Thursday, October 6, 2022


Ask Dave: My sani-flush is filling and overflowing my fresh water tank! How?

Dear Dave,
I have a 2009 Fleetwood Discovery 40X and something strange has started happening when I use my sani-flush system after emptying the black tank. After running it for a time, water starts coming out the overflow of my fresh water holding tank. The holding tank was about 3/4 full when I started using the black tank sani-flush system. I don’t know what’s changed but it has done this the last two times I emptied my black tank and used the sani-flush. I would not think any source of water to the black tank should be able to reach a potable water tank. Your thoughts? Thanks. —Marv

Dear Marv,
This is very strange. You are correct, water from either the black or gray tank should not be able to get to the fresh water tank! All three tanks should be closed systems.

The point of a sani-flush

The fresh water tank has a gravity fill port and a city water fill port, both located towards the top, a supply port/line at the bottom, and a drain valve, which sometimes is in line with the supply hose.

The black water tank has the flange and pipe from the toilet coming in the top, and sometimes a drain line coming from a sink. Then, at the side towards the bottom is the drain pipe with a spade valve. The sani-flush you refer to I believe is a black water tank flush valve that is installed on the side of the tank close to the top. It has a hose connection on the outside and sprays water around the inside, right? Some have a twirling nozzle while others are just holes. Here is a sample of one we installed for a demo video.

These are designed to spray pressurized water all around the inside of the black or gray water tank to clean the sides and probes. The pressure comes from the campground source, which could be 40-50 psi. This should only fill the black water tank and not get anywhere near the fresh water. If it did, it would start to overflow the toilet if you fill the tank up.

Sample the water after using sani-flush

There are two things I would look at. First, I would sample the water that is flowing out the fresh water tank overflow valve. If it is somehow coming from the black water tank, it would be slightly sewage colored and not sanitary. If it’s fresh water, I cannot see how it would be coming from the black water flush valve or tank to the fresh water. Even if the tank has a crack or one of the ports is loose, it would still be sewage water.

The second thing I would look at is the connection you are hooking the hose to. The female connection for the city water fill and the black water tank flush valve are sometimes identical.

Here is a photo of a 5th wheel we worked on for several videos. The upper connection is the city water fill and the lower is the black water flush valve. Both looked identical so we installed a quick disconnect on the city water fill to help identify it. If you inadvertently connect to this, it will fill up the fresh water.

Final steps

Verify you are connected to the right valve, what type of water is coming out the drain hose and if it’s actually coming out the hose or maybe just following it. Sometimes the flush valve connection is in the service center and a hose runs over to the black water tank. If that hose has become disconnected or cracked, it could be leaking fresh water from the sani-flush and channeling down the side of the fresh water tank.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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Phil J
7 months ago

Most if not all modern factory black tank systems have a backflow device between the water inlet and the spray dispenser in the black tank. I have two fifth wheel trailers and one is in the vanity cabinet and the other is in the wall of the shower accessible through a removable panel in the linen closet. If you have one you don’t need to use a dedicated hose for your black tank flush. Locate it to verify that it exists before using the same hose.

Ron Lane
7 months ago

Since it worked fine previously and there is absolutely no connection between the fresh water tank and the black water tank, the obvious problem is that he just connected the black tank rinse to the fresh water tank inlet. There is no other possibility.

Wayne C
7 months ago
Reply to  Ron Lane

Agreed. But unless a check valve has failed, or like the picture shows a fresh water fill valve is available, connecting to fresh water inlet shouldn’t fill the fresh water tank.

7 months ago

The water inlets need to be marked with ‘easy’ to read labels once the inlets are identified.
Although they may be marked, this picture does not show that. Also, having the black tank flush and the fresh water inlet so close together could result in using the same hose for both.
A definite NO NO!
A separate hose dedicated to black tank flush and sewer hose cleaning is needed.