I'm not sure if it needs to go at the highest point, that's just where mine was when I moved in - sitting on top of water tank on the upstairs level. I can't think of any technical reason why it needs to be there, but someone else might know otherwise. My question would apply wherever it is located (unless it was placed upside down lower in the system so the water would drain into the vessel (or at least the part where there is meant to be water!).

Darren