My system did the same thing. Existing t-stat controlled A/C , installed
Nest and the forced air heat worked but A/C kicked on and off within
seconds. I did have a blue "C" wire on old t-stat, and used that on
Nest. Unfortunately the common wire was ...
You have to see where the Common wire (blue wire) connects to the
terminal block on your HVAC unit. See the picture I posted; mine was not
in the "C" terminal. After connecting it to the "C" terminal my Nest
Thermostat worked properly and the A/C ran...
Went to the control board on unit and the common wire was not in common
terminal, was in R terminal. Switched wire to C terminal and Nest is now
cooling. Also CNYTeddy, pull the Common wire out of Nest terminal and
tuck away from the other wires & m...