Tank Flush - HOW?? 3761FL

[MuscadineMontana]

Ok folks. We’ve had this rig for 3 years and I’ve tried to flush the tanks twice following the instructions in the wet bay to a T.. both times I ended up flooding the secondary bathroom pipes under the sink. What am I doing wrong!?! Is the flush tank faulty? All valves are open when I’m have it running, per the directions. I can’t find a good video or any directions any where. Any help would be fantastic, because I don’t have to have to run the old spinning water hose down the pipes.. see pictures

[Gonefishing]

I have the same unit mine is a 2021 3761fl. I have two black tanks therefore i have two blank tank flush valves. I connect a water hose to the tank flush open the valve and turn water on to the hose. I let it run about 30 minutes. Then I switch to the other tank flush and do the same thing.

[MuscadineMontana]

So you have one valve closed and one open?. Then switch them around after 30 min? Are the grey tank valves closed or open, does it even matter?

[Carl n Susan]

The last pic of under the sink is different. Usually there are tow white Pex lines coming into an anti-siphon valve in addition to the Red and Blue lines connected to the sink fixture.

The water on the floor and on the TP. Is that from the sink line fittings or is it leaking in from behind the wall? Can you see the water dripping when the black tank flush is running?

The anti-siphon valve has been installed in the sink cabinet for a while. But it used to be hidden behind the shower wall. Maybe Montana has decided to place it somewhere new?

You only need the black tank valve (and not the shower or sink ones) open when running the black tank flush.

[Gonefishing]

Originally Posted by MuscadineMontana

So you have one valve closed and one open?. Then switch them around after 30 min? Are the grey tank valves closed or open, does it even matter?

I keep the gray tanks closed. Yes I do one black tank at a time.

Just to clarify you have two flush valves one for each black tank. They are side by side. You will need to connect your hose to each one separately and open the dump valve to see which one is for what tank. On mine when connected to the left one I open the left valve for the black tank. When connected to the right one I open the right valve for the black tank just remember when connected to the flush valve to,open you dump valve. I follow these steps 1. Open dump valve 2. Connect hose to flush valve 3. Turn on water. When done 1. Shut water off 2 disconnect hose from flush valve 3. Shut dump valve

[Carl n Susan]

The more i look at that last pic, the more it looks like the water is coming in from under the left wall. You need to find where the clear Pex lines are running. You probably have to go in through the pass through compartment and remove the back wall to see them.

[Mikendebbie]

This pic is NOT a 3761 frame - but this will give you a visual for how the tanks are mounted in the frame. Your unit has a mid-half bath and full bath in the rear. The white tank is the fresh water tank. The spec sheet for your unit says you have 88 gals black and 88 gals gray - which means you have two 44 gal tanks for each system. You may have a black and gray tank located to the rear of the fresh water tank in this pic. I have no idea if your single black tank flush connection is split to rinse both tanks or if it only rinses the front black tank - likely mounted under the basement like this photo shows. However, I have no idea where your tanks are located. I just like this photo!!!

[MuscadineMontana]

I do have 2 black tanks, but I only have 1 flush tank port. There is one to the left of it that says flush and the one to right says FILL.

[Carl n Susan]

Originally Posted by MuscadineMontana

I do have 2 black tanks, but I only have 1 flush tank port. There is one to the left of it that says flush and the one to right says FILL.

There are two black tank flush connections. The one on the left that you found. The connection to the right of it is the other black tank connection. The "Fill" label applies to the TANK FILL POWER FRESH) lever above there where you select where the shore input water will to go. That is to either the house water (NORMAL FLOW) fittings *OR* to "FILL" the Fresh Water Tank.

[BB_TX]

Classic symptom of failed antisiphon valve. I agree with Carl. From the pics it looks like water coming from under that wall.

Your vanity water lines are going into the left side wall. That is also the back wall of the washer/dryer cabinet. I would bet they brought all the water lines up inside that wall. Including them installing them antisiphon valve and pipes inside that wall.

It’s possible (maybe not likely) they put an access panel in the back wall of the W/D cabinet. If not (or if you can’t get behind the W/D), you will probably have to drop the belly cover under that area. Likely you will find a red pipe and a blue pipe going up thru the floor for hot and cold water for the W/D and the vanity. And also likely you will find two white pipes going up thru the floor to an antisiphon valve installed up in that same wall.

If the above is the case, the best thing to do would be cut the two white pipes, connect them together, and install an external antisiphon valve.

All speculation on my part of course. Different floorplan, but my valve was in the wall between my shower and my entry coat closet.

[Daryles]

You need to find the Amti Siphon vacuum breaker valve. Trace the white PEX pipes. That's the color they use for the black tank flush.
Once you find it, replace it with a quality brass one.

[Gonefishing]

Originally Posted by Carl n Susan

There are two black tank flush connections. The one on the left that you found. The connection to the right of it is the other black tank connection. The "Fill" label applies to the TANK FILL POWER FRESH) lever above there where you select where the shore input water will to go. That is to either the house water (NORMAL FLOW) fittings *OR* to "FILL" the Fresh Water Tank.

This is correct I have the same unit.

[Rexl1200]

I had to replace the anti-siphon valve on our 2021, it is located right behind the 1/2 bath wall accessible through the panel in the under storage.
Same exact issue. Get a good brass replacement.

[rohrmann]

Originally Posted by Daryles

You need to find the Amti Siphon vacuum breaker valve. Trace the white PEX pipes. That's the color they use for the black tank flush.
Once you find it, replace it with a quality brass one.

Not always white. They must have had an abundance of red PEX when they built our rig, and used the red to plumb the tank flush. May be hard to see, but our valve is between the shower wall and the back of the pantry cabinet.

[ScaryPapa]

Just had to replace the anti-siphon valve on our 2022. It was doing the same thing, leaking under the bathroom sink