Holding tank vent fitting options

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I believe that I used a 1 amp fuse on my Scad system. So it is really a low draw. And it was simple to install.
 
The old float type sensor on our original tank level system was stuck most of the time, so I abandoned it and installed a Scad sensor. Simple to install and use and it has been extremely reliable.

The “vent as overflow” discussion was mostly to arrive at the conclusion that lowering the maximum level in my tank drops it from a 30 gallon tank to a 28 gallon tank. Both are quite small for our ambitions.

Instead of the simple, effective, and inexpensive Uniseal, what about glassing in an elbow and hose barb much like the manufacturer did for the other tank connections? That would be at least as secure a connection and seal, and prevent the protruding pipe thing from reducing the usable tank volume.
 
HeadedToTexas said:
Instead of the simple, effective, and inexpensive Uniseal, what about glassing in an elbow and hose barb much like the manufacturer did for the other tank connections? That would be at least as secure a connection and seal, and prevent the protruding pipe thing from reducing the usable tank volume.
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I see no problem with that. A uniseal is pretty much the only practical option for a plastic tank unless you have inside access to use a mushroom fitting, but for a fiberglass tank you can certainly glass a fitting into the tank.
 
The "protruding" pipe need not extend more than 1/2" inch below the Uniseal.

If your existing tank fittings are in the end of your tank as they are on OEM and off-the-shelf tanks, you're already losing capacity because contents will overflow into the inlet line when the level reaches the bottom of that fitting and a pumpout will leave up to 2" in the bottom of the tank when the level falls to the top of the discharge fitting and begins to suck air. Relocating all the fittings to the top of the tank with a diptube on the discharge fitting that' long enough to touch the bottom of the tank and is cut at about a 25 degree angle (any flatter can trap TP and solid waste under it, any sharper leaves too much in the tank) allows you to use all of the stated (usable) capacity of the tank which is never every last cubic inch..

--Peggie
 
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HeadMistress said:
The "protruding" pipe need not extend more than 1/2" inch below the Uniseal.

If your existing tank fittings are in the end of your tank as they are on OEM and off-the-shelf tanks, you're already losing capacity because contents will overflow into the inlet line when the level reaches the bottom of that fitting and a pumpout will leave up to 2" in the bottom of the tank when the level falls to the top of the discharge fitting and begins to suck air. Relocating all the fittings to the top of the tank with a diptube on the discharge fitting that' long enough to touch the bottom of the tank and is cut at about a 25 degree angle (any flatter can trap TP and solid waste under it, any sharper leaves too much in the tank) allows you to use all of the stated (usable) capacity of the tank which is never every last cubic inch..

--Peggie
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Thanks for the info Peggy, this winter I plan on seeing if I can finagle my tank out and redo my vent line to a larger mushroom type. and I will check the discharge pipe also. Plan to replace all the waste lines also. Slowly working down my list but the waste smell is getting closer to the top of the list. If decided to add a second vent line on the opposite side of the boat I should have two fittings into the tank and not just tee off a single fitting?
 
HeadMistress said:
The "protruding" pipe need not extend more than 1/2" inch below the Uniseal.
Click to expand...
Very good points. So understanding the impact connections on the side of the tank have on usable volume, a vent tube that protrudes ½ inch below the top of the tank has no impact because that volume was not usable anyway. My connections are well below the top.

IMG_7860.jpeg


It makes me wonder about the reported 30 gallon capacity. I see specs on newer Mariner 37s and Helmsman 38s indicating larger holding tanks. Is that a mathematical difference based on hose connections, or a physical difference based on a larger tank. Any of you owners of newer models (Escape is hull 3) have pictures of your holding tank you could share?
 
CharlieO. said:
Thanks for the info Peggy, this winter I plan on seeing if I can finagle my tank out and redo my vent line to a larger mushroom type. and I will check the discharge pipe also. Plan to replace all the waste lines also. Slowly working down my list but the waste smell is getting closer to the top of the list. If decided to add a second vent line on the opposite side of the boat I should have two fittings into the tank and not just tee off a single fitting?
Click to expand...
I would go with aeration instead of a second vent line (which would need a second thru-hull) 'cuz unless your vent line(s) are relatively short, straight and don't rise more sharply than 45 degrees, a second one wouldn't help much.

Aeration doesn't have to be expensive...many owners DIY aerators using a bait box pump and some "perforated" (put holes in it) hose. that can deliver air throughout the tank. It does have to run 24-7 whether you're on the boat or not (see my previous posts on this), but the current draw is minimal.

--Peggie
 
HeadMistress said:
I would go with aeration instead of a second vent line (which would need a second thru-hull) 'cuz unless your vent line(s) are relatively short, straight and don't rise more sharply than 45 degrees, a second one wouldn't help much.

Aeration doesn't have to be expensive...many owners DIY aerators using a bait box pump and some "perforated" (put holes in it) hose. that can deliver air throughout the tank. It does have to run 24-7 whether you're on the boat or not (see my previous posts on this), but the current draw is minimal.

--Peggie
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What is your definition of relatively short? I could one side at probably a bit less than 5’ and the opposite side with maybe 7ish’ with a slight uphill definitely might much less than 45 degree.
 
My boats beam is about 14’. I put a 1.5” one to each side. They go up less than a foot each so it is pretty much a straight shot from side to side.
 
I have not measured it but it is about 1’ 6”.
 
Yes, 1 from each side so the air will come in one vent and flow across the sewage and out the other vent. It isn’t as good as going in one side and out the other end since my tank is rather long but it was the best I could do. Even then I had to have the mechanic come and hook the hoses up since I couldn’t fit back there with my back problems, I couldn’t contort my back to fit in there. In the past I never had anyone do work on my boat but now I have to.
 
CharlieO. said:
What is your definition of relatively short? I could one side at probably a bit less than 5’ and the opposite side with maybe 7ish’ with a slight uphill definitely might much less than 45 degree.
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If the hose(s) are straight, you have a textbook example of the ideal vent line: no longer than 5-7',straight as an arrow, doesn't rise more than 45 degrees. Go with just one that you've upgraded to 1" and add aeration and it couldn't be any better.

--Peggie
 
HeadMistress 2011
The vent line has everything to do with creating AEROBIC conditions in the tank.... when it's short enough, straight enough, and of sufficient diameter to allow a free exchange of air with the gasses in the tank.* But the "standard" 5/8" ID vent line can't do that if it's more than about 18" long. A vertical line even that short cannot do that because the gasses in the tank are heavier than air they don't rise, fresh air doesn't fall, so the twain never meet. 5' is about the maximum length that will allow a tank to "breathe."
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Peggy, In my search for answers I found your quote from June 2011. The bolded is what I am interested about. You often say straight, which appears to also mean level or up 45* max and no more than 5 feet.
Carbon dioxide weighs 1.87 kg/m3, so it is 1.5 times the weight of air. Sewage gas is heavier than atmospheric gas and it “sinks” to the lowest level in the house or in a room.
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I have always wondered why not slope downwards but stay above waterline?
 
SteveK said:
I have always wondered why not slope downwards but stay above waterline?
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You could, but many tanks aren't mounted high enough in the boat to allow it. The top of my tank is below the waterline, for example.
 
Between the weight of various gasses and the length and curvature of the path between my holding tank and the through hull, am I correct in understanding that my vent is not a gas exchange and oxygen supply mechanism? I would be happy if the vent line simply equalized the pressure inside and outside the tank by venting gas out as heads flushed and allowing air in during pump outs. Based on my understanding of chemistry and physics, I think a 1" line between a new Uniseal in the top of my fiberglass tank and a new stainless steel mushroom type through hull fitting would accomplish that venting even if it was more like 10 feet long with 3 roughly 90 degree bends (one at the Uniseal, one at the through hull fitting, and one going under the bulkhead).

When we are not having sanitation issues, we have always been happy with how the heads smell. I supply oxygen with periodic doses of sodium percarbonate. My only complaint is that the only venting we have right now is through the joker valves (which are overdue for replacement).
 
am I correct in understanding that my vent is not a gas exchange and oxygen supply mechanism?

Nope...you are incorrect, which is the reason why it's necessary for vent lines to be short enough, straight enough and close enough to horizontal to allow that to happen.

Tank vents have nothing to do with odor INSIDE the boat, it's odor OUTSIDE the boat being pushed out the vent from an anerobic tank when the toilet is flushed or the boat rocks that require enough air exchange to provide the oxygen needed to create the aerobic environment in the tank necessary to prevent odor from being created. When the vent can't allow that to happen, aerating the contents may be necessary.

--Peggie
 
rslifkin said:
You could, but many tanks aren't mounted high enough in the boat to allow it. The top of my tank is below the waterline, for example.
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Ambient air doesn't fall and sewer gasses don't rise...so in a downsloping vent or vents that have bends in them or rises closer to vertical than 45 degrees, or is longer than 6-7', the 'twain will never meet to make any exchange.

--Peggie
 
HeadMistress said:
Ambient air doesn't fall and sewer gasses don't rise...so in a downsloping vent or vents that have bends in them or rises closer to vertical than 45 degrees, or is longer than 6-7', the 'twain will never meet to make any exchange.

--Peggie
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That's exactly why I went for forced aeration over adding a second vent and further up-sizing the one vent beyond the 1" hose it currently uses. With the tank mounted so low (and on the centerline) it would be hard to get enough airflow through the vents unless they could be placed such that wind would be regularly forcing air through and out the other side.
 
HeadMistress said:
am I correct in understanding that my vent is not a gas exchange and oxygen supply mechanism?

Nope...you are incorrect, which is the reason why it's necessary for vent lines to be short enough, straight enough and close enough to horizontal to allow that to happen.
Click to expand...
Properly functioning vents perhaps, but the vent on MY boat is too long and too twisty to allow sewer gas out or oxygen in. It would be great if it did, but it never has since the boat was built in 2007. That's the understanding I am asking about.

Since my holding tank is low in the boat on the centerline with no straight 6-7' run to the hull above the waterline, I believe I am stuck either adding a bubbler or being happy I don't have an odor situation worth complaining about. That said, I do need to upgrade my vent to allow gas out when flushing and air in when pumping out. My plan is to upgrade my vent with about 10 feet of 1" hose, a Uniseal, and suitable fittings at each end.
 
OK, I will report back after repurposing a thruhull above the water line but below the top of my tank. Slope down will be about 30*.
My theory is simple, since CO2 and stinky air are both heavier than ambient air they will fall out down the pipe and in the process fresh air will bring in the oxygen needed into the vacated airspace of the tank. Nothing to lose but my time.
 
A downsloping vent won't work very well even with an aerator and 10' is too long. If you want to prevent odor out the vent you're gonna have to install a new vent thru-hull using an open bulkhead fitting instead of a "vent" thru-hull closer to the tank and at the level of the top of the tank or higher.

--Peggie
 
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