Negative pressure in water system

[egaito]

I've got a puzzling fresh water issue I thought I would run by the group....

I've been noticing that when I remove the cap to fill my water tank that there is a bit of suction that is released.

The vent line is clear, as water easily flows out of it when filling the tank. I'm not sure what else to check.....

Any ideas?

Thanks,

-Ed G.

 

[SKYCHENEY]

Does the vent line have a check valve in it? I don't know why anyone would install one here, but if they did, it would allow the water to come out, but it wouldn't let any air back in thus causing the vacuum in the tank. Just a thought.

Another idea: Do you have more than one tank manifolded together? If so, maybe the vent from the second tank is open, but the vent from the first tank(the one that you fill) is clogged.

 

[egaito]

Does the vent line have a check valve in it? I don't know why anyone would install one here, but if they did, it would allow the water to come out, but it wouldn't let any air back in thus causing the vacuum in the tank. Just a thought.

I checked for this, as the vent is kind of close to the water line, but didn't see one.

I'm wondering if there's a loop in the line that may be holding water, acting as a sort of check valve...that's the next check I think.

Another idea: Do you have more than one tank manifolded together? If so, maybe the vent from the second tank is open, but the vent from the first tank(the one that you fill) is clogged.

Just a single center tank I think on the 41C, unless someone knows different. The PO installed decking above the tank, so I've never seen the top other than where the fittings are.

Good thoughts, thanks for the reply.

-Ed G.

 

[Will]

I think your symptoms are probably a good sign that the cap seal is working properly and you have no leaks in your cooling system. The coolant expands when the engine warms up causing excess coolant or air to go out through the overflow line. When things cool down again, the coolant contracts resulting in a vacuum unless the seal allows air to leak back into the system.

Will

 

[egaito]

I think your symptoms are probably a good sign that the cap seal is working properly and you have no leaks in your cooling system. The coolant expands when the engine warms up causing excess coolant or air to go out through the overflow line. When things cool down again, the coolant contracts resulting in a vacuum unless the seal allows air to leak back into the system.

Will

The issue is with the fresh water tank, not the cooling system.

 

[trogers]

I had a similar issue on my previous boat. On that boat the water tanks, 4 of them had overflow valves plumbed to the transom. Small stainless 'buttons' that permitted the tank, when being filled to overflow, or when the fresh water was used for air to flow into the tank.

One time a "mud dauber" started a home in the 'button' on the transom. When the vent was cleaned with a piece of wire, the water flowed more easily and the vacuum created in the tank was negated.

Sounds easy to descrive in two paragraphs. It was a real bear troubleshooting. There has to be a breather vent on the tank. On my last boat, small and subject to dirt build up.

Given the brevity of my Hatteras experience, a whole three weeks now..... I'm certainly no expert, but I'd look for the vent and seek if it is clear.

 

[Trojan]

The only way you can have a vacuum on the tank fill tube and have an open vent line. Is the the fill line is extended into the tank and into the water. Or the fill might be on the side of the tank. Once filled as long as the fill cap is tight there is a vacuum on the fill line. It is like the trick of putting your finger over the end of a straw and removing it from a drink and the vacuum traps the drink up in the straw. Release your finger (CAP)and the vacuum is gone. If the vent is working as you say. There can not be a vacuum in the tank itself or you would not be able to draw any water.



BILL

 

[SKYCHENEY]

Or....how about the vent line being under water? That is that instead of having a vent that is attached to a hole in the top of the tank, it is instead hooked to a drop pipe which extends under the water level in the tank. This could also result in a vacuum at the fill pipe. I don't know why it would be like this, but it is another explanation.

 

[Eufaula]

It has to be in the vent system, maybe the pump installed is overpowering the vent, might be too small for the pump or like some one mentioned vent should be on top of the tank and terminate just below the top of the tank. James

 

[Trojan]

He said the vent works when he fills the tank. If the pump isn't running the vent should equalize the pressure. Weird. Something is being left out.



BILL

 

[Eufaula]

As mentioned the water flows when filling the tank. Yes, it will still flow if it is below the water level due to the filling press. It has to be in the vent system not letting the pressure equalize in the tank when the pump cycles.

 

[SKYCHENEY]

Okay... It just came to me. Here is what might be happening: The hose is old and flexible. With the pressure of the water filling the tank the vent hose expands to it's normal size and water flows out. When you pump water out of the tank with your domestic water pump, it collapses the vent hose onto itself and a vacuum builds. It may be that this happens at a bend in the hose. Have you looked at the top of the tank where the vent hose hooks on? Look there and follow that hose all the way to where it exits out the side of the hull. Do this after pumping a bunch of water out of the tank and see if you can find a collapsed section. Good luck and let us know what you find.

 

[egaito]

This is certainly an interesting thought.....the hose is heavy stuff, and "only" 10 years old or so, so I don't think it has collapsed, BUT, it may be kinked, with the kink being pushed open by water pressure when overfilling the tank, and sucked closed when the tank is drawn upon...

Thanks for all of the good ideas, I will let everyone know what the outcome is.

-Ed

Okay... It just came to me. Here is what might be happening: The hose is old and flexible. With the pressure of the water filling the tank the vent hose expands to it's normal size and water flows out. When you pump water out of the tank with your domestic water pump, it collapses the vent hose onto itself and a vacuum builds. It may be that this happens at a bend in the hose. Have you looked at the top of the tank where the vent hose hooks on? Look there and follow that hose all the way to where it exits out the side of the hull. Do this after pumping a bunch of water out of the tank and see if you can find a collapsed section. Good luck and let us know what you find.