Water tank vent

[Phil Nersessian]

I am having issues with the venting of my water tank. There are two hoses that go into the top of my tank side. one is about 2" wide and is obviously the fill line. There is another 1" clear w/ red and blue stripes, is this the vent line? Upon leaving the tank it drops down to the bottom of the compartment, if it is the vent I would think it would get blocked with water creating my issue.Thanks in advance.

 

[Connerkip]

Yes it sounds like the vent. On mine, it dipped down slightly and standing water in the line caused both ballooning and collapsing of the tank, depending on temperature and water use. I removed the refrigerator and improved the routing to allow a slight grade on the vent line. All is good now. No more air whooshing in when the fill cap is taken off.

To test to be sure it is the vent, take it off at the tank and blow through it. You'll know it's the vent when air comes out the vent port on the outside of the boat. This line comes out near the top of the tank. There is also a line feeding the water to the pump, but this comes off near the bottom of the tank.

 

[Todd]

This issue has come up before, you can search old posts. Filling was not a problem, when full water would come out vent but when using the water my tank would collapse to a point that the level gage would not work and I thought the repeated flexing of the tank would eventually cause it to crack. I could not reroute the vent on my 29 easily so I installed a small swing check on the top of the tank and connected it with 1/4" poly tubing to a fitting I installed on the side of the tank near the top corner. The only point I could easily access. The check lets air in and will not let water out. If it failed then a small amount of water might leak into bilge, no real problem. The flexing has stoped and the gage works so problem solved.

 

[Phil Nersessian]

Thanks Connerkip and Todd. My main concern is if the 1" clear line is, in fact, the vent. If it is, I will reroute it to take out the dip, but I have guessed wrong on other hoses and paid the consequences(trying to avoid a repeat of prior mistakes!). My concern with using a check valve is it potentially not letting air out.

 

[Connerkip]

Your concern of letting air out is valid. When my tank was near full after filling with cool
water, the next day in hot Florida, I noticed the tank was ballooned. Small pressures over large surface areas can result in a lot of force surprisingly. This was prior to my rerouting the vent hose.

 

[knotflying]

Here is a little twist on things. It is my understanding that thru hull lines like vents and bilge pumps etc. are supposed to have a loop in them to prevent outside water from just coming in when heeled over from large seas. I am not sure if height off the water line dictates when. With that said, if the vent line has a loop and it is below the fill line, when I fill my water and the tank is full I get a trickle of water running out of the vent. Therefore, if there is a loop, there is a possibility of water laying in the loop and blocking air from coming into the vent until pressure is strong enough to push it back down. I would also think that in the absence of a loop there should be a check valve to prevent sea water from backing down the vent into the freshwater. Perhaps someone can shed some light on my theory.

 

[Phil Nersessian]

Good thought Knotflying, Thinking maybe a high loop UP (inside the cabinets?) instead of down might accomplish this. Similar to a "bow vent" in plumbing terminology. Still would like to confirm that the 1" clear line is indeed the vent.

 

[Todd]

When filling the water tank I do not make a hard connection to the fill hose so venting during fill is not a problem. I will vent through the fill pipe. The vent is lower than the top of the filler cap so when full Water will come out the vent which tells me it is not plugged totally but the trapped water will not let air into the tank. The check valve was the easiest solution for my 29 as a reroute did not seam possible. The tank is left of center and the vent on starboard side so is has to dip down under flooring then back up.

Don't think the vent was 1" but I can't see it without removing cave wall.

 

[NorthernFocus]

Here is a little twist on things. It is my understanding that thru hull lines like vents and bilge pumps etc. are supposed to have a loop in them to prevent outside water from just coming in ...

Presumably when you say "loop" you are referring to a riser? Having a proper riser in a vent line has no negative effect(however it does defeat the vent acting as a "tell-tail" when filling). Having a low spot in a vent impairs (or defeats) its function. A literal loop can be irrlevant or can be bad depending on how it is configured. A loop that is above the run of the hose has no effect. If the loop is below the run it is effectively a p-trap and will allow pressure equivalent to the diameter of the loop to build in the tank.

All that being said, if you look up either CG regs or ABYC standards I think you'll find that the requirement/need for and height of a riser is dependent on how close the though hull is to the WL. Based on said proximity to WL, risers are not required in the vents for water and fuel tanks on most boats.

 

[ronnieg]

good morning Phil,
you are correct the 1/2" line coming out of the top of the tank is the vent line. if possible please route it so there is no low spot in the vent line this will help the tank vent and save you headaches on the water gauge working correctly. Please feel free to contact me with any questions

 

[R Baker]

I had the Same problem on my 2012 29. I removed the stove and tried to reroute the line but could not; nor could I reach far enough to reposition it. This subject has come up before. This is pretty basic boat plumbing that the factory cannot seem to get right. Are more recent boats still experiencing this problem ?