Originally Posted by Minnewaska
There should be a rise from the back of the head to a vented loop, to prevent back flow. That's what needs to be cleared, then it doesn't matter if the rest of the line is 100 ft long, if gravity does its thing. It's not the volume of the hose, its the amount of chase water necessary until it stops mixing and runs clear.
My holding tank setup doesn't require a vented loop, the bottom of the holding tank is above head level and the fill pipe is at the top of the holding tank. It is still only 3' long. It is just on the other side of a bulkhead from the toilet, so the whole line can be quite short.