Nest Support

Questions about Nest Learning Thermostat or Nest Protect: Smoke + CO alarm? We've got answers, videos, and plenty of resources to help you out.

When the Nest Learning Thermostat needs a common "C" wire

While we find that in the vast majority of homes the Nest Learning Thermostat can charge its built-in battery using the heating and cooling wires, there are a small number of heating and cooling systems and situations where the Nest Thermostat may require a common wire to bring power to the thermostat.

What’s a common wire?

A common wire is simply a low-voltage power wire to bring constant power from your heating and cooling equipment to the thermostat. Most common wires are blue or black, and you might have a common wire already.

Your HVAC professional can install a common wire for you if you don't have one already.

What are the symptoms of a situation where the Nest Thermostat might require a common wire?

For conventional heat-only systems:

  • Heat is always on or intermittently on when the Nest Thermostat is not calling for heat (the Nest Thermostat display background is black).
  • Heat is not on or intermittently turns off when the Nest Thermostat calls for heat (the Nest Thermostat display is active and the background is orange).
  • There is an unusual noise coming from the heating system.

For conventional systems with cooling:

  • Cooling is always or intermittently on when the Nest Thermostat is not calling for cooling (the Nest Thermostat display background is black).
  • Cooling is not on or turns off intermittently when the Nest Thermostat calls for cooling (but the Nest Thermostat display is active and the background is blue).

For heat pump systems:

  • Heating or cooling is always on or intermittently on when the Nest Thermostat is not calling for it (the Nest Thermostat display background is black).
  • Heating or cooling does not turn on or intermittently turns off when the Nest Thermostat calls for it (the Nest Thermostat display is active and the background is orange or blue respectively).

What systems are known to need a common wire?

  • Many zone relay panels or control panels.
    The Nest Thermostat requires a common wire to be compatible with many zone relay panels. Some systems have control panels or equipment interface modules that require a common wire to be compatible with the Nest Thermostat. Here’s a list of known panels that require a common wire:
    • Honeywell AQ25110B
    • Honeywell EMM-3
    • Honeywell TZ-4
    • Honeywell HZ311
    • Bryant 548F036
    • White-Rogers 36C03-300
    • Carrier HK42FZ011
    • Nordyne 624631-A
    • Nordyne 903915A
  • Some gas valves.
    Some gas valves on systems vibrate or buzz when the Nest Thermostat is installed. This can usually be fixed with a common wire.
  • Some micro-controller based systems.
    Systems that use micro-controllers instead of relays are sometimes more sensitive to power-sharing. If so, the Nest Thermostat will need a common wire to be compatible with these systems.

If you have questions about your installation and whether your home will need a common wire, please contact support using the information at the bottom of the page. We're here to help.