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I just bought a new home with a high-efficiency furnace. Both the heating and cooling are 2-stage. The wiring to the current thermostat is basic 4-wire (no C wire). I bought a used Ecobee 3 (1st generation), which includes the PEK for 4-wire installations. Replacing the thermostat wire would be a relatively short, easy pull.
My question is, what would I gain by wiring up the Ecobee 3 for 2-stage control?