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How to Fix RV Fresh Water Tank Siphoning

RV Fresh Water Tank Siphoning Fix

Have you ever filled up your RV’s fresh water tank, hit the road, and arrived at your campsite only to discover that half your water has mysteriously vanished? So, you top it off again—maybe a little too much this time—and water starts pouring out of the overflow lines. Then, by the time you park, you’re back to half a tank… again.

Sound familiar?

If so, you’ve experienced the frustrating issue of RV fresh water tank siphoning. It’s a common problem, and luckily, there’s a fix. In this post, I’ll walk you through what causes it and how to stop it from draining your precious fresh water.


What Causes the Siphoning Effect?


To understand the fix, we first need to look at what’s going on.

Most RV fresh water tanks are equipped with overflow lines that sit at the top of the tank and run down the side of the RV to exit underneath. These lines are designed to prevent overfilling—but they can unintentionally become the culprit behind water loss.

Here’s what happens:

  • You fill your tank and either overfill it or drive through hilly terrain.

  • Water begins to exit through the overflow lines.

  • Since these lines terminate below the tank’s fill level, gravity takes over.

  • As the water flows down the lines, it creates a vacuum behind it.

  • This vacuum starts to siphon water out of your tank, continuously pulling it out—even if you stop driving.

The worst part? Most RV tanks only have those overflow lines to vent air, so there's nothing to break the siphon. The water keeps draining until the vacuum is broken, which often doesn’t happen until the tank has collapsed enough that air is pulled back into the tank to equalize the pressure.


Why You Shouldn’t Just Put Valves on the Overflow Tubes


When RV owners run into the siphoning issue, the first instinct is usually to plug the overflow lines. A quick scroll through RV forums or Social Media groups will turn up plenty of advice to just slap on a shutoff valve and call it a day.

But here’s the deal: I do not recommend that approach. In fact, blocking those lines can lead to catastrophic damage to your RV’s fresh water system.

Let’s break down why.


Overflow Lines Serve a Critical Purpose

Those overflow lines aren’t just there to let out excess water—they actually serve a dual purpose:

  1. Preventing Vacuum Damage: When you run your RV’s water pump, it creates a vacuum inside the fresh water tank. The overflow lines let air back in, helping prevent the tank from collapsing or damaging the pump.

  2. Releasing Pressure During Filling: When you fill the tank, pressure builds up. The overflow lines allow that pressure to escape—protecting your tank from bursting.

If you block those lines with a shutoff valve, you remove the only way for your system to equalize internal pressure. That’s a disaster waiting to happen.


It Only Takes One Mistake

Even if you promise yourself you’ll always remember to open or close the valve, the risk just isn’t worth it. Travel days are chaotic. Maybe you forget. Maybe your spouse thinks you already opened the valve and starts the water pump.

That one mistake could cost you a cracked tank or a burned-out water pump. Either way, your relaxing getaway just turned into a costly repair job.


My Simple Fix for RV Fresh Water Tank Siphoning


When I ran into the siphoning issue on my RV, I knew I didn’t want to just slap a valve on the overflow lines. That felt like a band-aid solution—and a risky one at that.

Instead, I took a more reliable and permanent approach—just like how it’s done in home plumbing systems. I added a way for air to enter the tank above the water line to break the vacuum and stop the siphon effect. It was cheap, easy, and it’s worked perfectly ever since.

Honestly, I’m still wondering why RV manufacturers haven’t tackled this yet—it’s such a simple, common-sense fix.



Step 1: Access the Overflow Fittings

First, I cut an access opening to reach the two fittings where the overflow lines connect to the tank. Once inside, I looked for a spot where I could route an additional tube that would terminate above the tank’s water level. In my case, I found the perfect spot right over the frame rail. If that hadn’t worked, my backup plan was to run it through the floor behind a cabinet.

Step 2: Install a Tee Fitting

Next, I removed the hose from one of the overflow lines and installed a ½-inch nylon hose barb tee. That gave me an extra outlet to add a new line. I connected a ½-inch tube and started routing it higher than the top of the tank.

Step 3: Route the New Vent Line

I pushed the tubing over the frame rail and added a ½-inch nylon barb elbow to direct any potential water discharge downward. Since the outlet was already above the water line, I didn’t need a check valve—but if you plan to route it inside the RV (say, behind a cabinet), adding a check valve is a good idea to prevent any water from escaping indoors.


Problem Solved—No More Siphoning

Now, when I fill the tank or hit the road, water can still escape from the overflow lines—but it won’t siphon out. If a siphon starts to form, air enters through the new vent line I installed and breaks the vacuum before it can drain my tank.

Best of all? No more crawling under the trailer to mess with valves. The system just works—and I haven’t had a single siphoning issue since.


Parts List with Amazon Links

 

Click the in-text links to see the products on Amazon

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