[bncinwv]

There are two predominant ways to flush the black tank with the on-board flusher. One is to follow the instructions provided with the unit and to always leave the dump valve open when flushing until the discharge is clear (use a clear elbow at the hose hookup). The other is to bypass the instructions (which is the way we do it) and close the dump valve and allow the black tank to fill. The advantage of this method is the dump valve and pipe as well as the bottom of the tank now has the tank head (engineering term for depth of water above a point) working to assist with the flushing process. This allows for a more thorough, higher velocity flush than just using the sprayer. We also use the previously mentioned clear elbow at the end of the dump valve pipe so we can monitor the flushing results. As Ozz mentioned the magic number here is about seven minutes (I am not that brave and typically use five) to allow the tank to fill. If you go this route, do not get distracted, use some kind of timer, do not get distracted, and in case I forgot to mention it, do not get distracted. Using this process, it typically takes us four or five "flushes" to get clear discharge. Oh, and lest I forget, do not get distracted, hence you may be subjected to the dreaded "black rain" effect from the roof area or even worse, the potential for a flooded bathroom if your system has any leaks.
Bingo