Second Holding Tank Vent Location

Discussion in 'Boat Design' started by Pudulax1, Jul 11, 2013.

  1. Pudulax1
    Joined: Dec 2012
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    Pudulax1 Junior Member

    Hello All -

    I have a 40 gallon holding tank on a motor yacht with a 5/8 vent on the aft side towards the top. Unfortunately, that vent hose runs almost vertical to the thru-hull. So I am going to drill a second vent hole in the tank (1") and run it horizontal to a new thru-hull. I have Peggie Hall's seminal book on boat odors, but it does not recommend where to put the vent hole on a tank. Does anyone have an opinion on whether to drill the hole on the top of the tank (with a 90 degree elbow for the hose) or on the forward side near the top?


  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Most do run pretty much straight up to the inside of a stanchion or some such. You certainly don't want a side exit, you'll be sloshing stuff out all the time. Are you trying to get more air exchange? There are little carbon filters you can buy. And more sophisticated gadgets like ozone injection. I'd try installing a $9 charcoal filter in line that was intended for an ice machine and see how that goes. Messy to replace if you overflow the tank, though. Make sure you get a housing with a pressure bypass. I don't know what the USCG requires, but I wouldn't want any part of that vent line running more than 30 degrees from vertical.
    Last edited: Jul 15, 2013
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    What is the problem with the hose running vertical? It seems like the shortest run, which is good.
  4. kenJ
    Joined: Jul 2005
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    Location: Williamsburg, VA

    kenJ Senior Member

    I would put it on the top near the front of the tank. Perhaps you can get some cross flow of air between the two vents which will help. The important think is to keep the vent open, if it is on the side, the chance of getting plugged is greater.
  5. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    I have vents in 1" pipe running from the port side close to the top, they run at 45 degrees through lockers to the hullside and then up to a thru hull just below the deck level. And I use the large activated charcoal filters in the lockers. I've never had them block nor have I ever smelled anything from outside. Iv'e two tanks one fwd and one aft.

    You have to consider the max head rating of the tank if there's a non return valve in and you pump seawater in to flush it you could damage the tank as the top of the vent will be the max head.

  6. dskira

    dskira Previous Member

    I put a 3" pipe straight from the holding tank to the top of the pilot house. I put a vent cap on top for the rain and I never had any problem of odor even with a crew who had a bad diarrhea problem during a long cruise. Sorry for the detail.
    Bigger the vent, less odor you will have.
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