Gray Tank Vent Problem

[charliem]

I am reposting this info for new owners. The original discussion is archived but is separated from the graphics. The improper installation of the gray, and possibly black, tank vent greatly reduces the usable volume of the tanks, leading to premature drain backup and overflow. This problem was solved when the factory became aware of it, but is certainly subject to reappearance depending on installer knowledge, experience, supervision, training and the time/day of the week.

To further the conversation on the gray tank problem I submit the attached drawings.Figure 1 is a schematic of the gray water system on a 21RBS, but it should be representative of other models with exception of the bathroom sink. Gray water fills the tank through two entry pipes. One drains the shower and also serves as the wet vent for the tank. The two sinks are combined to drain through the second entry pipe. Each drain has a P-trap to prevent tank gasses from backing up into the trailer. The shower is directly vented by the wet vent. Each sink is vented by an Air Admittance Valve (AAV). These AAVs let air into the drain pipe downstream from the P-trap to prevent water from siphoning out of the traps. As shown in the drawing both entry pipes are connected to, but do not penetrate, the gray tank. This is the ideal installation. Air displaced by additional water from either entry pipe is vented out the wet vent to the trailer roof.  Note that the vertical scale is not accurate. The tank itself is only about six inches tall.Figure 2 depicts an installation error where the vent pipe extends some distance into the tank. The drains will perform properly until the water level reaches the bottom of the vent pipe. At this point the trapped air can no longer exit via the vent, but will be forced back up the sink drain pipe and out one of the sink P-traps. This causes the gurgling sound and water from the traps being splattered out of the sinks. I believe this is the condition we are seeing. Depending on how far the vent pipe protrudes into the tank, symptoms will appear at various levels less than 100% full.Figure 3 depicts a similar situation, but the pipe from the sinks is pushed too far into the tank. I believe this is physically possible during assembly, but I don't have any idea how probable it is. I'm still trying to think through the symptoms from this configuration. I think air trapped in the sink drain stack would be compressed and forced out one of the sink P-traps as water is added. This may cause gurgling, but it might just make the sinks drain very slowly. Either way the tank will act as if it's full long before it really is. Each time a sink is emptied the AAV will let a new charge of air into the pipe. More thought is required here. In a properly operating system the gray water should begin to back up into the shower first because the shower is the lowest drain. All this assumes the AAVs are working properly, admitting air in only one direction. If an AAV is stuck open, or not even there, the tank will be vented out the failed AAV. In Fig 2 the tank will appear to be filling correctly, but any tank odors will vent into the trailer. In Fig 3 it's probably similar, but I'm still thinking on this. If an AAV is stuck closed there is probably an effect on the sink P-traps. If sucked dry, odors will back up into the trailer through the dry P-trap.So what's the worst case? Figure 2 and a failed or absent AAV. The drains would work properly, but odors could be noticeable. If, as some campers do, the sewer hose is left connected to the CG sewer, the gray drain valve is left open, an AAV is stuck open, and a vent fan is running. Phewweee! All comments, suggestions, additions, or subtractions are welcome. My goal is to build an understanding for us and LL so this problem can be put in the history file.