I decided to upgrade the old thermostat control for heating/cooling, to a simple non-programmable digital unit. I simply attached the wires to the corresponding terminals in the new unit. After swtching the power back on, when I tried the heating and cooling, the furnace/ blower didn't startup. It showed on the new thermostat that it was calling for heat/ cooling, yet nothing was working. I triple checked that I had put the wires on the correct corresponding terminals, prior to switching back to the old thermostat. I had turned off the power at the furnace unit before commencing with the swap but, now that I think of it, perhaps I should have turned off power to the outside cooling unit....prior to swapping out the thermostats? One last thing, I noticed a subtle secondary click in the breaker to the furnace unit, when I turned it back on after installing the new thermostat. I know what a breaker sounds like when it trips....and this certainly didn't sound like a normal tripping noise....of course the breaker did not physically trip. I heard the same slightly delayed secondary click after re-installing the old thermostat. There is power to both sides of the breaker and to the furnace heating elements. I can't get a reading at the thermostat wires though. Ther is power going in the one side of the thermostat and no reading on the out terminal. Not really sure what the heck is going on....as it was working just fine prior to my messing with it. Is it possible that the transformer just happened to die at the point I swapped the thermostats out?? Any help/ suggestions would be greatly appreciated!