Fresh Water Tank Siphoning

Another guest post by Lee!


One of the odd little problems we have experienced with our rig is that when we fill the fresh water tank, water pours out of the overflow pex lines that stick out of the underbelly when it is full. Then when we turn off the fill water, the fresh water continues to flow from those pex lines and will continue until it stops, or until the fresh water tank is empty. Keep in mind this is not related to a full tank splashing water out while driving, it’s ONLY when filling the tank. Neat feature, huh? I poked around on the internet, and after getting distracted for hours by other unrelated but equally cool stuff, I have determined that it’s one of several problems. So let’s take a look at a typical fresh water tank so we know what we’re talking about! Here’s a picture.

As you can see, there is an inlet for filling with water, an outlet to the pump, a low point drain, and a vent. The vent allows air into the system when in use, otherwise a vacuum would exist, and the tank would collapse. When you fill the tank, and water reaches the vent level, it runs out the pex line at the vent.  Because the line opening is below the water level, it creates a siphoning effect which will continue until something breaks that effect, or prevents water from flowing out. This seems to be a common problem and there are lots of forum discussions about this issue, with various recommended solutions.  We first experienced it back in Quartzsite in 2015 and I installed a couple of push-on type quarter turn valves, similar to what you would see behind a toilet in a stick and bricks home. That allowed me to manually stop the water flow until the water level was low enough to break the siphon effect and I could reopen the valves.

UPDATE: A couple of folks have indicated they’d like a better explanation on how this phenomenon happens, so I am going to try it from a different angle. To begin with, in my attempt to locate a decent picture of a fresh water tank with labels for the various inlets and outlets, I chose one that doesn’t look like most fresh water tanks, and also doesn’t show the fact that the overflow line isn’t just attached to the tank, but actually goes inside the tank, and are often improperly installed or poorly designed so that they extend below the level of the fitting, sometimes by several inches. So there’s a more accurate representation of the tank itself, below this paragraph. In this picture, the 100 gallon tank is 81″ x 26″, and only 11″ tall. This is a crucial factor, because the vent hose goes in on the side, so it’s already BELOW the top of the water level if the tank is completely full. That’s how the siphoning gets started. Then, if the hose is improperly installed or poorly designed, and extends 3″ down, it’s maybe 4″ below the water line, which will allow 1/3 of the capacity to be siphoned out. Add to that the vacuum effect, because it’s a closed system, and it can pull down the center of the tank, which helps keep the siphoning going. Hopefully that makes more sense. The problem with my fix was that the valves were designed to sit in a house, behind a toilet, not bounce down the road, so the little handles fell off immediately. I bought another valve just for the handle, and kept it in my convenience compartment and just pushed it on when I needed to open or close the valves, and then took it off again and stored it. The problem with that fix was I could never remember if they were opened or closed, so I took to sticking a pen in the valve, and if it didn’t go far, I knew the valve was closed. If it slid in more than an inch, the valve was open. I’ve been doing this for over three years now. It’s funny what kind of weird things you get used to, even if they make no sense. Part of that is that we don’t boondock much, so we rarely use water from that tank.

When we got our Mor-Ryde suspension back in November my solution no longer worked. The pex lines come out of the underbelly, between the two axles, and I used to be able to just get down on one knee and reach under to them.  They were just behind the forward tire.


Now with the Mor-Ryde, the only way to get to them is to actually crawl underneath the rig on my back or side and reach up and around the new suspension components. Not ideal. Here’s a look at the view from underneath, and as you can see, the new suspension completely blocks the valves, which are the white plastic pieces attached to the red pex lines. The large mass of black steel to the right is where I used to be able to reach through.

And here’s a closer shot so you can see what I was using. The handles were not on, and I didn’t feel like crawling out to get them just for the picture. If he was really committed to this post he would have done that…just kidding hehehe – Trace

So, what I decided to do was extend the pex lines back to where the convenience center is, behind both sets of tires, and over to the driver’s side of the rig, so I could just reach down and open and close them easily. First I cut off the valves…


Then I added elbows and 5′ pex lines.


Where the pex lines crossed anything that could rub at them while in motion, I used a scrap of thin rubber to protect them, and zipped them together so they wouldn’t rub against each other or flap around in the wind.


Unfortunately, they didn’t reach to exactly where I wanted, but I didn’t want to buy 10′ of pex to use 7′, so I decided to live with the almost perfect location,  near the convenience center. I cut them back a little so I didn’t have to deal with the bracket for the rear jack, and offset them so I could easily reach the one on the inside.

Then it was just a matter of pushing on the new valves….

And wrapping and anchoring them.

Now they’re easy to reach, and I can open and close them with one hand.  I can also see from the position of the valve handle if they are open or closed. The whole process took about an hour, and cost around $36.00 with the parts I was able to buy at Home Depot.

ANOTHER UPDATE: I should have pointed out that this is NOT the best solution to this problem. It started out as a quick fix, and now I have just relocated my fix so I don’t have to crawl under the rig. If the valves are CLOSED and I fill up without opening them to allow the water to overflow, then I can rupture a fitting somewhere in the system, or my tank. If I forget to open the valves while using the pump, I can collapse my tank. The BEST solution is to have the vent higher than the top of the tank. However, if the siphoning gets started, that won’t solve the problem either, because of the vacuum. Some people have solved this problem by having the vent outlet higher than the tank, but of course then you have water spilling over inside the underbelly, which is not something I want. I’m also partly responsible for this, because I am using the fact that water starts flowing out as my “full” indicator. Otherwise I would have to keep running inside to check the water level indicator on the panel, and I don’t want to do that. For one thing, my panel only tells me water level in thirds, so “FULL” isn’t necessarily “FULL”. Also, I’d have to drop the stairs down and open the door, plus there’s all that running back and forth, which sounds dangerously close to exercise. In the past I have used my inline water meter to see how many gallons I have put in, but that only helps if I know I am starting with a completely empty tank, which I’m usually not. I think the real solution is a See Level system, with a remote display at the convenience center. Because it gives an extremely accurate reading, I can see what percentage the over flow begins, and the next time stop a few percent below that. And I also don’t mind remembering to open the valves, because to me the whole point is to be able to cram as much water as possible into the tank. 


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14 thoughts on “Fresh Water Tank Siphoning

  1. Lee,
    This is a great idea. I just finished crawling under rig to drain the freshwater tank. I also have the Mor-Ryde system. I already have the pex so I just need a couple fittings to route to a better location.

    • James – The a similar problem of not being able to reach applies to the fresh water drain valve, along with the fact that I hate that it just dumps water straight down to the ground underneath the rig. The next project is to extend that pipe off to the side, so I can put a fitting and either a gray/black water hose or an adapter for a garden hose on it and run it away from the rig to drain, as well as relocate the valve. Crawling under the rig is not something I enjoy doing. – Lee

  2. OK. Got me on this one. Not sure I understand what you are saying. “Then when we turn off the fill water, the fresh water continues to flow from those pex lines and will continue until it stops, or until the fresh water tank is empty.”

    Your drawing does not show any tubes or lines inside the tank. That would be required for a siphon.

    Which Pex lines? The vent line? if the vent line is an opening in the top of the tank, and there is no tube that drops down into the tank, how the heck does it siphon??? Our fresh tank has a vent at the top, It is simply a tube that attaches to a hole at the top of the tank, and then the tube drops down to below the coach. There is nothing inside the tank.

    Now, if I am not level, or if the fresh water slushes around when we drive, some will spill out of the vent tube. But there is no way it can suck the tank dry. That would require a tube all the way to the bottom of the tank, inside the tank.

    Can you help me understand.

    • Craig – Thanks for pointing that out. I’ve updated the post to address your questions. As for this phenomenon allowing a tank to be completely emptied, that hasn’t happened to me because I always stopped it before too much water drained, out of fear of the vacuum pressure damaging the tank. But in the many stories I’ve read about this, some people have hoses that do indeed go all the way to the bottom of the tank, which would allow them to empty completely. Another thing to keep in mind is that this problem as described is different from the spilling people experience from driving. This is exclusively a problem when filling the tank. – Lee

  3. Why not just blow a little bit of air into the vent outlet? That way any water that is in the vent line after filling is pushed back into the tank and the siphon can’t start. As long as there is a point in the vent line that is higher than the fill tube and the vent line downstream from that high point is dry, the siphon can’t start.

    Maybe I’m not seeing it clearly.

    • Russ – That makes sense, but keep in mind that this occurs once the tank is full, so water would already be flowing out of the tubes. If I don’t fill the tank completely, then I’m defeating the point of having a fresh water tank because the only way to know it’s full is when water runs out of those overflow/vent tubes. I can’t trust the indicators on the console because right after I did this mod I filled the tank, while constantly checking the console. Once it indicated “full” I zeroed out my inline water meter and put in another 32 gallons (!) before water started to flow from the vent tubes. So that’s no good. And the siphon action is happening because the beginning of the vent tube is below the top of the tank, which is either a design flaw, or an incorrect installation, so there’s really no way to both fill up the tank AND prevent the siphoning. Either way it’s a much bigger job to correct it than this solution. The next step is a SeeLevel system that will actually accurately tell me the status of all three of my tanks, which will be better than the “shot in the dark” system I currently use to manage the gray and black tanks, as well as help me to fill the fresh tank to a point before the siphoning starts, and just in general it will be nice to know to what percentage any of the tanks are full within a few percentage points. But that’s a little down the road. – Lee

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  7. Lee, you might try finding which of the two overflows is the siphon-er. (On my OR, it is just one of the two). Put the valve JUST ON that one line, so you still have one vent/ overflow and you will be less likely to be pressurizing the tank.

  8. FIXED– Update, 2016 OR Light 319rls. Mine has a white and a red overflow. Mine would siphon off about 1/3 of a tank out of the red pipe if filled. I tried inserting a small (8mm) tube up inside the red pipe (into the peak of the red tube) to act as a vacuum breaker but it did not work, even when it was longer than the bottom of the red pipe end.

    I can’t recommend this for everyone yet, but here’s what stopped the siphoning—–
    I took about 18 inch length of 4mm drill rod ( a straight stiff wire) and pushed it up to the peak of the red pipe, then put a cordless drill on the bottom end of the wire and used it to drill a small hole in the peak ( elbow?) of the red pipe, to let in enough air to break siphons.
    It worked, and without cutting open the belly cover. I doubt the small hole will ever leak , since any water in the red pipe will fall out the bottom of the pipe due to gravity ( the end of the red pipe is open) and only air should ever go into the small hole. I previously had installed a valve on JUST my RED pipe, like you did, but I forgot to open it during a fill up, and I got worried I might bust the tank, even with the white hose being left open. I don’t think one vent will be enough to prevent tank pressure buildup.

    I also ran a cable release ( like used on the gray tank) to the fresh dump valve, so now I don’t need to crawl under to dump, just pull the handle. If I was doing this over – I would just run a brass welding rod through a piece of PEX pipe to this valve, since it needs to be PUSHed closed.

  9. I just had the same problem and came up with a simpler solution. Put in a T after the drain port and run a tube from it above the tank. This stops the siphon and relives the vacuum in the tank. It also doesn’t need manual intervention.

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