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Old 03-13-2012, 04:11 PM   #1
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Dolphin very slow gray tank drain

Does anyone have any suggestions on how to improve the flow draining the gray water tank? My black water drains very fast and I thought I had figured it out by draining the black water then the gray and leaving the black water open for venting. Wrong it did not work a second time. When I say slow it takes at least 10 to 15 minutes to drain the gray water.

Has anyone else had this issue and found a cure?


If it is a blockage how do you correct it?
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Old 03-13-2012, 04:22 PM   #2
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Yep, our gray tank drains much slower. The drain pipe is 1/2 the size of the black tank. Our's runs good the first couple of minutes and then slows down.

fred
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Old 03-13-2012, 04:37 PM   #3
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I have never had that with either my 30 gal grey, in my TT, nor my 60 gal tank in my current MH.

You might have something stuck in the grey tank pipe that needs to be cleared out. Have you ever back flushed it? I have had this happen with my black tank and one back flush fixed it.

Walmart.com: Camco RV Dual Flush Pro: Automotive

I use this, regularly.

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Old 03-13-2012, 04:38 PM   #4
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Yep, our gray tank drains much slower. The drain pipe is 1/2 the size of the black tank. Our's runs good the first couple of minutes and then slows down.

fred
Ouch, I was really hoping there was some sort of clog and/or someone would have a magic cure. It is not logical to me that it is so fast for a few seconds then goes to a drizzle.
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Old 03-13-2012, 04:43 PM   #5
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I have never had that with either my 30 gal grey, in my TT, nor my 60 gal tank in my current MH.

You might have something stuck in the grey tank pipe that needs to be cleared out. Have you ever back flushed it? I have had this happen with my black tank and one back flush fixed it.

Walmart.com: Camco RV Dual Flush Pro: Automotive

I use this, regularly.

Sheila
The only thing I have tried is a short "snake" but it could not make the bend in the pipe. I'll check into the Dual Flush.
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Old 03-13-2012, 04:49 PM   #6
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This might be the answer......I have a 14 year MH.....one owner, until I bought it. Recently, on my move from AL to FL for the winter, I put about 5 gal of water and some tank cleaner in both tanks. When I arrived, I drained BOTH tanks when I connected to the sewer system. Then I left the gray tank open and closed the Black tank, which is my normal practice. Two days later, my gray water backed up into the shower, it was full. The gate valve was open but the drain was blocked with large globs of solid grease, etc that had accumulated over the past several years. They had beeb broken loose by the cleaner I put in there.

Maybe you have a partially blocked drain that needs cleaning! Rook
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Old 03-13-2012, 04:56 PM   #7
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If you can get the RV parked close to the house, you might try hooking up a hose to the hot water spigot for the washing machine.

Run as much HOT water into your gray tank as you can, along with 2 cups of Dawn dish liquid. If you can then move the rig back and forth a bit to slosh the hot water it would be good. Drain while still hot.

A lot of grease can coagulate on the bottom of the gray tank, sometimes mixed with sand and other crud. The hot water helps move this stuff out.
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Old 03-13-2012, 05:32 PM   #8
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Good thoughts, I am going on a short trip Thursday and I think I will add some form of sludge buster and drain when I get there.
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Old 03-13-2012, 05:44 PM   #9
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Yep, our gray tank drains much slower. The drain pipe is 1/2 the size of the black tank. Our's runs good the first couple of minutes and then slows down.

fred

Same with ours
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Old 03-13-2012, 05:53 PM   #10
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I have had the slow drain issue with mine since new. NRV confirmed, as I did later, on that the vent was not blocked or into the tank too far. Draining was always a long time process. I had even gone as far as cutting a large 2" diameter hole in the drivers side of the tank to look for objects of blockage such as the cutouts that may have been left inside after prepping the tank at the factory. None found. Also observed water flow out of the drain with the tank half full. That was when I could see water lazily moving into the 1 1/2" drain pipe.

There just is not room in the short height to generate good flow. Your tank is probably draining as best it can.

1) The tank is typically only about 4 1/2 inches high so not much water pressure even in a full tank to force great flow thru the 1 1/2 inch pipe.
2) The exit from the tank is along the front to back center line of the MH
on the back side of the tank so being level front back or even low at the back will help flow.
3) Because the tank is wide and not very tall, being level side to side is also very important for maximum drain rate and completeness.

I have changed out my level monitor and can now see more accurately water left in the tank.

Following the 3 recommendations above, the bulk of water is gone after only about 5 minutes of draining. However the small amount of water left will still take up to 10 more minutes.

Hope this helps,

Marty
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Old 03-13-2012, 06:20 PM   #11
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I would have to agree that it is a design flaw. Sometimes when I drain mine, it will start fast and then slow to almost nothing. Ten to fifteen seconds later you can hear a suction and belching sound at which time the tank drains fast again. Somehow it is creating a suction in the tank and then lets go. I've check all the vents under the sinks and they are all good.
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Old 03-13-2012, 07:15 PM   #12
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Thanks Marty, you confirmed my suspicions. I had wondered about the size of the gray pipe. The main problem with your 1-3 is most of the time you are at the mercy of the elevation at the dump stations.

Fred, I haven’t noticed the suction or belch but do need to locate vents you are referring to.

Seems like National would have fixed the design flaw between 2003 and 2005 and later, but alas this is the best made coach I could buy for the price.
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Old 03-13-2012, 10:51 PM   #13
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Thanks Marty, you confirmed my suspicions. I had wondered about the size of the gray pipe. The main problem with your 1-3 is most of the time you are at the mercy of the elevation at the dump stations.
lenjen, I often use the jacks at dump stations when they can offset the local terrain to the advantage of better drainage.

I believe the following may explain some of the observations made in the above posts..

Additional design flaw in the NRV grey tank is its ability to create traps on either side of the center vent to the roof. This can be easily seen with the side panels to grey tank space removed.

The top surface of the tank sinks downward away from the central vent. This prevents the vent from freely releasing water in those areas closest to the sides of the tank. When the level drops enough for the vent air to get to those spaces, a sudden surge rate occurs in the draining.

Additionally, the gurgling sound makes it seem like more water is draining but is mainly a result of air mixing in the 1 1/2 " exit pipe as the lowering level of water exposes the top of the exit pipe.

Marty
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Old 03-14-2012, 07:29 AM   #14
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same here. use jacks. up in front, up on curbside.
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