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Old 03-17-2016, 08:01 PM   #1
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2012 27' FB International
Austin , Texas
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Posts: 38
Unhappy Strange problem with graywater tank

I have something really weird going on with my graywater tank. It was refusing to drain into the septic system two days ago. I tried plunging the shower where the water was backed up, I poured biodigester down the shower drain, I tried opening and closing and opening the valve that starts the flow into the septic over again. I did everything short of roto-rooting it.
After awhile I decided to try emptying the black water tank which was about 2/3 full at the time. It whooshed out with no problem.After emptying the blackwater tank I tried the graywater tank again and it whooshed right out, solving the problem.
Now, two days later, the problem is back, with the graywater backing up in the shower with a distinctive septic smell.
Now, I thought the black and gray water tanks were completely separate, so I don't understand how emptying the blackwater tank allowed the graywater tank to empty. I'm baffled!
Can anyone explain what's going on and offer a simple solution? Right now the blackwater tank only has about 5 gallons in it, so I don't think it's a good time to empty it just yet.

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Old 03-17-2016, 08:16 PM   #2
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1974 31' Sovereign
Ottawa , ON
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My guess? The air vent is somehow blocked.

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Old 03-17-2016, 09:59 PM   #3
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2015 30' FB FC Bunk
Ayer , Massachusetts
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I'm curious if this problem has suddenly occurred and you have a history of it otherwise working correctly or if this trailer is new to you and it has never worked correctly. If it is the later, a former owner may have tried a mod that conected the tanks for some reason.
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Old 03-17-2016, 10:17 PM   #4
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2012 27' FB International
Austin , Texas
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Ted S.

I've had the trailer since August in continuous use and this is the first time this has happened .
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Old 03-17-2016, 10:19 PM   #5
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2012 27' FB International
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Originally Posted by Aage View Post
My guess? The air vent is somehow blocked.
How would I go about unblock in the vent?
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Old 03-17-2016, 11:05 PM   #6
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Okay let's think about this. The pipes join lower down to a single outlet. Is it at all possible that when you pulled the gray water valve open, the valve really didn't open? I assume when you opened the valve on the black water tank, you had the gray valve closed. Then when you opened the black valve, the pressure of the discharge also pushed against the gray water valve. Then you pulled the valve and this time it opened? Could there be either something where the pull rod on the gray water tank is not actually opening the valve. To prove that back, I'd attempt to use a drain clean out cable and if the gray water valve is open but not draining, thread the cable from the outside discharge back into the gray water tank and see how far you get. It just may be that the valve is closed even though you think you have it pulled open? Even if the vent is clogged, you would get water out since technically the tank could vent through the sink drains in the trailer.

My gut feeling is either something stuck in the lower pipe or that drain valve isn't really opening.

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Old 03-17-2016, 11:12 PM   #7
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That's my first take too. Something is messed up in the drain valves assembly. Too bad there isn't an easier access to examine and service them. Anyway, that's where I would start. Good luck and keep us informed.
The ability to follow instructions is highly underrated.
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Old 03-18-2016, 06:31 AM   #8
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Berlin , Maryland
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Just my thoughts as i am definitley no expert. Based on what I read it appears to me that you may have a blockage or a sticky valve. If it was me I would do the following: I would fill both tanks with water then drain the black water tank followed by the gray water tank. When both are empty then I would flush with hose. Another option is after you fill the tanks take the AS for a short ride to loosen any possible residue in the tanks and then flush.
I was also taught when i first purchase our AS to dumb either some water/ or ice in the black water tank to loosen any gunk that might remain.
Again just a thought
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Old 03-18-2016, 10:49 AM   #9
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sounds like the air vent on the black tank.
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Old 03-18-2016, 10:55 AM   #10
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1977 31' Sovereign
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Sounds like you've got a clog somewhere.

Flush out both air vents with a garden hose. Then fill and flush both grey and black tanks. If necessary, back-flush both grey and black tanks.

This will get messy if the clog persists and you hafta back-flush with 3" sewer fittings removed. So make preparation accordingly.

As a matter of routine maintenance, I use a garden hose for this sort of thing. Chances are, your plumbing will have a built-in pressure regulator. So, connect the hose to an outside faucet that has good pressure.

BTW - Harbor Freight has some handy drain flushing kits for this sort of thing.

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Old 03-19-2016, 04:29 AM   #11
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2014 27' FB International
Lewisville , Texas
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If you left the grey tank valve open when you opened the black tank valve some of the black tank water could have back flushed into the grey tank through the open valve.
Sounds like a clog in the grey tank which is unlikely, unless one of the tank cut outs has been floating around in the tank since it was new and just now it made its way to the valve. But I wouldn't think that would clog it completely.
I think the above suggestion to open the grey valve and shove a garden hose up their is a good start.
Let us know what happens.
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Old 03-19-2016, 06:49 AM   #12
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2011 25' FB International
Carencro , Louisiana
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I have a 2015 27fb as well. I believe if you have black water in the tank flowing out with the valve open and open the grey water valve at the same time with not much in the grey tank, it is possible for some of the black to make it's way to the grey system. The pipes are smaller and can become clogged from tissue etc. I would try above what "No Noize" recommends. There are 3 vacuum valves in your unit. One under the closet, for the shower, one under the galley sink, for the sink and one in the water closet to the right of the sink in the wall cutout. The one near the shower may have gotten some paper caught up in it. you can find this by opening the door under the right closet door. (this would be the 1st place I check and might even replace this valve, they are cheap and avail at Lowes/Home Depot. If you had grey water in it during the winter, it could have froze and damaged the valve. The next thing I would do is go on the roof and check the two sewer vents an make sure they are clear, no bug/animals made a home there. Don't ya just hate sewer issues! Let us know how it is going.
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Old 03-19-2016, 04:18 PM   #13
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I've previously read that the gray tank drain pipes were smaller than the black, but just thought that must be older rigs. Does a new 27' really have a smaller drain pipe leading from the gray tank?
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Old 03-19-2016, 04:40 PM   #14
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Spearfish , South Dakota
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My 27 has a vent pipe running up both sides of the bathroom door in the triangulated wedges on the inside door frame. Why one is bigger, it hides the step set in for the folding bedroom privacy screen. Both should be 1 1/2 abs for venting sewer gas to the roof on both grey/black tanks
Your issue can't be about one plugged vent pipe for both tanks. It has to be on the discharge/blockage side.
The vacuum breakers, closet, sink, wash basin, work as a vent pipe for the P traps. In a stick house/older RV there would be a vent pipe to the roof for every P trap too.
If the shower won't drain while holding water as the grey tank over fills, I would look at the vacuum breaker in the closet, along with the main discharge for blockage.

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