Holding tank vent pipe

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I am just about to fit a Holding tank in my Southerly and am planning the route of the vent pipe which I will fit below the toe rail outboard of the deck. As the diameter of the pipe is 1.25 inch to match the input/output of the tank I was wondering whether I could get away with using a standard re-enforced water hose which is much more maleable than the conventional white heads pipe. Can anyone forsee this arrangement causing a problem. Waste will never enter this pipe, it will be simply be to allow a free passage for the input/output of air. The pipe run will be less than 6 feet.
 

NormanS

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I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.
 

MoodySabre

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I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.

If the pumped outlet is 1.25" then the vent pipe has to let in air at a reasonable rate. Ta
Ektamks recommend all pipe work at the same size. I thought it was 38mm.
 

VicS

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I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.

It needs to able to ventilate the tank freely to prevent anaerobic conditions developing .......... which is what leads to nasty smells.

As large as possible / sensible and as short as possible
 

macd

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The purpose of the vent pipe is not simply the allow material to enter and leave the tank, but to allow a free exchange of air between the tank and atmosphere. (Hence they're also called breather pipes.) This reduces the likelihood of anaerobic decomposition, the type that gives rotten eggs smell. A convoluted 0.5" pipe six feet long would vent OK, but is unlikely to be very good as a breather. Mind you, neither is a 1.25" pipe. Ideally, they should be short and straight. (Ours is about 200mm.)

...as VicS wrote whilst I was typing.
 
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NormanS

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Wow! 200mm. :D
OK, I do know what you mean.
My boat actually has a holding tank, and its vent pipe is only 0.5", fitted by the builders, but as the tank is never used, I am in no position to say whether it is adequate or not.
 

FlyingGoose

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It needs to able to ventilate the tank freely to prevent anaerobic conditions developing .......... which is what leads to nasty smells.

As large as possible / sensible and as short as possible

This is the best solution and from the Head Mistress herself Peggy who wrote the book on odors in boats
the Largest hose you can fit 38mm in recommended for large tanks
Mine is 38 mm
But like anchor , Nav lights and Batteries there is always an opinion
https://www.amazon.co.uk/Get-Boat-Odors-Peggie-Hall/dp/1892399156
 
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Thanks all.
Very valid point about odour so will probably stick to the white pipe as it is a job I only really want to do once. In terms of the discussion on diameter, as I understand it, it is mainly to do with the capacity of the pumped input of black water. If the vent pipe is too small then as you pump the holding tank will pressurise.
 

macd

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it is mainly to do with the capacity of the pumped input of black water. If the vent pipe is too small then as you pump the holding tank will pressurise.

"Too small", in that sense, might be something like 6mm diesel pipe, and even that would probably cope. Air is not very viscous.
As said, several times, the issue is permitting a sufficient exchange of oxygen with the contents of the tank to preclude anaerobic decay.
 

vyv_cox

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"Too small", in that sense, might be something like 6mm diesel pipe, and even that would probably cope. Air is not very viscous.
As said, several times, the issue is permitting a sufficient exchange of oxygen with the contents of the tank to preclude anaerobic decay.

Exactly so. These days I have even seen it suggested that two large vents should be fitted with fittings maybe opposite sides of the boat to enable a through flow of air.
 

Sea-Fever

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Exactly so. These days I have even seen it suggested that two large vents should be fitted with fittings maybe opposite sides of the boat to enable a through flow of air.

i have just finished fitting a 60L tank in my Westerly 33. The vent is short at about 500mm in 19mm ID sanitation hose. This is less than tek tanks would have me fit but a 38mm thru hull is a big hole to put in the boat so I am trying the 19mm and will see what happens. If it starts to pong I can always go bigger.....not so easy the other way round.
 

Sea-Fever

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i have just finished fitting a 60L tank in my Westerly 33. The vent is short at about 500mm in 19mm ID sanitation hose. This is less than tek tanks would have me fit but a 38mm thru hull is a big hole to put in the boat so I am trying the 19mm and will see what happens. If it starts to pong I can always go bigger.....not so easy the other way round.

If you read the forum contributions on the subject some say 16mm is fine....

A point to note is that my Jabsco flushed perfectly normally with just a 19mm vent - it doesn't have to be the same size as the heads outlet.
 
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Hi sea fever. Can I ask whether you already have this 19mm fitted on your holding tank already and it is working OK. I also don't want to fit a 38mm fitting. Trouble is there are so many conflicting bits of advice. Tek tanks say 1.25 inch (same as input/outlet) but as many of the contributors have said this is not necessary. I would love to hear from someone who actually has this small size already fitted and it is working ok.
 

RichardS

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Both our holding tanks have 19mm vent pipes which run from the top of the tank down to the waterline so probably a metre long. Both work perfectly and we have never had any smells even in regular 30 degree temps.

The delivery pipes to the top and from the bottom of the holding tank are both 38mm.

Richard
 

Sea-Fever

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Hi sea fever. Can I ask whether you already have this 19mm fitted on your holding tank already and it is working OK. I also don't want to fit a 38mm fitting. Trouble is there are so many conflicting bits of advice. Tek tanks say 1.25 inch (same as input/outlet) but as many of the contributors have said this is not necessary. I would love to hear from someone who actually has this small size already fitted and it is working ok.

Unfortunately when I said I had just fitted the tank I meant it literally. I christened it about an hour or so before replying to your post. I doubt there was time for anaerobic conditions to develop... it was only a number 1 as well :)
 

Resolution

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OK so this is a touchy subject for me. We have a single holding tank down at seat level, with a long vent pipe up to an outlet just under the toe rail. Probably about 40mm diameter. Last season, chasing a bit of unusual sniff, I noticed that the holding tank was bulging under internal pressure. An emergency trip offshore enabled us to blow the tank and release the pressure. Diagnosis is that we had overfilled the tank sometime and the vent pipe had become blocked, dried out in the heat and is now cement strong. None of our efforts to flush the blockage back down have been successful, it is of course located behind some fixed joinery, so will require major works to remove and clear. This is on the agenda for next month, what joy :disgust:.
Morals of the sad story: don't overfill, and fit the biggest size of pipe possible.
Peter
 
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