Hi Gordon
I think you maybe questioning this to avoid the possibility of an air lock. I think it won't really matter as the water path within the ESC is through a pipe so water will just flow through and drive out any air.
Depending on the final orientation of the ESC, in an ideal world I'd generally choose the lower pipe as the inlet, higher pipe as the outlet, this has best chance of filling any internal spaces with water.
Hope that helps.
David.
