The issue is that I want to be able to equalise the temperature of my home without running the furnace fan continuously and in particular control the differential between upstairs and downstairs. This is especially problematic overnight in the summer when the AC tends to cycle less and I can end up with a ~7F-10F differential.

What I would like is a thermostat/controller that takes input from multiple sensors and uses the average reading to determine when to run the heat/AC. When the differential temperature, max-min, of all the sensors is above a certain threshold the fan will be run.

Simple? Is such a beast available? If so what words should I be using when searching for one?