Monday, October 18, 2021


Check your water tank venting and save water

By Greg Illes

It’s surprising at times how much impact a simple oversight can be. In our Itasca motorhome, the fresh water tank had a factory-installed venting tube. This is a necessary feature so that the tank can “breathe” with capacity and altitude changes. However, the way the designers chose to install this vent was not ideal.

The vent was routed immediately out of the tank and down below the RV floor. Consequently, normal driving would slosh our fresh water out of the vent. On one occasion, we arrived after a long winding road at our destination with nearly 30 percent of our water already spilled overboard!

This venting style is not uncommon, especially if you have dual-fill capability (gravity tube and city-water connections). So it’s a really good idea to check the design of your venting system and make corrections if necessary.

If your venting system is designed with an uphill vent like the top image in the diagram, you’re probably in good shape to keep all of your water in the tank. But if you have venting which goes immediately downhill, your fresh water will spill out on turns, slopes and the typical jostling which accompanies motoring down the road.

If your venting system is a wasteful one, the fix is fairly easy. You can interrupt your vent line and make it much more difficult for the water to exit. CAUTION: Don’t think about simply capping it off — that would result in tank collapse or rupture. Always make modifications with equal-diameter tubing. And always be sure any changes you make leave the vent clear and unobstructed.

The two basic fixes are to either rerun the vent uphill (for example, to where the gravity fill entrance is), or to make a racetrack of tubing around the top of the tank before exiting downwards. Either method will prevent or drastically reduce water spillage. A variation would be to run tubing upward a couple of feet and then back down again (no, this will not cause a siphoning action).

Every installation is different, so it takes some study of your available space and maybe some experimentation. But if you’re losing precious water, it’s definitely worth the trouble and a few bucks’ worth of tubing.

Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his blog at



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1 year ago

Hi! I am currently building my water system and am considering the venting placement. I believe there is a good argument for venting below the tank (poor in the picture). Since the vent is open to the environment, dust, pollutants and other matter can enter or hang out in the venting tube.

If you vent below the tank, the siphon effect guarantees that any water entering the vent while the tank is moving will spill and exit below the tank.

If however you keep the vent above the tank, water that enters the vent tube during movement will then drain back into the tank. If you have dust or other buildup in the vent tube, this will not enter your tank, possibly polluting the water.

From what I gather, having the vent go upwards is only ok if it is isolated, say inside the vehicle.

What do you think about this?

1 year ago

Install a T-Fitting at the overflow and route a new hose higher for vent relief.

Install a valve at the hose under the RV. When it approaches full, you can close the lower valve to prevent water from siphoning out while still benefiting from the upper hose that prevents a vacuum lock. That way, you don’t get a cargo hold full of water when the tank is full.

1 year ago

I had the same problem with my fresh tank a few years ago. I could only raise the hose about 2″ above the level of the top of the tank, so I did that, but then continued the hose down into a “J” trap, ending up above the level of the tank again. This set up keeps water in the “J” trap, just like in your sink drains. That keeps bugs from entering the hose & migrating into the fresh water tank. If any bugs did get as far as the “J” trap, they would get flushed out the next time you fill the tank & it overfows.

Bob Wilson
1 year ago

Lazy Daze RV’s are vented properly to begin with, sad other mfgs do not get it

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