[cabmandone]

Okay, but that doesn't explain why your secondary heat didn't kick in. Your chronotherm should have a red light that illuminates when stage 2 (emergency heat) activates. You should have a swich to go between Heat, Cool, Auto and E Heat. Set it to E heat and see if the electric resistance heat is working. I'm guessing it isn't because at 5 degrees below set point your auxiliary heat should have kicked in. I installed Carrier for several years and can do some research on the system but I'm thinking yours should have an Optimizer for dual fuel (heat pump and electric resistance) but I can't recall exactly anymore. It has been several years since I've installed a heat pump with and air handler and electric resistance heat.

Correction: I think the older chronotherm had a red light on the sub base that displayed when auxiliary heat is on. On yours it might say "aux" or "E heat" when the backup resistance heat is on. Question, did your A/C work okay throughout the summer?

From the manual for the stat
STATUS LIGHTS—Lights that show system operation
settings.
SYSTEM light (yellow)—This light glows whenever
the thermostat is calling for heating or cooling
(located on thermostat).
AUX HT light [T8631R only] (green)—This light
glows whenever the thermostat is calling for operation
of the backup heater. Backup (auxiliary) heat is
more expensive to operate than the heat pump and
typically is used only when the heat pump is unable
to handle the load (located on subbase).