What options do I have if I don't have the “C” or common wire?

The "C" wire or Common wire is only required to provide 24-volt power to your Honeywell connected or Wi-Fi thermostat. If the original thermostat did not have a C wire or common wire installed and you need one to power your Wi-Fi or connected thermostat there are several options listed below.

  1. Look for an unused wire in the wall. If you find one, trace the wires to where they end at the heating or cooling equipment terminal and connect wire to the C or common terminal. If you would like help with setup, please reference our Contact A Pro page to find an HVAC service professional in your area.
  2. If the thermostat and system allows, a Wire Saver Module (THP9045A) can be installed. The wire aver is designed to be used with applicable thermostats in which a 24V C wire is required but the homeowner does not have a C wire connected to their current thermostat. Honeywell's wire saver acts like a splitter and replaces the need for a C wire to be run from the HVAC system to the thermostat. This is the best recommendation to supply power to your connected or Wi-Fi thermostat without the presence of a C wire. Since this module does require you to open up your furnace, it is recommended to Contact A Pro for proper install.
  3. A completely new wire can be installed by an HVAC professional from 24 VAC common at the equipment to C terminal at the thermostat. Visit our Contact A Pro page to find a service professional near you.
  4. If the thermostat is controlling a two wire, heat only system, a second 24 VAC transformer can be connected to the RC and C terminals. Please Contact A Pro in this situation to help safely install and wire a transformer.
  5. Follow the steps in this instructional video:
Last updated: July 27, 2018
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