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Old 01-01-2020, 04:00 PM   #1
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Holding tank vent locations

I got the standard holding tank issue, can't pump it out, #1 cause is plugged vent, so I'm trying to locate the vents on this new to me 1990 48 Carver/Californian MY. My pump outs are on the Port side walk way and just below that are 2 very small fittings that could be the vents? If these are the holding tank vents they are way to small,as Peggy would point out. Coming from the inside I have been unable to locate much of anything.
Before tearing up the inside floors I'll ask for some guidance in trying to locate the vent hoses. Any ideas?

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Old 01-01-2020, 05:52 PM   #2
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That would be the right place for them, and I don't think they are too small. We had a vent like that on our 210 gallon waste tank and it worked just fine. Just need to maintain. Are you saying you don't have ready access to the top of your tanks? That'd be a problem.
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Old 01-01-2020, 06:06 PM   #3
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Just a guess but it sure looks like the tank vents for 2 tanks. Many / most manufacturers use the same vents they use for fuel and fresh water but they are less than ideal for black tanks. Ideally you want the largest straight thru hull and straight largest dia hose possible to get adequate air I to the tank to get aerobic activity and no foul odor.
If you can get to the tanks and the inside of the vents I'd replace them. Straight fittings w/o screens let you easily flush them with a hose.
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Old 01-01-2020, 06:20 PM   #4
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They appear to be the standard 5/8" vent line used on all tanks, along with the same vent thru-hull that boat builders put on all vents that's designed to keep sea water out of the fuel and fresh water. They're adequate size to provide an escape for air displaced by incoming contents and source of air to replace contents as they're drawn out, but are easily clogged and don't provide enough air exchange to provide the aerobic environment needed to prevent odor from occurring.

A blocked vent is the most common reason you can't pump out...the most likely place for the blockage is the thru-hull. So use a screwdriver blade, ice pick--whatever works to scrape it out.

Backup (bubbling, "burping") when the toilet is flushed is another clue that the vent is blocked...and causing the system to become pressurized. So don't use the toilet again until you've solved the problem.

So if scraping out the vent thru-hull doesn't clear the vent you're gonna need access to the tank to clean out the other end of the vent line.You shouldn't have to tear up floorboards to gain access to it, but if you do, re-install 'em as a hatch.

If a blocked vent isn't the problem...have you had any landlubber guests aboard recently? No matter what you tell 'em not to flush, there's always one who decides that "just one won't hurt" and flushes a d'd wet wipe...which, if it makes through the toilet into the tank, can create a blockage in the tank discharge fitting.

Once you've located and solved this problem, it would be a good idea to replace that vent thru-hull with an open bulkhead or "mushroom" thru-hull that you can put a hose nozzle against and backflush the vent line every time you wash the boat and/or pump out, which will prevent future vent blockages.

Happy New Year, y'all!

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Old 01-01-2020, 10:01 PM   #5
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You can tell a fresh water tank vent from a fuel tank vent from a holding tank vent by the taste.

If you you don't want to do that, maybe you could pull the ER vent grill or other nearby panel/component to access the area for a look-see. You might be able to tell by the type of hose mounted on the vent.
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Old 01-01-2020, 10:21 PM   #6
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If the chrome plated screen has corroded enough, it will block the vent. Hold a small mirror under the vent for a quick look. If corroded, remove the screens. Not needed but wasps may build a nest in there so vents will need to be checked periodically.

If you have access to top of tank, undo vent hose. Blow low pressure compressed air through hose. Might get lucky or might blow hose off and then a 2 minute job turns into a 4 hour job. Poke a wire through the top of the tank vent. Might get lucky.
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Old 01-02-2020, 11:07 AM   #7
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If you have access to top of tank, undo vent hose.

Only AFTER you've opened the deck pumpout fitting to relieve any pressure in the tank!!! Compressed air won't break up a clog in the fitting, but can cause worse problems...And use a screwdriver blade, not a wire!


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Old 01-02-2020, 11:49 AM   #8
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I can stick a coat hanger in both vents 3-4 inches before it hits something. I wouldn't think a screen is that deep. From the best I can tell there is probably a 90 degree bend right inside the hull. I've had some experienced people onboard trying to track this down without much luck. Seems the holding tank in under the fuel tank that is under the water tank.

This systems was converted over to a vacuflush system a few years ago so this is making things more difficult as we find old hoses capped off.

I can get to the vacuum generator as it's under one of the beds. Can I disconnect the hose to the holding tank, after depressurizing the system and and at least get a pump out done?

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Old 01-02-2020, 12:58 PM   #9
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Do you have an overboard discharge pump? Maybe go offshore and pump out there. Too bad about the Vacuflush system...
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Old 01-02-2020, 01:13 PM   #10
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No overboard discharge or "Y" value that we can find. Amazing a 48 foot Carver built in 1990 didn't have that installed. We surveyed 5 of these boats, same year and model and each one had a discharge, all in basically the same place.

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Old 01-02-2020, 01:19 PM   #11
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Line TO holding tank goes in at top. Discharge is at Bottom or dip tube so your work- around for pumping tank won't work.
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Old 01-02-2020, 04:27 PM   #12
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Quote:
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Line TO holding tank goes in at top. Discharge is at Bottom or dip tube so your work- around for pumping tank won't work.

Actually I think it might if the only purpose is to provide a source of air into the tank so he can pump out. Just make sure the level in the tank is well below the inlet fitting.



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Old 01-02-2020, 04:49 PM   #13
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Will flushing toilet allow air in?

Peggy,maybe you can answer this. If the OP just holds the vacuflush in the flush position won't this allow for air to enter the tank? All the duckbills should be in the correct orientation to allow that. Right? And if the vacuum generator is running it will be actually pushing some air in? Just shut the water supply to the toilet and flush away while pumping.
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Old 01-02-2020, 05:13 PM   #14
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Peggy,maybe you can answer this. If the OP just holds the vacuflush in the flush position won't this allow for air to enter the tank? All the duckbills should be in the correct orientation to allow that. Right? And if the vacuum generator is running it will be actually pushing some air in? Just shut the water supply to the toilet and flush away while pumping.

Won't work for a couple of reasons: 1, the vacuum pump only runs AFTER each flush long enough to re-establish the required amount of vacuum between the bowl and itself. 2.It does that by sucking all the air OUT of the system...it doesn't pull any in.


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Old 01-02-2020, 06:20 PM   #15
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Hmmm, if your holding the peddle down, and keeping it down, water continues to flow. Where does all that water go? Doesn't it continue to the tank and there by keep an open a passage for air to enter for a pump out at the same time?

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Old 01-02-2020, 06:41 PM   #16
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Brockets, that was my thinking. Also when the vacuum generator pulls a vacuum it is only on the toilet side of the vacuum generator. On the holding tank side it must push everything down. That is where the air has to go that it pulls out of the vacuum side.

Of course this was only meant to be a temporary solution to get the tank pumped. The vent will need to be opened up if the vent is indeed the problem.
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Old 01-02-2020, 07:16 PM   #17
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Yes, all this is temporary, I'm just trying to figure out if it's the vent(I think it is), or a something else like a plugged pump out line. This isn't going to be fun tearing up the back of the boat to get to the tank and fittings.

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Old 01-02-2020, 08:14 PM   #18
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I can stick a coat hanger in both vents 3-4 inches before it hits something. I wouldn't think a screen is that deep. From the best I can tell there is probably a 90 degree bend right inside the hull. I've had some experienced people onboard trying to track this down without much luck. Seems the holding tank in under the fuel tank that is under the water tank.

This systems was converted over to a vacuflush system a few years ago so this is making things more difficult as we find old hoses capped off.

I can get to the vacuum generator as it's under one of the beds. Can I disconnect the hose to the holding tank, after depressurizing the system and and at least get a pump out done?

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I'm wondering if what you are hitting is a vent line filter?? maybe installed by PO when head was converted? possibly accessed from exterior by removing the ER vent housing?
While you won't be able to pump out from the transfer line from vac to holding tank if you disconnect that line it should provide air to the holding tank and allow pumping out if your problem is blocked vent... if blocked pumpout line that's another story. A snake down the deck fitting openning should tell you if that line is blocked and could help open it up if blocked.
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Old 01-02-2020, 08:21 PM   #19
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I have nothing constructive to add, just a question about the tanks.

Why 2 pump out openings? why two vents? Do you have two tanks, one for each head? Just seems funny to me. If you have one tank for each head why are they so close together? shouldn't one be nearer the forward head and one nearer the aft head? If the tanks are far removed from the heads they each service don't you have awfully long sewage runs?

Just can't wrap my head around it.

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Old 01-02-2020, 09:30 PM   #20
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I would guess that the second vent is for diesel or water tanks.
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