Six-wire thermostats are commonly seen with heating systems that have two stages of heat and one stage of air conditioning. A two-stage heat system means that when the thermostat calls for heat, the furnace will turn on using the low setting first. If the temperature set is not reached after a particular amount of time, the furnace will ramp up to a higher output of heat and will run at this higher setting until the temperature is reached. With a few tools, it is fairly easy to replace a six-wire thermostat. Certain safety precautions must be taken, however.
Electricity Safety Basics
Before you begin work on any project involving electricity, let anyone who might be around you know what you'll be doing. Ensure that everything you'll be interacting with is unplugged or disconnected. Turning off the power to the unit at the circuit breaker box is always a good idea. Using high-voltage gloves and tools with insulated handles is also a safe bet when you're working with a thermostat.
Thermostat Color Code
When it comes to thermostat wiring, you will find that most models follow a set color code for ease of use. Usually, the red wire is connected to the "R" terminal, the white wire is connected to the "W" terminal, the green wire is connected to the "G" terminal, the yellow wire is connected to the "Y" terminal, the black wire is connected to the "C" terminal, and the brown wire is connected to the "W2" terminal. Note that this is only the case on six-wire thermostat systems, and may differ if you have a system with a different number of wires. If you have manufacturer instructions, they may include a thermostat diagram that may be helpful in your wiring.
Thermostat Wiring Basics
Turn off power to the furnace and air conditioner by switching the correct breakers off in the circuit breaker panel. Failure to do this could result in a short circuit on the furnace control board.
Next, remove the old thermostat from the wall by gently pulling it off the wall plate. Disconnect each thermostat wire and label it with what terminal it is connected to.
Remove the old wall plate from the wall and mount the new thermostat wall plate using drywall anchors and screws. Connect each thermostat wire to the new thermostat. Make sure they are connected to the same lettered terminal that they were connected to on the old thermostat.
Once you have finished, restore the power to the furnace and air conditioner. Set the thermostat to your desired setting.