Sunday, December 4, 2022


Ask Dave: When connected to city water, why does gray water tank start to fill?


Dear Dave,
We are in our sixth year of full-timing in this coach. I’m completely baffled by something that’s come up. When connected to city water, 3-5 gallons of water is bleeding into the gray tank over an eight-hour period. We have three sinks and a shower with no leaking faucets. I don’t see any other connections to the gray tank. I saw something online that mentioned the fresh water fill valve bleeding, but I don’t see how this could be connected to the gray tank. —Blane, 2016 Holiday Rambler Endeavor 40DP

Dear Blane,
From what I can see on all the used RV sites and forums, your HR has only one gray water tank that has a capacity of 75 gallons, and one black water tank with a capacity of 50 gallons. The shower is typically the main drain for the gray water, and the toilet is for the black. However, not all sinks dump into the gray water tank due to the location and plumbing difficulties around the outriggers and risers in the foundation. That being said, it’s hard to tell which sinks go to the gray water tank.

Figuring out what sink drains where

When you are connected to an outside pressurized water source, the city water connection in the service compartment has a cold water line that goes to the onboard pump supply and then “T’s” around it and connects with another “T” on the outgoing supply line from the pump. This supplies pressurized water to all the cold faucets as well as the water heater. There is some back pressure to the onboard water pump from the outlet side when connected, which is held with a check valve on the water pump. If the valve leaks due to calcium or lime, or just gets weak, water will backflow into the fresh water tank. So that would not get to the gray water tank.

The shower drains directly into the gray water tank as well as some of the faucets. Below is a 3D schematic of a Winnebago unit that has one gray and one black water tank. You can see how the plumbing goes to the tank.

This is not identical to your plumbing setup. However, I don’t think there is any way a line from the city water system can backflow anywhere to fill the gray water tank. Either the shower or one of the faucets have to be leaking to get to the gray water tank.

How to tell if your RV’s faucets are leaking

One of the best ways to tell if you have a faucet leak on any of the sinks or showers is to put fresh water into the tank, turn on the onboard pump, and listen for the pump to cycle. If all the valves are shut off, there should be constant pressure in the lines and the pump will not run. If there is a leak anywhere, the pump will “cycle” on and off and then you know there is a leak.

I would also plug all the drains in sinks and even the shower with a rag or towel and actually take off the shower drain screen and plug the drain a couple of inches below the surface. If the shower handle has bad washers, it could leak and travel down the inside of the wall and go into the shower pan and down the drain and would not be noticed. If you plug the lower part of the drain, the water will pool in the basin.

One last item to check: If you have a gray water tank flush valve that can be supplied by the city water connection, the valve could be stuck. Typically there is only a black water flush valve and it has its own connection. However, I have seen stranger things!

 You might also enjoy this from Dave 

Why is my gray water tank leaking and how do I fix it?

Dear Dave,
I have a 2006 Coachman Epic. The gray water tank is leaking from the bathroom only. How can I get to it and fix it? —David

Read Dave’s answer

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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2 months ago

Gremlins are hard at work here!

2 months ago

Could be gray tank vent on the roof is receiving some extra water somewhere?

2 months ago

Hi Dave,
When checking for leaking faucets by using your 12VDC pump, you mentioned if the pump is cycling on & off there is a leak. Well if the pump pressure adjustment isn’t set right, it will stay on till it’s catches up & also cycle. So even though thats a great way to test for a faucet leak, it’s not a perfect way. I’m thinking that Blane may need to ward off the evil ‘drain” spirits out of his coach!
Just a thought

Last edited 2 months ago by Snoopy

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