Is the CT3355 thermostat compatible with heat pumps or baseboard electric heating?
No, the CT3355 is not compatible with heat pumps, multistage equipment, or baseboard electric heating systems that operate on 120/240 volt line voltage. It is also not compatible with gas millivolt systems. If you have six or more thermostat wires, you likely have a heat pump or multistage system and should return the product. (Pages 4-7)
From the official Honeywell CT3355 Home Programmable Thermostat manual, page 4
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