Is the CT3355 compatible with heat pumps or baseboard electric heating systems?
No, the CT3355 is not compatible with heat pumps, multistage equipment, or baseboard electric (120/240 line volt) systems. It is also not compatible with gas-millivolt systems or any 120/240 volt circuit. For information about which programmable thermostats work with incompatible systems, you can call Honeywell Customer Assistance at 1-800-468-1502. (Page 4)
From the official Honeywell CT3355 Home Programmable Thermostat manual, page 4
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