HVAC Troubleshooting
Symptom: I would like to put in a new programmable thermostat but I don't have enough wires.

This is a common problem when replacing a mechanical thermostat or a thermostat powered by batteries. These older thermostats require 4 wires, but newer more sophisticated thermostats require a 5th wire known as the Common wire to power the digital display. There are two ways to add the common wire to a four wire installation:

Note: Before starting, we recommend that you take a picture of the original wiring to the old thermostat as reference to help remember what terminal each thermostat wire was connected to.

Option #1: The easiest and least expensive option is to sacrifice independent control of your indoor fan.
  1. First, disconnect power from your heating and/or cooling system.
    Caution: if your heating and cooling system is powered during the thermostat installation process and you accidentally touch the R (red) power wire to the C (blue or brown) common wire, then you will permanently damage either the equipment fuse or the transformer in the equipment.
  2. After disconnecting power from your heating and/or cooling system, move the wire connected to the G terminal (typically green) that controls your indoor fan to the C terminal (common) on the thermostat.
  3. At your heating and cooling equipment repeat the same step and move the G terminal wire to the C terminal.
  4. At your heating and cooling equipment place a jumper wire from the Y equipment terminal to the G equipment terminal. This will allow the fan to turn on when the thermostat is calling for cooling or heating but it will not give you independent fan control from the thermostat.
  5. See diagram below for wiring instructions.

Add a wire diagram

Option #2: The second way to add a wire is just put in an add a wire model #acc0410 This product which cost less than $20 allows you to make 1 wire work just like two wires. It is very easy to install and works great. Make 4 wires into 5 or 5 wires in to 6 or even 7 wires by using 2 add a wire. You will again as above need to move the green wire from the G terminals to the C terminals (shown as brown in the diagram) and then use the Yellow wire to carry both signals(shown as the blue wire in the diagram).

Add a wire diagram

Add a wire
Add-A-Wire Diode
Wired to thermostat
Add a wire
Add-A-Wire Model #ACC0410
wired at control box in furnace or fan coil
Add-A-Wire Model #ACC0410
In applications where additional wiring cannot be run, the Add-A-Wire accessory can be used to add a wire to the thermostat.
  • Easy to install
  • Easier than running new wire
  • Allows 5 wire thermostats to use only 4 wires
  • Also allows 5 wires to work like 6 wires