How to test black tank vent pipe

[djw2112]

Hi, i have a 79gal black tank which is about 80% full. I have noticed that even after i dump and use the restroom the odar in the RV is much worse than before. It used to take just a little bit of spray afterwords, but now im having to spay two or three times aftewords to get the smell out.. and even then it lingers.

I have also noticed that when i flush the water tends to want to spit drops back up in the air out the toilet drain. So i put some dish soap in the toilet and when i flushed the bubbles floated upwards out of the toilet.

I believe this means that my tank vent pipe is not working correctly, is this a correct assumption? If so how do i test it from the roof pipe exposure? Do i stick a water hose down the pipe to see if it fills with water and overflows (meaning its stopped up)?

 

[Gary RV_Wizard]

A hose in the vent pipe can sometimes give you clues, but it won't tell you much if the vent pipe has dropped down into the tank, below the typical sewage level. Water will flow freely into the vent and tank, but air cannot get back out once the tank level covers the bottom of the vent pipe.

 

[Rene T]

A hose in the vent pipe can sometimes give you clues, but it won't tell you much if the vent pipe has dropped down into the tank, below the typical sewage level. Water will flow freely into the vent and tank, but air cannot get back out once the tank level covers the bottom of the vent pipe.

You should be able to see that if the pipe is no longer flush with the roof.

When flushing do you have the ceiling vent fan on? If it’s on then it could suck fumes out from the tank

 

[djw2112]

Thanks Gary, I did check the plumbing under the toilet via the storage bay and all looks great and no leaks and no smell in storage bay, and the toilet itself is properly sealed via the floor seal so it has to be the pipe.

The pipe runs inside the wall right next to the toilet so if the pipe is not plugged up it might have a crack or hole in it which would explain why it smells for so long and so strong. I will inspect the pipe to see if its lose on the roof, or if its plugged up. If i dont find anything ill have to open up the wall to inspect the pipe there.

I was just thinking, many years ago when i put up a shelf in the bathroom i wonder if i nicked the pipe with a screw.

@Rene T no sir i never use the fan, its plugged up with a solar pillow, always has been.

 

[JayArr]

Fill the tank half of water (buckets into the toilet)

Add a bunch of dish soap.

Attach the output end of a shop vac to the rooftop vent so that you're blowing air down the pipe.

If you get a lot of suds and bubbles then the pipe has probably shifted down so that it's below the surface of the water, sort of like blowing bubbles into your milkshake.

If you get air coming out of the toilet but no major bubbles or foam then the pipe is likely above the water and OK.

 

[Gary RV_Wizard]

You should be able to see that if the pipe is no longer flush with the roof.

Yes, but that implies you have removed the vent fitting at the roof to see the pipe itself. It's quite possible the roof fitting is in place but the pipe has disconnected from it.

 

[Gary RV_Wizard]

A vent pipe problem is one of those things that are promptly & frequently brought up on the internet but rarely the cause of odor issues. I'm not suggesting you forget about the vent, but keep your mind open to other possibilities. Those include a "poop pyramid", an accumulation of sludge on tank walls and floor or perhaps a crack in the top of the black tank.

You might try using a septic treatment on the tank, e.g. Roebic K57 or RidX, filling the tank with fresh water + the treatment and let it cook for 3+ days before rinsing out.

The poop pyramid problem can be difficult to verify unless your system has a straight-down pipe (no 45 degree bend) so you can see it with a flashlight. And even more difficult to remove.

 

[JayArr]

Umbilical cameras can now be had for less than $100. They don't have screens anymore, they just link to your phone by BlueTooth. If you can't solve this any other way you could buy one and "look inside"

You can also rent sewer snakes at the rental store that have cameras with lights but I think that may cost $100 just for a rental.

 

[Rene T]

Yes, but that implies you have removed the vent fitting at the roof to see the pipe itself. It's quite possible the roof fitting is in place but the pipe has disconnected from it.

The vent caps I’ve see on my rv’s have a small hole on the top of the cap that’s filled with silicon. Removing the silicon reveals a screw head. Remove the screw and the cap comes right off so the pipe would be visible.

 

[djw2112]

Great help everyone, thanks so much. Turns out there were two issues. The first issue was that there was a 3 inch thick spider web starting at the top of the pipe, it was so thick im sure it impeeded the venting.

The second issue is that the vent cap that is on there is not the OEM cap style. Looks like the cone that goes around the pipe broke at one point so they just stuck a different kind of flat cap on there (it had no legs to insert into the cone). There is not much clearance between the cap and the cone with the cap on there, and then if the cap is pushed all the way down it seals the cone, not letting any venting out. I suspect that when i was cleaning the roof a few months ago i may have pushed down on the cap by mistake at some point and sealed it.

So i will modify this cap to have legs like the OEM one and then there should be plenty of gap for it to vent.
I did rinse out the pipe with the waterhose just to do it and no issue there.

So now when i flush there is hardly any odor at all..

Thanks so much for the great help

 

[Rene T]

That’s awesome. Thanks for getting back to us.

 

[Kirk]

So i will modify this cap to have legs like the OEM one and then there should be plenty of gap for it to vent.

You may want to consider replacing the cap as there are many options and some of them improve the ability to vent the waste tanks.

 

[djw2112]

Kirk true, but i would also have to replace the cone (since its broken on the top) which i dont want to do right now. The beginning of fall is when i do roof patching and that sort of thing so ill do it then

 

[RNS]

Interesting resolution, critters get into the darnedest places thanks for the update

 

[chuckbear]

We had wasps build a nest and completely block off the blackwater vent. Chuck

 

[lavarock1210]

That’s awesome. Thanks for getting back to us.

X2 so often on forums many possible fixes are presented but the OP never comes back and let the forum know if any were of any help.