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All About The RV Fresh Water Tank Overflow Valve

There are all sorts of little features in your RV that might not have expected or didn’t really know about. Many of these features come from your RV operating differently than your home, especially within it’s water and septic system. Your RV could have innumerable other features that are specific to it’s use as a mobile home.

Your RV’s septic and water system is certainly one unique thing about how to live in an RV. Water is relatively easy to add, use and drain, but has other functions beyond a typical home system.

One feature is the RV fresh water tank overflow valve. This valve ensures you have the right amount of pressure in your RV fresh water tank. We will discuss this unique valve in detail below along with all the purposes it serves.

What is the RV fresh water tank overflow valve?

Let’s start with a little about your RV’s fresh water system. Most RVs have three tanks: A gray tank for sink and shower wastewater, a black tank for toilet waste, and a freshwater tank for storing water to use in the toilet and other faucets. The freshwater tank can be filled externally, generally with a hose or with the water lines from an RV park.

The tanks themselves are sensitive to pressure and are designed to hold an amount of water. The overflow valve is part of a relief valve for the fresh water tank.

Drain Excess Water

It’s really quite easy to overflow your fresh water tank. The hose itself nor water system have any idea how full your fresh water tank is, and most RV owners might be doing something else while filling their tanks.

The RV overflow valve is a tube with a mesh screen that allows water to come in when the tank is completely full. The freshwater tank is designed to hold a certain amount of pressure, and anything in excess of that pressure can cause the tank’s structure to get damaged.

So, if water is flowing out of the overflow valve when filling it, it’s time to stop.

Drain Excess Air

Water and air both create pressure. When you are filling up your water tank, the water starts to move the air in the tank around, and it has to go somewhere. The extra oxygen can freely slow out of the overflow valve.

This is another situation in which excess pressure between water and air content can damage the tank and potentially rupture it. A ruptured tank will likely lose all your fresh water and likely result in needing to purchase and install a new fresh water tank.

Adding water

During your experience using an RV, you might come across a time when you have issues with the small pieces of equipment that get water into your RV. For example, you might lose your water fill hose, have an attachment break, or have other issues filling your tanks.

The overflow valve has access to your tank. Simply popping the valve cover out and placing a funnel in it can allow you to add water from containers, hoses, or whatever you are using. These can be a huge help if these issues occur when you are nowhere near a place to get the hardware to fix water problems.

You’ll get the same immediate feedback from your fresh water tank if you fill via the overflow valve. The water will simply stop going in.

Where is the overflow valve?

Many RVs have a set of water line related tools on an access panel on the tank side of the RV. The overflow valve is usually near here. You’ll normally see a circle with an aluminum mesh cover. Your first discovery of the overflow valve might be the first time filling the water tank – there is likely to be water coming out as you complete your first water fill.

Do any other tanks have an overflow valve?

No, just the fresh water. To be fair, you probably wouldn’t want either the black tank or the gray tank to have an overflow valve.

Can I add a water overflow valve?

Your RV tank probably has one to deal with changes in pressure, but you probably could. You would need to create a hole in the main tank as well as include a tube that reaches to the outside of the tank, with some kind of screen or hole that allows water through.

Can I cover the RV overflow valve?

You probably won’t ever need to. The overflow valve does mostly positive things for your RV, including letting excess water go and relieving excess gas and oxygen. 

If you ever discover that your water tank is leaking excessively and can’t find another source of why, you could consider covering the valve to ensure that is not the problem.

Can my overflow valve clog?

This is unlikely, but it could. If debris were to enter through the valve or get pushed into the valve through the tank, it could cause a problem. You are likely to notice issues with the same debris getting into your water.

Debris is among reasons why the valve is often covered with mesh – to keep big particles out and animals from trying to live in the tube.

Does the overflow tank has a similar item in a house?

Your sinks and tub tend to have overflow drains that allow water to escape when the sink or tub are getting too full. These are also automatic and are quite helpful in the event that you forgot to turn the sink off or it leaks. 


The overflow valve is a unique part of an RV and is meant to ensure the long term health of your water and the tank its stored in. The tank can help release extra air and water, which is a good signal you have too much water stored.

Take a look at yours, locate it, and try to experience it. It could come in handy at some point when you need to fill the tank with water.

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