Air vent from main water tank seems clogged

JeffMac

JeffMac

Member
Nov 6, 2018
41
Seattle, WA
Boat Info
320 Sundancer, 2004, twin v-drives
Engines
350 Mag 300 hp inboard
320 DA air vent coming from mid berth water tank area is not working. Seems clogged. Any ideas?
 
When I fill my water tank from the fill cap outside, my water tank swells up and expands and nothing comes out of the water vent.
 
JeffMac said:
When I fill my water tank from the fill cap outside, my water tank swells up and expands and nothing comes out of the water vent.
Click to expand...
I have taken apart the vent hose and blown on it and flushed water through it from both sides and it’s still not draining.
 
The problem seems to be that water and air can enter the vent towards the tank, but not the other (correct) direction. Seems like someone installed a one way flapper valve or something in the wrong direction from the vent connection into the water tank.
 
It should be venting both directions so that it needs to expand and contract with different temps.

maybe you have a delaminated hose, or mud wasp nest acting like a check valve
 
You didn't bother to give your location, so this may not help you, but it is common for dirt-dobbers to build nests in the vent line outlet. Try cleaning the outlet fitting and I suspect you will find your problem.
 
Sublimetime said:
It should be venting both directions so that it needs to expand and contract with different temps.

maybe you have a delaminated hose, or mud wasp nest acting like a check valve
Click to expand...

This was my my thinking, including expansion and contraction while filling and when using.

MM
 
Something doesn't sound right, he says his tank 'swells up and expands'. That can't happen from filling it, unless the tank is partially collapsed already by the water pump when you use water and the tank vent isn't breathing.
 
Woody said:
Something doesn't sound right, he says his tank 'swells up and expands'. That can't happen from filling it, unless the tank is partially collapsed already by the water pump when you use water and the tank vent isn't breathing.
Click to expand...

Why wouldn't a tank expand if water is being put in and the air is not venting? Where would the pressure go? My fresh tank is 5 feet long, the center of the top and sides can certainly pop up an inch or so. That is what I perceive the OP is saying.

MM
 
When my potable water vent hose was clogged with dirt daubers I could not efficiently fill the tank because it would spew water out of the filler neck. So something is definitely going sideways if the tank is expanding while filling.
 
I filled my tank and watched it swell before venting. It think the top of the tank lifted at least an inch when full, then it began venting through the overflow. I was putting pressure on it by filling looking for leaks. Air compresses, water does not. (Stay away physicists)

OP, are you sure youre filling it all the way? Water is backing out through the fill cap I assume.

Might be something inside the tank. Something floating near the vent connection. Possibly a water level float that broke apart. There arent check valves or anything on that line.
 
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MonacoMike said:
Why wouldn't a tank expand if water is being put in and the air is not venting? Where would the pressure go? My fresh tank is 5 feet long, the center of the top and sides can certainly pop up an inch or so. That is what I perceive the OP is saying.

MM
Click to expand...
I'm wondering why there'd be any pressure. It's not like your screwing the water hose into the fill hole and putting 60lbs of city water pressure on the tank. But even if you did, when you disconnected the hose wouldn't the pressure return to normal, the fill hose doesn't have a check valve in it, I don't think.
 
If the water tank is completely full, the water, itself, may be clogging (for lack of a better term) the full hose. Also, if either of the two hoses happen to have a low spot (or a long horizontal run), they can gather standing water that way (creating a "clog"). I'm not sure a vent hose could all of the sudden get kinked, but that's another possibility - unless you or a previous owner has replaced the vent line and didn't route it properly.

Jeff, when you said you "flushed water through it from both sides", can you better explain that? Did you flush through the hull vent? The most common thing to happen, along the lines of what's been mentioned, is for the hull vent to get clogged (caked up wax is another common reason). Also as mentioned, there is no type of one-way valve built into any of the hoses (although a delaminating hose could certainly cause that... but rare).

If you've verified that you can get air/water through the hoses in both directions (your "flushing), as you said, then there is no one-way valve installed by some prior owner.

Verify that the pathway from the fill/vent is completely clear to the tank fittings. If that is good, then the issue could be with the actual tank fitting.
 
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Agree with Woody here and I'd like to add that while adding water to a unvented space the water in the fill hose would just erupt back out.

If the water is coming out the overflow, which is the same on my boat as the vent. The vent sounds to be somewhat blocked. Put some soapy water in the vent and see if air is escaping while water is being added.

Spiders or mud dauber would be my guess. Grandfathers pipe cleaners will remove the spider webs.
 
Im just explaining my understanding from what I saw when I was working on a leak discovery.

Hose pressure is fairly significant. The 1” hose pressure is greater than your 1/2” tank overflow relief hose allows. This will cause your wayer tank to expand as the pressure inside increases and the volume of air is low. I keep filling until my overflow triggers, but by this time the water tank has taken a little more water than it wants. (Its just a piece of plastic and has some give.). When you stop filling the with the hose, the tank pressure now pushes back at the overflow vent AND the hose fill port until the pressure is equal at all 3 places...top of tank, hose fill, and overfill vent.

If you are filling your tank the air has to go somewhere. Are you getting air pressure at you overflow vent? Is your hose burping?
 
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