Grey Tank Burp

[Steven@147]

Well I was tanking a shower and heard some gurgling noise, then whoosh. Seems air/water shot up through the left (dual sinks) bathroom sink, splashed on the counter and on the bathroom mirror. Well crud, we got the dreaded tank burp on grey tank 1. been ok for a little over two years now we got it as others have experienced. Yup searched and read up on what others have found.
It takes a little while after dumping the tanks but its been doing it the last few times when the grey 1 gets a little in it.
Got after it today checking the usual things. Cleared out the storage compartment , took down the wall, (gosh I just hate looking in there) and sketched how the drain and vent lines are run as best I can tell, some things you just can't see, but made my crude sketch. I'll clean it up later but for now gives me some reference. got up on the roof and took the little black vent caps off. One had some spider webs in it and the roof TPO "U" flap they cut out was partially covering the vent. No indication the vents slid down into the tank any more than what it was.

Going to have to take apart the skeleton frame the back wall screws to in order to reach the shower trap to see if its partially plugged. I'm going to try and pull the shower drain screen and see if I can fish goop out instead of taking the trap apart, I can't reach it. Took lots of pictures for reference. The weird thing is the whoosh comes from the far most bathroom sink in the line, (dual sinks) you would think if it was going to do something like that it would be the sink closer to the AAV, which are fine by the way. I replaced them long ago with better valves.

Well just one more thing too look after. Hey until I got to it today I just stuck the sink plug in tight, be my luck the plug will become a missile next time or if the tank burps again.

 

[boyscout]

If you did before, you probably no longer think that only your black tank is the stinky tank.

Sure seems like air in the tank isn't getting from your tank into the stack and up to the roof as the tank fills. You've obviously thought things through well, but the stack doesn't have to fall noticeably from the roof in order to get deeper below the top of the tank. The top of the tank can be forced up if it's ever over-filled, forcing the stack deeper into the tank, and that could increase the space between the top of the tank and the bottom of the stack. The net effect of either would be the same: an air pocket could form above the bottom of the stack and pressurize as the tank fills until it just has to blow out somewhere. The nearby sinks with small p-traps may be an easier path out than through a column of waste in the stack and all the way up to the roof.

It's so easy to theorize about another's troubles, and that's all this is, idle theorizing. Hope this helps. Good luck.

 

[Steven@147]

To finish my thread and give you some pictures and my theorizing of what was happening.
https://www.mygrandrv.com/forum/showthread.php/19507-Another-Plumbing-Puzzler?p=187565#post187565 here is another thread you can read as well

Now this started happening to us grey tank 1 burp after over two years of use. observations - as you can see in one of the pictures the grey tank 1 vent line also has the shower drain connect to it. That vent/shower connection has a long horizontal (@3-3.5 feet) line laid across the floor internal basement, then a 90 and straight down into the tank. The long horizontal line laying on the interior floor will prevent the vent line from slipping down lower into the tank. i am confident the grey tank 1 vent line hasn't moved since installed, its stuck in the top o the tank but good. It doesn't matter what level the tank is at, it will burp after the tank was just dumped or it can do it after the tank has some in it. You can recreate it easily just by placing your foot over the shower drain for a few seconds, take your foot away and the drain gets a big gulp of water down it, then you get a gurgle and tank burp out of the bathroom sink.

Now why did this start happening. I think there are several factors at work here, just my theory. When new the plumbing lines were all open, no sediments in the plumbing lines. After a couple years of use the lines have retained some sediment, residue, soap scum etc and are not completely open as when new.
The grey tank 1 vent line has a long horizontal level (@3-3.5 feet) piece of pipe going across the internal basement floor, actually it needs a minimum fall toward the tank( not be level) to always keep the line partially open while water is going down it and air is escaping out of it at the same time from the shower. Get too much water (obstruction) in this line and you will get a tank burp because air can not get past the water, soap, suds, etc in that line. So when the pressure gets too great it comes out anywhere of least resistance, that just happens to be the bathroom sink drain. Also an observation when this started happening the rig was really level 0.0 tilt and angle on the ground control (keep this in mind). Any minute obstruction in the shower drain like hair or whatever causes water, soap, suds etc to back up and then all of a sudden overcome the obstruction and a big gulp down the drain and obstructs the vent line.


Now I did some repair measures, checked the vent tubes out through the roof - OK. I ran one of those small whirly-gig plastic drain cleaners in the shower drain down through the trap, didn't get much "stuff" out, but some. We keep our feet away from the shower drain. And now that we have moved to a different campground, it has stopped burping. For S&Grins, I checked the tilt angle on the ground control, we are - 0.2 on the tilt (RV nose down ?) and 0.1 angle. This puts the 3-3.5 foot piece of plumbing vent line at a down angle towards the tank helping to keep the vent line water flowing and air to pass by through the line at the same time.

Well that is my theory, in any event the tank has stopped burping.