[HiHoSteveO]

For those considering upgrading their existing "Carrier Edge" thermostat.
After research, I chose the ecobee3 Lite. $169 at Amazon.

I discovered that my existing original (6 year old) Carrier Edge thermostat for my Carrier heat pump was wired with only two wires to an interface control board which is mounted on the outside of the air handler in the garage. What I thought was going to be a problem, was not.

The two wires from thermostat connect directly to the interface control board along with the other necessary wires for a smart thermostat, except for a required common "C"wire.

The cable between the thermostat and interface board contained 5 wires. (3 are spare) Exactly what is needed.

The ecobee thermostat comes with an adapter included called a "PEK" that provides the C wire. It is connected along with the other wires using wire nuts on the outside of the air handler. No need to even open the cabinet.
The interface control board that is mounted on the side of the air handler is completely bypassed and can be removed.

The ecobee is better than expected and all functions including emergency heat still work.
The reports online even show in graph form, when (and how long) air conditioning, heat, emergency heat and fan were on. Along with humidity levels.