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Old 10-30-2020, 08:56 AM   #1
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Gray tank won't drain

After a couple month trip in the new (to me) RV, my gray tank won't drain at all. It has been a slow drain since I bought it, but now the gray tank is 3/4 full and I've tried multiple times to dump it ... unsuccessfully.

The gate valve appears to be clear. I can snake past it with no obstructions and up the 2" drain pipe to the first bend. I'm assuming there is some sediment, debris, hair, grease combo that is at the bottom of that tank, plugging the exit ... or plugging the 2" drain pipe somewhere between the tank and the gate valve.

Anyone use one of these contraptions to pump water back into the tank?? Any success with them?? Will they break up an obstruction or clog?? Any other ideas??

Thanks for any help
Karl
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Old 10-30-2020, 09:27 AM   #2
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I have not used one of those, but your problem may be a remnant of the hole that was drilled in the tank to accept the drain pipes fell into the tank and has drifted to the opening. That has happened before to others. The water pressure may work, but a plumber's snake should be able to probe all the way into the tank opening as well.
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Old 10-30-2020, 09:56 AM   #3
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I'm not sure what the problem might be, but I have found with the 2 National Dolphins we have owned, both have had a slow draining grey tank, so this seems to be normal for this design. Part of this has to do with the tank covering a large area but is not not very deep and of coarse the smaller 2" drain pipe. I have found it drains better if I tip the RV to the left using the jacks when dumping. Even then, as it empties, the grey tank flow slows right down as it empties. I never had the grey tank drain plug up, but I could see it happening since it doesn't rush out like the black tank.
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Old 10-30-2020, 10:10 AM   #4
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Quote:
Originally Posted by GREGORYJ View Post
I'm not sure what the problem might be, but I have found with the 2 National Dolphins we have owned, both have had a slow draining grey tank, so this seems to be normal for this design. Part of this has to do with the tank covering a large area but is not not very deep and of coarse the smaller 2" drain pipe. I have found it drains better if I tip the RV to the left using the jacks when dumping. Even then, as it empties, the grey tank flow slows right down as it empties. I never had the grey tank drain plug up, but I could see it happening since it doesn't rush out like the black tank.
If the tank is as you describe, the it is certainly possible that a build up of grey waste has accumulated. Have you ever had the tank thoroughly flushed and cleaned? Once you get the tank drained, that may be the next step to take.
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Old 10-30-2020, 10:23 AM   #5
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2007 vintage that is 'new' to you
Grey tank draining as been slow trickle since you bought it

Thoughts:
1) Check grey tank VENT is clear/unobstructed and has NOT slipped down into tank
Air MUST go in for fluids to go out
2) Should be 1 or more Air Admittance Valves....under sink cabinets.
Unscrew/remove them and try draining again. W/O them more full venting is provided
3) Drain angle as suggested .....tilt RV towards drain outlet
4) Cut out disc left in tank...floats then gets sucked down to outlet opening and sticks cause obstruction
5) Previous Owner did NOT drain/flush tank, allowed debris to accumulate ---those end valve flushers CAN Help but with a FULL Tank they can OVERFILL quickly
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Old 10-31-2020, 11:25 AM   #6
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Thanks for all the good inputs and advice.

I've narrowed down the possible causes to a blockage at the tank connection to the 2" drain line, possibly some combo of tank sludge and maybe the "donut hole" from drilling the tank. I've discarded the vent blockage theory, because the blockage would have to be absolutely airtight to stop all flow. Over the past week, there has not been a dribble out of that line.

I like the idea of snaking the drain line ... but I don't like the idea of dealing with 30-40 gallons of gray waste letting loose on me while I try to retract that snake and close the gate valve. A good, but messy solution.

One member suggested an inexpensive version of the contraptions in the pictures. It was a sewer cap with a hose fitting on it. It would allow water to be forced in the sewer fitting in the same manner, but didn't incorporate an extra gate valve. Connect the end cap with a flushing hose, open the tank gray gate valve, pump water "upstream" into the gray tank, hopefully dislodging or breaking through the blockage. Then close the gray gate valve, disconnect the end cap and flushing hose, connect my sewer line, then open the gray gate valve to (hopefully) dump my 30-40 gallons of gray waste.

I like the idea. If a "T" or "Y" connection was put between the flushing hose and the end cap, one could add a pressure gage to monitor how much pressure the waste line was seeing. When the blockage breaks free, the pressure should immediately drop, letting me know success. Cheap and easy ... and I already have the end cap with hose attachment, the "Y" and pressure gage.

What are your thoughts? See any problems?
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Old 10-31-2020, 02:25 PM   #7
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I have the FLUSH KING and it does a great job on both tanks, much better than the built in black tank flush.


Also clean and easy
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