Fresh Water Tank Continually Drains

Hi, I am new to Rving so bear with me.  I have searched your site and did not find a reference to my particular problem, but maybe I missed it.  I overfilled my fresh water tank recently and now it leaks continually out of the overflow pipe under the coach.  So I read that it might be siphoning so I allowed it to drain all the way out and then filled it again.  As soon as I start filling it, it starts draining out and continues draining until it is empty.  Now it has started draining out the low point drain as well, and that drain is in the closed position.
Any ideas would be helpful.  This is a new coach and still under warranty.

Hi Rick, welcome to The RV Forum!

If it's under warranty, your best bet is to take it back to the dealer and let him fix it.  Make sure he shows you what went wrong so you'll be able to deal with it yourself if it happens again.

Lou, thanks for the quick reply, I thought that might be what you would say.  The problem is my dealer, they are a long way off and I have to leave the coach there while they scratch their heads and try to figure out what's wrong.  The service they have given me in the past has been much less than satisfactory.  I would rather fix it myself, (I am kind of a hands on guy), but don't understand what could be going on.  I may try a different dealer than the one where I bought the coach.  Thanks for your help.

Contact the manufacturer and explain your situation with your dealer. They may authorize you to call in a mobile repair guy and they would foot the bill.

A leaky low-point drain is just plain defective, unless it is the type that is a simple threaded cap and has come loose.

The siphoning is strange - I not heard of any that didn't stop after the tank is partially empty.  I'm wondering if that is actually an overflow, or something else is leaking.  It is conceivable that something broke under the pressure when the tank was overfilled and now water is leaking out. It may be running along the overflow of tank drain hose without actually running through it.

Plumbing leaks are usually considered an emergency repair, so you might contact the manufacturer and see if they will authorize someone nearby to at least investigate.

Welcome to the Forum!

Sounds like a defect, maybe a cracked tank.  Contact the manufacturer.

And, no, I do not remember this specific issue on here before.

Thanks guys, for all the input.  I think I will take your advice and contact the manufacturer.
Cheers, and Happy RVing!

I hate to say it...but it SOUNDS like the tank drain valve is open...or at least not completely closed.

(let's hope it's something this simple)

Our low point drains are capped with pex caps. I almost lost the gasket inserts. I did not realize there was a gasket as I was not familiar with the pex-type plumbing. One had fallen off in the drive-way and I was lucky to notice it. Without these, I suspect the caps will not seal by themselves. The gaskets like to stick in the end of the pipe when the caps are removed.  We have also had the siphon thing happen.  Our TT has a single water connection with a valve to turn for "city water" and "tank fill." I help prevent siphoning by returning the valve to the "city water" position after I equalize any pressures on both sides of the valve by pushing in on the inline check valve at the fill connector after disconnecting the hose and rotating the valve through both positions a couple of times. Stand to the side as you will get a squirt of water. I believe the siphon thing begins when you fill past the level of the overflow hose that is in the tank. This will pressurize the air space above the water level. A little movement as you start to travel down the road enhanced with some pressurized air at the top of the tank will get it started. If you leave it in the tank fill position it will allow air to enter the tank through the fill connection that will contribute to the siphoning. If I see evidence of water spilling out the overflow at my first safety check stop, I repeat the check-valve valve position sequence. Not sure but I may have picked up some of this in the manual or on here in another post. The other thing that can help is not overfill the tank. Shut the water off as soon as you start to see water beginning to come out the overflow. I am a bit of a "if some is good, more is better" kind of guy and that may not b the best approach with the fresh water tank!

You said 'Overflow pipe UNDER the coach"

You have a tank drain line that is UNDER the tank.. also low point drains in the water system. All should have valves or plugs/caps.. Sounds like the tank drain valve line is open to me. And if it is a "Cap/Plug" type then the dealer owes you a part.

How do you fill the tank?  Pressure fill systems send water from the city water port thru a valve and then to the tank.  Gravity fill systems simply fill thru a port, after removing a cap, using a garden hose or a container.

If you are using a pressure fill system and the venting is inadequate, the tank may burst.  That "overflow" in a pressure fill tank is the vent and some will wrongly plug it to prevent water loss.  A kink or other obstruction can cause a pressurization of the tank, so many use two vents.

There is no such thing as a siphon.  In order to have a siphon, the offending pipe would need to dip down to the bottom of the tank in order to empty it.  That word has been used often to describe water loss and it is not what is happening.

There is no such thing as a siphon.  In order to have a siphon, the offending pipe would need to dip down to the bottom of the tank in order to empty it.  That word has been used often to describe water loss and it is not what is happening.

Click to expand...

I'll have to partially disagree with that. The only thing needed to create a siphon action is for the tank intake end of the hose be below the water surface and the outer exit end a bit lower than the intake end.  The siphon action will occure naturally whenever that is the case.  An overflow siphon is rather common in RV fresh tanks when overfilled, but they rarely empty the tank completely, for the reason lynnmor cited.

Since the siphon-action cannot drain the tank any lower than the end of the intake hose, I suspect the problem here is NOT a siphon via the vent/overflow hose.  Typically the overflow/vent is simply a port in the upper side or top of the tank. Some tanks use the same hose for vent and overflow, while others have separate ones.