Gray tank venting problem

[olympicman]

Gray tank vent problem:
A couple of weeks ago water backed up in the shower while wife was showering , when I returned an hour later I looked in shower with 3" of water in it I said no big deal tank needed dumped.

Now when we fill the kitchen sink with water for dishes and drain it backs up in the shower if the dump valve is closed, but drains fine with dump valve open. Thought ok vent to tank is plugged, so I removed vent cap at roof of RV and ran water down vent pipe to clear and make sure it ran clear and it did.

Clearing the vent pipe did not help and I tried stuffing a towel into shower drain so water could not come up there, filled both kitchen sinks and drained them with dump valve closed and water came up in bath sink drain. There is also a bad smell when draining sink with dump valve closed, but no smell when valve is open.

My gray vent pipe does not go straight down at the top it has 2 45 fittings to move the pipe over to miss the slide-out.

Any ideas to help would be appreciated.

[Travelin2]

http://www.buspartsexperts.com/KAZOO-DRAIN-VALVE-1_p_1195.html
If you connect a small drain pipe or hose fitting where you put that screw hole and install a duckbill valve on the end of it will take care of releasing any accumulated water without needing your attention. These can be found in many different sizes and types for many different applications....

http://www.buspartsexperts.com/KAZOO-DRAIN-VALVE-1_p_1195.html

[enblethen]

I was thinking that you may want to put some drain cleaner down the vent into the trap. It could clean the pipe so it will not clog. Maybe leave it in overnight and then flush it out.
Make sure it is good for plastic pipe.

[77rollalong]

olympicman wrote:
Yes I have ran a hose down the vent and ran water with dump valve both open and closed probably 20 to 30 gallons. If you look at my drawing you can see that the vent pipe does not drop straight down into the tank, but drops down from top of tank before going up to the roof top. The full wall slide makes the pipe configuration nessesary.

if you could some how drill a hole into that vent pipe to put a drain tube to let out any liquid that could build up in it drain out under the motorhome, not a good design at all
, as it is the tank builds pressure...

[Harvard]

Bobbo wrote:
If that was the problem, I would think that when you dump the grey tank, there would be enough suction to suck the water out of the p-trap. ....

I believe there are multiple (4?) p-traps in parallel, the one(s) with the least resistance would give way in this manner.

[olympicman]

Final update,
Drilled small hole and drained out the stinky water in low point of the vent pipe. I filled both kitchen sinks with 4 and 8 gallons of water and pulled plug on both at same time, no backup into shower and no stink. Problem solved. I also put a screw in to plug the hole I drilled.

[olympicman]

Update 1/24/2019,
Yes low point is accessable, so I am going to drill a hole for it to drain and plug with a screw. I will update later today how that works.
I am also looking into installing the SeeLevel gauges to better monitor tank levels, bt that will have to wait until I get home to Oregon in March where I have all my tools.

[starcraft69]

Any update ??????

[stevenal]

Is the low point accessible? If so, I would consider drilling a drain in it and plugging it with a screw. Also make sure you have a decent vent cover to keep rain water from entering the trap.

[Dusty_R]

Bobbo wrote:
If that was the problem, I would think that when you dump the grey tank, there would be enough suction to suck the water out of the p-trap.

Rig a collar sealing the top of the roof vent with an air compressor hose inserted. WITH THE DRAIN VALVE OPEN turn on the compressor and blow out the vent like you would blow out water lines to winterize.

I agree except, I would use the output of a shop vac instead of an air compressor.

[Terry78]

Vapor lock?

[Bobbo]

If that was the problem, I would think that when you dump the grey tank, there would be enough suction to suck the water out of the p-trap.

Rig a collar sealing the top of the roof vent with an air compressor hose inserted. WITH THE DRAIN VALVE OPEN turn on the compressor and blow out the vent like you would blow out water lines to winterize.

[olympicman]

Harvard, yes a P trap in the vent pipe, poor design by Rexhall not a regular p-trap, but it acts like one.

[Harvard]

olympicman wrote:
.....you can see that the vent pipe does not drop straight down into the tank, but drops down from top of tank before going up to the roof top.......

So this vent is a p-trap that is filled with water.....blocking the flow of air....right?

[olympicman]

Yes I have ran a hose down the vent and ran water with dump valve both open and closed probably 20 to 30 gallons. If you look at my drawing you can see that the vent pipe does not drop straight down into the tank, but drops down from top of tank before going up to the roof top. The full wall slide makes the pipe configuration nessesary.