“Can this be connected using two wires for heat only? I don't think my old 1977 Honeywell has a "c" wire.”
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The C wire supplies the 24V required to run/charge the thermostat. There is some documentation out there that states you can still hook it up to older systems without a C wire but you will have to buy and run a separate transformer wire to put on the C wire line to supply the voltage.
Sorry, there was a problem. Please try again later.The Nest Thermostat is designed to work with most 2-wire, 20V - 30V systems, and even older ones. We recommend using our online compatibility checker first to make sure that the Nest Thermostat is compatible with your system. You can find it here: https://store.google.com/us/widget/compatibility/thermostat Typically, smart thermostats don’t require a common (C) wire, but every system is different. Without a common wire, your Nest thermostat will charge itself through power sharing using your existing heating and cooling wires. However, when your heating and cooling wires are not able to deliver the needed power for your Nest thermostat, you may need to connect a common or C wire since it helps deliver enough consistent power to your thermostat. When there’s a C wire, it becomes the thermostat’s go-to for power backup when power sharing doesn’t work. This can also conserve battery life since the batteries will likely only be in use when there’s a power loss to the thermostat. Also, Nest Thermostat (2020) has a backup battery. For systems that are not using a C wire, the average battery life is three to six months. However, depending on the Wi-Fi activity on the thermostat, it may be possible to extend or reduce the battery life. You can also contact our support team for assistance: https://support.google.com/googlenest/gethelp
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