4 Wire or 5 Wire Thermostat Wiring Problem - Wifi Tstat 4 Wire or 5 Wire Thermostat Wiring Problem - Question: We have a 19-year-old condo with an old Carrier mercury thermostat. We have both a natural gas furnace and an AC unit. There are only four wires going into the existing thermostat - red, green, blue, white. How do I wire my thermostat? | Sensi Thermostat Support Learn more by downloading the Sensi Installation Guide which walks you through wiring and connecting to Wi-Fi. If you are not connecting your Sensi thermostat to Wi-Fi, follow the instructions below to wire and correctly configure your thermostat for your heating and cooling equipment.
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