Is a C wire (common wire) required to install the TH6320WF2003?
Yes, a 24 VAC 'C' wire is required for the TH6320WF2003 thermostat. The thermostat is not compatible with electric baseboard heat (120–240V) or millivolt systems. If you only have four wires, you can use the optional THP9045A1023 Wire Saver Module to provide a common wire. (Page 1)
From the official Honeywell Lyric T6 Pro Wi-Fi Smart Thermostat manual, page 1
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