vent tube us up to the roof or if it has slipped down into roof (Means it has slipped down into the tank)

If it is slipped down you might be able to grab it with something and pull it back up.

If the pipe is pulled up high maybe try having someone down near or under the empty black tank. Drop a few ice cubes down the pipe and see if they hit the bottom of the tank. This will at least tell you if there is a plug of sorts in the pipe.

[SalorJoe]

MFL wrote:
^ Could be what Rob describes. If bathroom roof vent is open, and especially if exhaust fan is running, it will draw odor when flushing, since that is a direct vent.

Jerry

Yes Jerry.. Sounds like Skibane and Second Chance hit the nail on the head. Makes perfect sense to me and I hadn't thought of that, but yes obviously when the tank is only 1/3 full the gas that collects in the top 2/3 escapes out the vent as designed; but once it hits 2/3 full and the vent pipe, which is stuck too far into the tank, gets below the water level, now the gas would collect in the upper 1/3 and remain there and then escape out the open hole when the toilet is flushed, exacerbated of course if the roof vent is pulling air up from the tank. Great reply. I have written Vanleigh to see if they agree, but given the conditions I have described and the air pressure which built up when I pressed the tank up almost full, this makes perfect sense. Thanks Guys!

[MFL]

^ Could be what Rob describes. If bathroom roof vent is open, and especially if exhaust fan is running, it will draw odor when flushing, since that is a direct vent.

Jerry

[Skibane]

^Yep.

In some cases, the RV manufacturer simply cuts a hole in the top of the holding tank, sticks the vent pipe part way down it, and then seals the opening with silicone sealant.

The problem with this approach is that unless the pipe is positioned so that its end is just barely inside the tank, it gets submerged as soon as the tank starts filling up.

You may have to cut away the sealant around the pipe, shorten the pipe enough that it just barely enters the top of the tank, and then re-seal it.

[Second_Chance]

Sounds like the black tank vent pipe was installed too far down in the tank or has fallen there.

Rob