Holding tank vent position ?

[affinite]

I'm currently fitting a holding tank and I'm generally happy about the plumbing except location of the 38mm open vent. Looking at the diag in PBO this month it seems this should be in the hull as high up as possible. (Presumably this dumps bad air over the side rather than rolling down the decks!) Even if I fit this just below the toe rail I'm worried what will happen when heeled over as my vent will be fwd of the beam. Any advice ?

 

[snowleopard]

that's huge for a vent, 3/4" would be more than adequate i would have thought, certainly it works for me. to avoid filling from wave action, have a loop above the exit point so any water has to go uphill. it doesn't matter too much if some water gets in at sea because you can pump it out again.

bear in mind that when someone uses the head there will be foul air expelled which pongs mightily!

 

[HeadMistress]

A loop in the vent line will actually increase odor each time the toilet is flushed, because it prevents the free exchange of air needed to keep the tank aerobic. For a more detailed explanation of what causes odor and how to vent a tank to prevent it, read "Odour Control" and "Venting" in the sanitation article on the Tek-Tanks site here: http://www.tek-tanks.com/Pages/frameset.htm

Where will the tank be located in the boat? If the head is forward, the ideal location for a holding tank on a sailboat is under the vee-berth. This allows for a short straight vent line that exits the hull far enough forward to stay out of the water at maximum heel. However, if the head is aft, that would make the head discharge line unacceptably long, so we'll have to work out the best way to vent it without taking on water while heeled.

 

[jerryat]

Hi1

My holding tank is forward under the 'V' berth and the 22mm (3/4") vent goes to a postion just under the toerail, i.e. as high as possible in the hull side. There are no loops or other form of 'trap' and, as others have pointed out, any seawater that does enter (very rare in our experience) can be simply pumped out again.

Cheers Jerry

 

[scarlett]

As you will be using the holding tank at anchor mostly, I recommend that the vent be down wind of the cockpit i.e. at the stern.

 

[HeadMistress]

[ QUOTE ]
As you will be using the holding tank at anchor mostly, I recommend that the vent be down wind of the cockpit i.e. at the stern.

[/ QUOTE ]

Keeping odor off your boat, but gassing anyone unfortunate enough to be anchored downwind of you. /forums/images/graemlins/smile.gif

Many people believe that it's just the nature of holding tanks to stink, impossible to prevent, which isn't true. When organic matter--ANY organic matter, including waste--breaks down aerobically (oxygenated), it produces only CO2, which is odorless (so is methane, btw)...it's only when organic matter breaks down anaerobically that it produces smelly gasses. That's why compost piles must be tossed and aerated, or they compact, rot and stink...why stagnant ponds stink and running streams don't--the reason why decorative ponds in office buildings and shopping malls always include fountains and waterfalls. A boat holding tank is no different from any other "pond"...it too must be oxygenated to prevent odor.

On boats on which the tank is maintained aerobically, it's literally possible to be standing on the dock next the vent thru-hull when the toilet is flushed and not know it unless you hear the toilet. And it's not as hard to do as you might think...a short straight vent line going to a thru-hull that's always into the wind forces enough air exchange in the tank to keep the tank aerobic...and odorless...which makes the bow the ideal location for a waste tank--but that's only practical if the head is forward.

Unfortunately, though, when a tank is buried under the sole in the bowels of a boat, the vent line cannot be short enough nor straight enough to accomplish this...which makes the keel the worst possible location for a holding tank. An aeration system in a tank may be the only solution. In other locations, it may be possible by installing two vent lines, with a low voltage exhaust fan in one to pull air through.

Read the articles at link I posted above...and then I'll take questions.

 

[affinite]

Peggy
The fwd V berth has a water tank under it so the holding tank will be in the heads compartment itself (behind the front cabin) in what was the wet locker. This is just fwd of the beam. The route to the toe rail is currently only about 12" but it sounds as though I should route it fwd to get it out of the water when heeled?
BTW I bought my parts from TekTank. The vent I have bought is the one shown on your link.

 

[roger]

A holding tank peril

I heard a story recently from a friend who saw a motor cruiser at a pump out station. He undid the deck fitting and was immediately submerged in a mighty fountain of poo!
The mechanism is speculative but I suspect the following;
a. The pump out pipe has to go to the bottom of the tank to empty it completely.
b. Gas pressure above the tank contents caused by fermentation gave the operating pressure.
c. The vent pipe may have had an anti -stink filter which was blocked when the holding tank got too full.
Lesson - either dont have a filter or keep tabs on how full the tank gets.

 

[HeadMistress]

Re: A holding tank peril

His tank vent was blocked, Roger.

The vent on all tanks--water, fuel, waste holding--has two essential functions: it allows air IN to replace contents as they are drawn out, and it allows air displaced by incoming water, fuel or waste to get out. If the vent is blocked, air displaced by incoming waste has no means of escape, so the tank becomes pressurized...and a pressurized tank can have disastrous consequences--anything from a geyser when the deck fill or deck pumpout cap is removed to a backup in the toilet, to blown out fittings, to a cracked tank.

It's also impossible to pump out or dump a tank against a blocked vent, 'cuz unless air can be pulled in through the vent to replace the waste (or anything else in ANY container) being pulled out, a vacuum is created in which nothing more can be removed--but in which the suction from a strong pumpout CAN crack the tank.

If you suspect a blocked vent, immediately cease all use of the toilet until you can clear the vent and pump out or legally dump the tank. If you have a manual toilet, stop flushing and cease using the toilet when you feel any backpressure or see any waste backing up into the toilet. If you have an electric toilet, stop flushing the toilet and cease any further use of it if you experience sluggish discharge or see waste backing up into the bowl.

Or...you too can experience a thrill of being bathed in sewage. /forums/images/graemlins/smile.gif

 

[HeadMistress]

Affinite, whether to route the vent line far enough forward to keep the thru-hull out of the water at max heel depends on how the vent line would have to be do that. I would take it far enough forward to limit the vertical rise to no more than 45 degrees...a clamshell cover should prevent taking on water through the tank vent.

 

[Swagman]

I found on our last yacht, with a flexible waste tank fitted far forward, placing the vent exit inside the anchor locker which itself drains overboard, not only worked fine but looked (ie one can't see the vent) better.

And interestingly, even at anchor with a head wind no bad smell was ever experienced.

 

[HeadMistress]

Not sure what UK and European law requires, but US, Canadian and Australian safety regulations require that all waste tanks be vented to the outside of the hull because methane is flammable...and also because anaerobic hydrogen sulfide gas ("sewer gas") is highly toxic, even lethal.

I realize that the odds of a methane explosion in the anchor locker are pretty remote unless an electric windlass has faulty wiring...but I don't make the rules, I'm just the messenger.

 

[syneraida]

I have a very similar issue except its with a gray water tank, the tank was decommissioned a long time ago and all pipework removed, so now i'm left with a empty tank and no connections. The problem is that the tank is located midships in the bilge, so a vent line forward or aft will be looooong (minimum 8 meters), the old vent was simply to the adjacent bilge well.

If I run a vent to the upper most part of the hull in that area it will be regularly submerged whilst sailing, any ideas or suggestions would be really appreciated!

 

[Chris_Robb]

I'm currently fitting a holding tank and I'm generally happy about the plumbing except location of the 38mm open vent. Looking at the diag in PBO this month it seems this should be in the hull as high up as possible. (Presumably this dumps bad air over the side rather than rolling down the decks!) Even if I fit this just below the toe rail I'm worried what will happen when heeled over as my vent will be fwd of the beam. Any advice ?

Steve - I have more or less the same set up with the vent coming out just under the tie rail and foreward of max beam. I have angled the vent so that it faes backwards - to my knowledge I have had no water down it even on a long starboard tack. 38mm is the right size - as big as poss and as straight as possible. No need for a loop as any water will go into the tank so does not matter.

What pump out arrangements do you have?

PS - I have never smelt the tank except very briefly when someone is pumping the heads!

 

[Mistroma]

Steve - I have more or less the same set up with the vent coming out just under the tie rail and foreward of max beam. I have angled the vent so that it faes backwards - to my knowledge I have had no water down it even on a long starboard tack. 38mm is the right size - as big as poss and as straight as possible. No need for a loop as any water will go into the tank so does not matter.

What pump out arrangements do you have?

PS - I have never smelt the tank except very briefly when someone is pumping the heads!

I expect that Affinite has finished the installation he was starting 11 years ago. Syneraida may be in the lead for YBW 2016 Lazarus award.

 

[essexboy]

The bigger the better, this also applies to tank vents.

 

[Graham_Wright]

Not sure what UK and European law requires, but US, Canadian and Australian safety regulations require that all waste tanks be vented to the outside of the hull because methane is flammable...and also because anaerobic hydrogen sulfide gas ("sewer gas") is highly toxic, even lethal.

I realize that the odds of a methane explosion in the anchor locker are pretty remote unless an electric windlass has faulty wiring...but I don't make the rules, I'm just the messenger.

Welcome back Peggie!

My tank also vents to the anchor locker and also high on the hull. The anchor locker is vented through the hawse pipe and the drains. It's second cousin to the outside world!

 

[Chris_Robb]

I expect that Affinite has finished the installation he was starting 11 years ago. Syneraida may be in the lead for YBW 2016 Lazarus award.

Ha Ha - I was surprised Steve had not got a holding tank....... Have been following him around for years! Teach one to look at the date!!!!

 

[Quandary]

Our tank vent goes up then runs back through the aft locker and down to discharge through a manifold with other vents and drains under the transom about 150mm. above static water level. When the boat is moving fast the stern wave covers the outlet and at the same time the tank contents are agitated so it is necessary to pump the tank every time we get under way in anything other than calm conditions, not a major problem once the habit is established as the pump handle comes though the bulkhead into the head above the pan.

 

[vyv_cox]

I expect that Affinite has finished the installation he was starting 11 years ago. Syneraida may be in the lead for YBW 2016 Lazarus award.

This one is a fine example of the genre. The boat Affinite was sold around 3-4 years ago, Steve now has something rather different. Peggie Hall retired some time ago and AFAIK does not post any longer, at least over here.