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Old 16-03-2011, 21:05   #1
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Holding Tank Vent

Can a spring loaded check valve be installed to an unused fitting on top of my holding tank to aid in venting the tank during pump out? I know it physically will work, just unsure if it will seal to keep from leaking odors within the cabin. Is there a check valve made for specifically for waste systems? My existing 3/4" vent hose, Sealand odor vent canister and thru hull vent are not capable of passing as much air as needed to prevent from creating a vacuum in the tank when I use the dock side pump out facilities. If I remove the nozzle right away, I get a small amount of inrush air, so I assume that I have to be collapsing the tank a bit. Never looked, but seems highly likely. I prefer not to install a 2nd thru hull vent if possible, but not totally against it if it is the only cure.

Simple design, dock side pump out capabilities only.

Thanks
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Old 16-03-2011, 22:21   #2
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Re: Holding tank vent

SeaLand makes a "pressure relief" valve Dometic Sanitation Systems but there's a better way: get rid of the filter--which may actually be causing your problem 'cuz filters iinterfere with the flow of air in and out of the vent...and if it ever gets wet, can completely block the vent. And also install a larger vent. You'd have to put a new hole in the tank to install the SeaLand doodad, but if the unused fitting on your tank is at least 1", you can use it (seal the existing one with a threaded plug, available from any hardware store)...enlarge the vent thru hull...problem solved. With the added advantage of a vent that can now supply ehough air excharge to keep your tank aerobic, which is the key to PREVENTING odor...eliminating the need for a filter.
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Old 16-03-2011, 23:14   #3
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Re: Holding tank vent

As I understand it,that item wouldn't work as it is for preventing pressure build up within the tank, not a vaccuum. I installed the filter because the odor was very noticeable outside the boat, and it took care of that issue. I understand the filter may be the restriction. I have no idea if it created this problem or not, as I installed the filter right after the boat purchase, so no base line. I will look at the tank for the size of the unused fitting and aim for the larger vent hose route as that will be more appealing than 2 thru hull vents. If I can eliminate the filter as you say, even better.

Thank You
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Old 17-03-2011, 06:30   #4
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Re: Holding tank vent

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Originally Posted by US1Fountain View Post
As I understand it,that item wouldn't work as it is for preventing pressure build up within the tank, not a vaccuum ...
Not according to the very brief description per the Dometic/Sealand link.

"Protects holding tank from excessive suction by dockside pumps."
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Old 17-03-2011, 07:25   #5
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Re: Holding tank vent

wow .. you have a 3/4" hose and still not getting enough air? sounds like you have some kind of obstruction in the hose or the filter.
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Old 17-03-2011, 07:27   #6
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Re: Holding tank vent

I would like to up the diameter of the vent on my tank but have not seen vent fittings that accept anything larger than the 5/8 standard. Any suggestions on a vent fitting that will accept larger hose?
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Old 17-03-2011, 07:38   #7
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Re: Holding tank vent

I have that charcoal holding tank filter in my system and it wasn't long before it seemed to stop allowing air into or out of the tank.
- - My MSD is hooked directly to the holding tank. So when pumping out the holding tank I get significant air flow into the tank through the MSD rather than the vent.
- - Depending upon what brand/type MSD you are using, it might be the solution to allowing adequate air into the tank during deck pump out. After all, it has a 1.5 inch hose attached to the unit and the holding tank and any "back flow" is prevented by the joker valve.
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Old 17-03-2011, 08:18   #8
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Re: Holding tank vent

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To learn everything you will likely need to know about marine sanitation go to this site for information written by Paggy Hall, the head mistriss:

Link deleted

Included is information on the need for an adaquate sized vent line to prevent odor. Very good article written by a knowledgeable lady.
Nooooooooooooo!!! Please do NOT go to that article! I haven't updated it since 1996, making most of the information in it so obsolete now as to be BAD information! I've tried for at least 5 years to get boatbuilding.com to remove it, but they just ignore my requests. They show me as a member , but I've never joined...I tried once, but it wouldn't let me. and any more recent "updates" are theirs--changes to my email address and/or physical address.

It wouldn't be much of a problem if it were only on that one site...but others keep finding it and post a link to it on their own sites/blogs...or even copy/paste the whole thing! It's like a vampire...I can't kill it!
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Old 17-03-2011, 08:31   #9
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Re: Holding tank vent

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wow .. you have a 3/4" hose and still not getting enough air? sounds like you have some kind of obstruction in the hose or the filter.
He prob'ly has the standard 5/8" hose...many owners don't realize that hose sizes are always the INNER diameter, and so mistakenly give the outer as the size. 3/4" would be the OD for a 5/8" single wall sanitation hose. A vent line filter is another clue that it's most likely only 5/8"...that's the standard barb size on a vent.

However, it's a pretty good guess that he does have a vent line blockage. If it's not in the filter, most likely in the thru-hull 'cuz that's the most common location. Dirt daubers love thru-hulls...dust, pollen and corrosion block screens...and overflowing the tank out the thru-hull will clog the thru-hull...clog a filter even faster.

I'm not a fan of the SeaLand "relief valve" 'cuz it misleads owners into thinking it protects the tank against both implosion due to suction AND EXplosion due to pressurization...and also that it relieves 'em of doing any tank vent maintenance. Vents should be backflushed--which cannot be done if a filter is in the line--every time the boat is washed and/or the tank is pumped out.
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Old 17-03-2011, 08:34   #10
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Re: Holding tank vent

You'll have to use a plain ol' bulkhead or "mushroom" fitting instead. That may require a clamshell cover on sailboats that bury the vent thru-hull when heeled.
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Old 17-03-2011, 08:39   #11
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Re: Holding tank vent

The toilet shouldn't be a substitute for a working vent line...it's not good for the toilet pump...particularly strong suction can misalign the seals, o-rings and valves...and doesn't work with all toilets. Nor is it reliable if the toilet discharge line is clogged or reduced by mineral buildup...or a y-valve is closed. Maintaining the vent line is easy...far better to do that instead of relying on work-arounds.
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Old 17-03-2011, 08:44   #12
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Re: Holding tank vent

My sanitation guy stated that dried bits of TP often clog the vent at the waterline of the tank. The only way to fix this is to put pressurised water back down the vent line. He uses a garden hose connected to what looks like a plunger head stuck up against the thru hull to wash these vent lines.
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Old 17-03-2011, 08:44   #13
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Re: Holding tank vent

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Originally Posted by peghall View Post
You'll have to use a plain ol' bulkhead or "mushroom" fitting instead. That may require a clamshell cover on sailboats that bury the vent thru-hull when heeled.

Thanks Peg. I guess not having a screen on the fitting is not an issue as anything that wants to crawl down that hose is more than welcome to what it will find.
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Old 17-03-2011, 08:58   #14
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Re: Holding tank vent

I also suggest getting rid of the filter, as it should not be necessary. I wouldn't install a check valve either... The splash resistant water maze fittings, where the vent hose goes through the hull, may have an obstruction? (dirt dobbers, salt?)

The US standard for holding tank vent hoses is too small, and other countries use larger hoses as their standard.

In my case, I made the tank out of the hull itself, so a clogged vent could result in much more than a bad day! I ran two separate vent hoses, with two separate through hull water maze fittings. I use no filters, and have never smelled the vents at all. There is no vacuum problems on pumpout either...

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Old 17-03-2011, 09:04   #15
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Re: Holding tank vent

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Originally Posted by capn_billl View Post
My sanitation guy stated that dried bits of TP often clog the vent at the waterline of the tank. The only way to fix this is to put pressurised water back down the vent line. He uses a garden hose connected to what looks like a plunger head stuck up against the thru hull to wash these vent lines.
This is a good point! My wife flushed the head into a 100% full tank once, and it goes right up the vent hoses and down the hull! I used a homemade fitting & garden hose to blast a strong stream of water in the maze fittings, (from the outside), and down the vent hoses. This clears the clog nicely.

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