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Old 19-10-2013, 08:10   #1
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Incomplete pump out of holding tank

I have a dometic pump, the kind with a diaphragm, and when I pump our holding tank, it pumps out the top third of what's in the tank and then begins to suck air. The pump out service does better, but still does not completely empty the tank. The pump is new, I have checked repeatly that the vent is clear, the holding tank indicator and gauge are working, and the duckbills are correctly installed. There are no visible leaks, and no smells. I have a 1997 Hunter 40.5. New cap installed at deck pump out, and new 3 way valve between holding tank pump and deck pump out. Any ideas?
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Old 19-10-2013, 08:43   #2
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Re: Incomplete pump out of holding tank

How are you able to tell 2/3 remains after pump out? Visual on the tank itself? Or are you using the gauge for that?

If the latter, gauges and be "working" but inaccurate at the same time...

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Old 19-10-2013, 09:08   #3
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Re: Incomplete pump out of holding tank

If the pump out discharge hose is at the bottom of the tank then the tank is empty. Must be the gauge.

How much does the tank hold and how much do you think is sucked out. We do not have a gauge and into 18 years have not question if the tank is empty when it sucks air
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Old 19-10-2013, 09:50   #4
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Re: Incomplete pump out of holding tank

I have removed the sender from the top of the tank and visually saw the amount remaining in the tank. Fluid level is about two inches or so from the top of the tank when it begins to suck air. The dometic pump can suck a vacuum and remove about a third of a tank full when the holding tank is full, and then sucks air at that point. The tank holds 35 gallons.
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Old 19-10-2013, 10:31   #5
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Re: Incomplete pump out of holding tank

Had same problem with macerator pumpout.Quality TP would clump over outlet and stop flow.Use cheapest single ply then no issue.
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Old 19-10-2013, 10:34   #6
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Re: Incomplete pump out of holding tank

Is the discharge hose at the bottom of the tank? Is the pump located at the bottom of the tank?

Have you tried a marine pump out station. If a marine. pump out station can. empty the tank then its the pump.
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Old 19-10-2013, 11:27   #7
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Re: Incomplete pump out of holding tank

Okay, so it appears the marine pump out can only pump out to the same level as we can, then the tank draws air. We removed the level sender from the top of the tank and pumped as much of the tank from there. It is original equipment, aluminum tank. Hunter owners.com has a replacement tank, aluminum again for 600 bucks. Then comes the fun of tearing the boat apart. Does anyone know of a plastic replacement tank? I am in Ensenada, Mexico getting ready to depart on the HaHa so I can still get up to the US and get parts. Any ideas would be much appreciated.
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Old 19-10-2013, 11:42   #8
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Re: Incomplete pump out of holding tank

Sounds like the dip tube for pump out has a hole / crack in it. Perhaps it is possible to get a 1-1/4" PVC/ABS pipe and insert it into the existing dip tube, sealing the edge between the new PVC and old dip tube, at the top . Be lots cheaper then a new tank. Might need to cut/ sand the exterior of the PVC pipe to fit inside the drop tube. Look around the plumbing section of the hardware stores. Might try a PVC sink connector or drop tube, the ones with a small flange on it.
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Old 19-10-2013, 12:16   #9
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Re: Incomplete pump out of holding tank

sailorchic24 probably has the answer if both the onboard pumpout and a pumpout service can only partially empty the tank. Not unlike a hot water tank when the pick up line in the tank breaks off and even though the tank heats up, you run short of hot water quickly.
Not sure why, but most black water systems are renown for undersized/flimsy tube sizing for waste removal and venting. A robust system will not only work better but be much more trouble free. Replacing the pump out piping system is no fun but once done it will pay immediate dividends. Commercial pump out services do have a common sizing for deck fittings but a larger pick up pipe tied into a common waste removal deck fitting will work well considering the vacuum pull commercial systems use. For your onboard diaphgram pump, oversize the piping, pump and through hull fitting. For an existing system, thoroughly clean the tank by flushing with water and disinfectant will clean the tank sufficiently to replace the piping with a minimum of smell and contamination. Good luck with a sh*tty job... Phil
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Old 19-10-2013, 13:09   #10
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Thanks to all of you for your input ;-)
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Old 19-10-2013, 14:09   #11
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Re: Incomplete pump out of holding tank

Here is the vacuum generator tank and the drip tube for that tank.

As has been stated, the drip tube needs to be intact, leak-free and end just above the bottom of the holding tank.

On my boat the holding tank has two drip tubes, one for the macerator overboard pumpout, one for the deck plate pumpout.

When I maintain my holding tank, I start w/ a pumpout, then scrub the interior w/ powdered cleaner and a long brush. I also disassemble the vacuum generator for cleaning. The level indicators are cleaned at that time.

This periodic maintenance helps ensure that this system operates properly.




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Old 19-10-2013, 19:03   #12
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Re: Incomplete pump out of holding tank

Is the tank discharge fitting at the bottom of the tank, or is it on the top of the tank with a pickup tube inside the tank? If on the top with a pickup tube, what's the tube material--pvc or metal?

I should have read further before I posted...your problem has to be a pickup tube that urine has eater holes in...in fact, the bottom two thirds may even have fallen off....and your aluminum tank isn't far behind in being turned into a colander by urine.

The simple temporary solution: a piece of 1" hose through what's left of the pickup tube, plus 1"-1.5" adapter to connect the 1.5" hose needed to connect to your Dometic discharge pump (you haven't mentioned a VacuFlush, but even if you have a VacuFlush toilet it wouldn't have anything to do with your holding tank unless it's one of those gawdawful "sailvac" tanks that does double duty as both a vacuum tank a holding tank). Wrap the fitting in the top of the tank with lots of duct tape or whatever you have handy to prevent odor from leaking around the hose.

When you get back to the US, you'll want to replace the tank...and plastic is the ONLY material to use for waste holding. Ronco Plastics Ronco Plastics Marine Catalog is your best source. They're in Tustin CA...build TOP quality tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non rectangular...and they install fittings--including pickup tubes--in the sizes and locations specified by the customer when they make the tank.
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Old 19-10-2013, 19:21   #13
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On mine the vacuum generator is for placing material in the tank. Discharge is through a pump only. Same pump as vacuum pump but does not have all the extra stuff. Just a pump and motor.
If it can't discharge it's because it is blocked.you say its drawing air. Means the pump is sucking but its intake is not below the sewage. If you are pulling from the top of the tank the pipe that should extend through the tank to its bottom is no longer getting to the bottom. Like cracked or let loose.
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Old 19-10-2013, 19:30   #14
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Re: Incomplete pump out of holding tank

Having built in my own fiberglass waste tanks I agree with Sailorchic34 that you've got an air leak in your dip tube. My tank has a stainless steel access plate with an 1-1/2" couple welded through it. The dip tube is 1-1/2" abs with a threaded adaptor screwed into the plate coupler. The discharge / pump hose attaches to the top side of the plate. I'd like to see waste eat it's way through abs.
Forget aluminum replacement. When the time comes fiberglass in place.
Why do manufacturers have to make boat systems so complicated? Simple is much easier to troubleshoot and fix.
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