The common wire is required for a 5 wire thermostat. If there are only 4 wires to the existing thermostat here are the steps you can take:
1.Verify if 5th wire is available
2. If convenient, you can pull a new wire
3. Use the add-a-wire
“Add a wire” for existing thermostats without a common wire (4-wires available in wall) so that it can work with a thermostat requiring a common connection. If it is convenient, you can always pull new wire that allows you to hook up the common wire.
Input: Voltage rating: 18 to 30Vac, 50Hz or 60Hz
Current consumption: 30mA at 24Vac when W & Y are active Terminals: RC, RH, Y, W, C, G Output:
Voltage rating: 18 to 30Vac, 50Hz or 60Hz Current rating: 2 amps per terminal Terminals: RC, RH, Y, W, C, G