Black Tank Venting

[Grandesigner]

So we had terrible smells in the bathroom this last trip, even after flushing the black tank with lots of water and adding more solution. I'm wondering if the vent pipe is not working right, so yesterday I confirmed which one it is while filling the tank with fresh water again.
After removing the vent cap I ran water down the tube and had my wife listen for water going into the tank thru the toilet while open. She heard nothing...what? Where did the water go?
Turns out it went into one of the gray tanks as evidenced by the gage showing 2 dots lit up. Sure enough, clean water came out when I pulled the lever. So obviously the black and gray tank share the vent. Still not sure how to verify the black vent is clear except by now mild smell that came out the vent. It would be nice to confirm a clear path some other way!
Anyway, I replaced the vent cover with one of these which had worked well in a previous trailer.
https://www.amazon.com/gp/product/B0...0?ie=UTF8&th=1

Any ideas on this?

[jkwilson]

Pour something in the black tank with a strong smell and see if you can smell it at the vent. Ammonia?

[Stilly1155]

So we had terrible smells in the bathroom this last trip, even after flushing the black tank with lots of water and adding more solution. I'm wondering if the vent pipe is not working right, so yesterday I confirmed which one it is while filling the tank with fresh water again.
After removing the vent cap I ran water down the tube and had my wife listen for water going into the tank thru the toilet while open. She heard nothing...what? Where did the water go?
Turns out it went into one of the gray tanks as evidenced by the gage showing 2 dots lit up. Sure enough, clean water came out when I pulled the lever. So obviously the black and gray tank share the vent. Still not sure how to verify the black vent is clear except by now mild smell that came out the vent. It would be nice to confirm a clear path some other way!
Anyway, I replaced the vent cover with one of these which had worked well in a previous trailer.
https://www.amazon.com/gp/product/B0...0?ie=UTF8&th=1

Any ideas on this?

It might be the vacuum breaker for the grey tank under your sink. We had a foul odor that I thought was the black tank vent also and after "sniffing" around no pun intended we replaced both vacuum breakers under the kitchen and bathroom sinks and the foul odor disappeared.

Home depot had good ones for about 30 bucks each as I remember. GD uses ones that cost about 5 bucks, hence the problem with them.

Just a thought and worth checking. YMMV

Tim

[Second Chance]

It is common - in foundation homes and RVs - for tanks/appliances to share roof vents when the system has been designed with AAVs (air admittance valves). This is the case with our Grand Design RVs. The most common cause of bad smells in the bathroom or under the kitchen sink is a malfunctioning AAV. Sometimes they are loose enough to fall off (we've found this on both of our fifth wheels); sometimes they just stop working correctly. Just this past week we had a bad smell in the bathroom. That AAV had been replaced 18 months ago with a good aftermarket brand and was still tight. I removed it (capping the pipe with plastic and a rubber band temporarily) and soaked the AAV in very hot water with dish detergent. After soaking, I agitated the valve in the water and then gave it a good rinse with hot water. I then verified that the valve would admit water when submerged upside down in a container and would then retain the water when lifted out of the container (still upside down). This verified the one-way function of the flexible flapper valve. After reinstalling the AAV in the bathroom, no more bad smells!

This link has some good explanations and diagrams:

https://www.oatey.com/faqs-blog-vide...ittance-valves

Rob

[WhittleBurner]

I 2nd checking/replacing the AAV in the bathroom. Likely won't be long before the kitchen AAV stinks as well[ so if there is 2 pack, buy it

[Redapple63]

X3 for the AAV

Bill

[Grandesigner]

Thanks for the good thoughts, I will check them out.
A little more detail might help narrow it down.
The only time we noticed the smell was when we flushed the toilet, which opened the big valve to the tank. I started blaming the new and less effective black tank treatment w/o formaldehyde. We did not have this issue before the old stuff was no longer available to purchase.
Yesterday when I was filling the black tank via the rinse port on the nautilus, I went on the roof and could smell faint black tank odors coming from the same vent I could hear water sounds from. That is how I identified which vent was the candidate for the new fixture. I just wanted to make sure there no tp or something blocking it, so that was why I poured water down the pipe. It flowed freely but did not make it to the black tank, presumably due to the plumbing junction to the gray tank.

For the AAV issue, was the smell constant? We only smell it after flushing...

[Second Chance]

Thanks for the good thoughts, I will check them out.
A little more detail might help narrow it down.
The only time we noticed the smell was when we flushed the toilet, which opened the big valve to the tank. I started blaming the new and less effective black tank treatment w/o formaldehyde. We did not have this issue before the old stuff was no longer available to purchase.
Yesterday when I was filling the black tank via the rinse port on the nautilus, I went on the roof and could smell faint black tank odors coming from the same vent I could hear water sounds from. That is how I identified which vent was the candidate for the new fixture. I just wanted to make sure there no tp or something blocking it, so that was why I poured water down the pipe. It flowed freely but did not make it to the black tank, presumably due to the plumbing junction to the gray tank.

For the AAV issue, was the smell constant? We only smell it after flushing...

We really only had the smell after running water down the sink(s). We've also never used chemicals in the black tank and have never had issues with black tank smells.

Rob

[traveldawg]

On my rig there are 3 holding tanks; 2 grey water and 1 black water. All 3 share the same vent pipe to the roof. They join together somewhere in the wall between my pantry and shower. And in the case of the bathroom grey water tank the shower drain also shares the vent pipe.

It's a heck of a way to do plumbing, but I guess it's cheap and sort of works.

It is so convoluted that if you overfill the black tank (ala forget the back flush is on and walk away) the bathroom grey tank will fill after the black tank fills and then water starts coming up the shower drain.

Like I said - heck of a way to plum drains!

oh - I also changed out the AVVs in my rig.

[Grandesigner]

Looks like the bathroom gray tank and black tank share a vent and have an AAV under the sink. The galley sink does not have one, I guess since it has its own vent.
I'm jealous and confused as to why and how not using black tank solution does not result in normal human biological 'smells' for some...what is the secret?