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Old 05-29-2018, 07:41 AM   #1
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What is this holding tank vent?

The holding tank on my Grand Banks Eastbay 47 has what looks like some sort of valve on it. The boat is 'new to me' so I'm getting used to how everything is set up. The owners manual makes no mention of the system details beyond hose diagrams.

After either of the vacuflush systems operates that valve on the holding tank is letting out air, which of course smells foul.

I've done nothing to the set up yet. The area looks wet because I was rinsing out the bilge area just aft of this (that has the sink/shower sumps) and splashed a bit of water up onto the holding setup. The moisture is not from within the tank itself. One of the shower sump pumps had failed and the tank had overflowed. That's been fixed and I was just rinsing out the bilge under it.

The system has has two vacuflush pumps leading to the tank. (one from each head, on separate breakers). The hose from each head goes into a small tank ahead of the pumps and then into the holding tank.

It has a charcoal filter mounted right to the top of the tank, screwed into an elbow. From the looks of things this hasn't been changed recently, not unless whoever replaced it also reattached various bit of the wiring back to it again using zip ties. I intend to correct this and keep the wires secured and free of the filter.

It also has two overboard discharge pumps. The overboard units are selectable using the toggle switch marked 1 & 2 on the junction box atop the tank. They're controlled by a keyed panel at the helm. I tested both of them while offshore during delivery from Rhode Island to Maryland and they worked fine. No zip tie on the through-hull, but it's on my list. If just to avoid giving anyone inspecting it a point to complain about it. We boat regularly in areas where we can't discharge, so this won't see regular use.

To try and find the source of the smell I poured a bit of dish soap on what looks like a flapper of some sort in the well of that valve. When I operate either of the vacuflush pumps and flush their respective head the soap on the valve bubbles up a bit. Which leads me to believe that's the source for the stink.

The previous owner was using West Marine's "PURE OCEANS Bio Odor Bioactive Head Treatment"

I'm assuming the charcoal filter needs to be changed and is probably not allowing effective airflow. I'm not keen on how close the filter is to the top of the tank as tilt from rising on plane and a full tank seems likely to allow it to get contaminated. I do not know if the overboard vent line is clear or not, but intend to check that also.

My question is what is that vent? And under what circumstances should or shouldn't it operate?
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Old 05-29-2018, 07:56 AM   #2
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Greetings,
Mr. wk. My, my. Certainly is "busy" in there. The only thing I can comment on is the charcoal filter. Personally, I would take it out and insure that the tank has a lot of ventilation to encourage growth of "good bugs"(aerobic bacteria).


Why two over board pumps I wonder?
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Old 05-29-2018, 07:59 AM   #3
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I should also add that the keyed lockout for the overboard system has empty/low/mid/full indicator lights and never reads anything other than empty, regardless of the amount in the tank. There are three sensors plugged into the tank. You can't quite see them in the previous pictures. You can make out one of them directly above the right-hand side of the charcoal filter. I'm assuming they're individual floats that have gotten gunked up and stuck in the dropped (empty) position. That'll be a crappy task to clean them...

I'm looking to change the tank monitoring to something capable of NMEA 2000 integration, so this is less important at the moment. My water and fuel tanks use a Computank setup (which isn't compatible with anything else, so I've learned). Integrated tank monitoring is on my to do list. But that's not in the scope of this thread.

We're fortunate that our marina has a pump out service. They coordinate with the harbormaster each Monday (or Tuesday since it's after the long weekend) and will do pump outs without us having to be there. I had pumped out this past Sunday (while re-fueling). Before leaving yesterday I then added quite a lot of fresh water back into the tank to help dilute whatever else might still have been in there. I did this through both heads to clear both their sets of hoses and vacuflush tanks and pumps. I just pointed the shore water hose at an open head and let it pump through.
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Old 05-29-2018, 08:06 AM   #4
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Quote:
Originally Posted by RT Firefly View Post
Greetings,
Mr. wk. My, my. Certainly is "busy" in there. The only thing I can comment on is the charcoal filter. Personally, I would take it out and insure that the tank has a lot of ventilation to encourage growth of "good bugs"(aerobic bacteria).

Why two over board pumps I wonder?
Belt and suspenders, I guess? Yeah, I was likewise puzzled at what seems like overkill.

The two overboard pumps feed to just one overboard through-hull. That and the pump selector switch being right there on the tank seemed odd. But the hatch for this is very accessible, right in the main companionway, so it's relatively easy to flip the switch's three positions of 1-off-2. The through-hull is accessible from another hatch just aft of this (where the shower sumps are located).

I don't know of there's enough slack in the vent line to just eliminate the filter. I'll probably have to fashion a short hose to bridge the distance.

My past boat had no overboard setup and just one head/vacuflush setup. So I have no idea what's typical for this sort of stuff.
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Old 05-29-2018, 08:13 AM   #5
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I may actually already have that cheater hose in my storage locker somewhere.

My past boat was plagued by tank stink. I think I finally figured out why just before we sold it. The design of the vent setup had the vent line leading out from the top/stern end of the tank and then ran quite a distance back to the engine compartment. Popping up on plane no doubt lead to effluent running back into the vent hose. Which seemed likely to have at least one area where it dipped enough to potentially end up with a water trap. Which would then have blocked what little flow might have existed. It was a bad design all around, but I sold it before this revelation occurred to me. We just nuked the tank with blue and pumped out frequently.

This current setup does not seem nearly as defective and I'd very much like to operate it as chemically-free as possible. Sans stink, of course.
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Old 05-29-2018, 08:42 AM   #6
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What you're describing isn't a true "vent" thru-hull, it's a "pressure relief valve" that only opens when the toilet is flushed to prevent the tank from becoming pressurized and when the tank is pumped out to provide a source of air to prevent the pumpout from pulling a vacuum. Dometic/SeaLand has created a system that causes the very problem filters are needed to prevent: odor out the vent (Why couldn't I have invented something that does that???).

The system has has two vacuflush pumps leading to the tank. (one from each head, on separate breakers). The hose from each head goes into a small tank ahead of the pumps and then into the holding tank.

That's the VacuFlush toilet system. You have two separate systems, each consisting of only three components--the bowl assembly, he vacuum accumulator tank and the vacuum pump. Both are connected to a single holding tank, which is NOT a part of the VacuFlush system.

Here's how it works: The small tank is the vacuum accumulator tank. The accumulated "vacuum" (suction) only pulls the bowl contents TO the pump..the pump has to push it the rest of the way. So the vacuum pump has to do two things simultaneously: it suctions the air out of the plumbing between the toilet bowl and itself while simultaneously pushing the flush the rest of the way to the tank, treatment device or thru-hull. The vacuum tank has a switch and a sensor on it that starts the vacuum pump when there's a loss of "vacuum" (toilet is flushed or an air leak in the system) and turns it off when the correct amount of negative pressure has been reached. (This is from a piece I've written called "VacuFlush 101" that I'll be glad to send you if you'd like to send me a PM that includes your email address--no way to attach anything to a PM). We can also discuss why your tank level indicators are stuck and other issues, and what to do about those.

So if I were you,
I'd replace the 'pressure relief valve' (there may also be one on the tank ahead of the filter) with a new 1" or even 1.5" open thru-hull that will provide enough air exchange with the gasses in the tank to make a filter unnecessary. Just adding a new vent fitting on the tank may be the easiest way to do that...and it's actually pretty easy, thanks to a li'l gizmo called the Uniseal.
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Old 05-29-2018, 09:34 AM   #7
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Random thoughts...

I am confused as to why there would be two overboard pumps?

The vacuflush system seems a bit overly complicated to my simple mind.

Get rid of the charcoal filter and check the vent line to be sure it is open. While you are at it, make sure the vent line doesnít have any dips in it.

That valve on the top of the tank seems really odd. Peggie has seen it all so she knows about it. Adding another vent line would be a great idea if you can find a way to route the line to a place high enough on the hull the hull fitting.

The stuck level sensors may not be as bad a job as you imagine. Iíve done it a few times. You may find that all four of them are mounted in a single plate that you can unscrew and remove. Disconnect the wires to each (label them) and then take the sensors out of the boat. Iíve used a 5 gallon bucket with warm water and detergent to let them soak. After a while dispose of the bucket contents by flushing it down the head. Then I used a hose to rinse off the sensors and ensure they moved freely. There may be better ways but that is what worked for me.
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Old 05-29-2018, 10:49 AM   #8
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The mystery valve is, as Peggie describes, a vacuum breaker, a flavor of check valve, if you will. It will only allow air INTO the tank. Prevents vacuum damage to the tank in the event of vent blockage during pumpout. The filter is accessible, but if it is to remain, it would be preferable to locate it with a vertical orientation, as high above the tank as practical. In the event of overfilling, the vent line can puke into the filter, any effluent will effectively block flow through the filter and render it non-functional.

Someone else mentioned the vent line and insuring there are no traps in the line that can collect liquid. Important as it can render the vent ineffective, and if effluent enters the vent piping and later dries, it can create a rather obstinate blockage.

The same sort of dried debris is likely what is causing the malfunction in the level floats. If they are the typical SeaLand floats, Ythey consist of a float with an internal magnet that operates a reed switch in the float probe. The floats are susceptible to accumulation of "stuff" that can dry out, especially if the tank is idle for long time periods. Then the float jams. They are easy to clean if the whole float assembly is unscrewed and removed for cleaning. They are also susceptible to buildup of salts on the floats. Vinegar, muriatic acid or On & Off will remove those deposits. Vinegar takes many hours, On & Off a few minutes.
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Old 05-29-2018, 10:54 AM   #9
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Plus one on deleting the charcoal filter. Get more air into the tank. Groco Sweetank is a great solution if you cannot get a large enough vent hose in. Listen to Peggie and you canít go wrong.
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Old 05-29-2018, 05:07 PM   #10
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Old 05-29-2018, 05:16 PM   #11
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On the 3rd (labeled) pic, I see a hose from the port toilet to the pictured tank... but I don't see a hose from a starboard (or other) toilet. Is there another tank somewhere, possibly nearby? (And maybe another vent line, and another charcoal filter?) Are the two overboard pumps servicing two separate tanks?

Or have I been struck blind and just can't see something obvious?

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Old 05-29-2018, 06:18 PM   #12
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Yeah, donít mess with Peggie, she knows her $h-t
Heh, when I mentioned searching for answers my wife said, "the head mistress" when I said there was a woman named Peggy that knows all this stuff. The setup on our old boat was never right so I'm tasked to "get sh!t together" on this one... or else.
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Old 05-29-2018, 08:19 PM   #13
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And I'm glad to be able to help you do that. But we need to TALK, 'cuz I have a whole bunch of questions that need answers before I can offer much more than I've already posted. So send me a PM that includes your email address and we'll work out a mutually convenient time to do that.

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Old 05-31-2018, 11:26 AM   #14
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What you're describing isn't a true "vent" thru-hull, it's a "pressure relief valve" that only opens when the toilet is flushed to prevent the tank from becoming pressurized and when the tank is pumped out to provide a source of air to prevent the pumpout from pulling a vacuum. Dometic/SeaLand has created a system that causes the very problem filters are needed to prevent: odor out the vent (Why couldn't I have invented something that does that???).
So under normal operation that relief valve should only open if there was no other way for tank to avoid being over pressurized during a flush, or under pressurized when being pumped out from the deck fitting.

The former seems like it should normally be handled by the existing line running to the charcoal filter and the existing through-hull vent. The latter, a pump out, I could see potentially overwhelming the size of that filter and vent line, especially if the filter was blocked (due to neglect/poor design).

I'm familiar with how the Vacuflush setups work, but thanks for covering it again. Those parts seem to be working OK.

Quote:
So if I were you,I'd replace the 'pressure relief valve' (there may also be one on the tank ahead of the filter) with a new 1" or even 1.5" open thru-hull that will provide enough air exchange with the gasses in the tank to make a filter unnecessary. Just adding a new vent fitting on the tank may be the easiest way to do that...and it's actually pretty easy, thanks to a li'l gizmo called the Uniseal.
Tank ahead of the filter? You mean the tanks ahead of the Vacuflush units? There's two of them. The plumbing follows a route from:

Head->hose->tank->vacuflush->pipe->holding tank.

That's repeated on each of the two toilets. Each has their own pump and both elbow into the same holding tank.

The only place I recall seeing a valve is that one on the holding tank itself. But I'll examine the two vacuflush tanks again to be certain.

Running a new vent line to the outside is not on my near-term to-do list. At least not until I get a better idea of how accessible the spaces are that could accommodate it.

My initial impression is the existing filter is probably blocked, and possibly the existing vent line from it. I'm going to remove the filter this weekend and check that the vent is clear. Once that's clear I'll recheck the bubbling at the pressure relief valve. If that still leaks under normal flushing of the heads then I'll have to take further steps. Augmenting the existing single vent with another line seems among the most logical means to improve the situation.
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Old 05-31-2018, 01:51 PM   #15
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Has the design of the tank valve always used the same diameter? As in, will the current Dometic Tanksaver valve fit what's in there presently?
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Old 05-31-2018, 02:25 PM   #16
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Tank ahead of the filter? You mean the tanks ahead of the Vacuflush units?

No...the HOLDING tank. The filter is in what may be a vent line in addition to the pressure relief valve. I'm unclear as to whether it's on the tank or is a thru-hull terminating another "vent" line coming off the tank.

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