Standard for vented loop height
is 8-12" above the waterline at any angle of heel. On some boats, that can put it 2' or more above the toilet. But whether you put it 1' above the toilet or 10' above the toilet, you can't change the laws of physics: water runs downhill. The only way to prevent waste/water from running back down the hose from the top of the loop to bowl is by pumping enough times to push all of it over the top of the loop. It doesn't matter what brand the toilet is, the pump moves bowl contents the same distance with each stroke: approximately 3", which means it takes 4 pumps to move it 1' (and that's only if the toilet is working to factory specs...inefficiency due to worn seals increases the number of strokes needed). So even if you only put it 1' above the toilet, if you only pump 3x, some waste/water will be left in the line to run back downhill toward the toilet. If it's 2' above the toilet and you only pump 6 times, that much more will be left in the hose to run back downhill. The higher the loop, the longer the hose between the toilet and the top of it, and the longer the hose, the more water it can hold to run back downhill to the toilet. How quickly it floods the bowl depends upon how worn the joker valve is. But just pumping enough to push it over the top of the loop won't send anything but liquid the rest of the way to the tank. Because solids fill the hose, they won't freefall...they have to be pushed along. If you don't push 'em all the way to the tank, they sit in the hose to permeate it and the risk of clogs increases. But we both know you're NOT gonna pump your toilet 30-40 times every time you flush it. It's your boat...but if it were mine, I'd find another location for the tank that simplifies flushing the toilet and minimizes potential problems instead of causing 'em.
