Here is a explanation from Coleman Aircel.
Heat Pump Lockout
If the system has both gas heat, (LP furnace or hydronic heat) and electric heat,(electric strip heat or heatpump) appliances installed in the same zone, then the system will automatically switch from Heat Electric to Gas Heat if the electric heat can not satisfy the desired setpoint temperature. Because of the nature of the electric heating systems, they tend to be less effective the lower the outside temperature. Therefore, at low temperatures, your electric strip heat or heatpump may not be able to satisfy the setpoint. The system switches from Heat Elec to Gas Heat when the actual zone room temperature is five degrees or more below the desired setpoint temperature. If this happens three times in a row, the electric heat is locked out for two hours and the gas heat is the primary heat source. When this happens, the LCD will continue to display Elec but the word Gas will flash on the LCD to alert the user that the electric heat source is locked out. See table below for an example of how the heatpump lockout system works.
Example of Electric Heat Operation to call for Backup Heating
Setpoint Indoor Operation Temp.
69 Electric Heat turns on (Primary Heat Source) (“ELEC” is displaying on T’stat)
71 Electric Heat turns off (Thermostat satisfied)
69 Electric Heat turns on (“ELEC” displaying on T’stat)
65 Gas Heat turns on (“ELEC” is displayed & “GAS” is flashing on T’stat) (Electric
Heat unable to satisfy Thermostat) (First strike for backup heat counter)
71 Electric Heat & Gas Heat turn off (Thermostat satisfied)
69 Electric Heat turns on (“ELEC” is displaying on T’stat)
65 Gas Heat turns on (“ELEC” is displayed & “GAS” is flashing on T’stat) (Electric Heat again unable to satisfy T’stat) (Second strike for backup heat counter)
Nothing is operating