4 Wire or 5 Wire Thermostat Wiring Problem - Question: We have a 19-year-old condo with an old Carrier mercury thermostat. We have both a natural gas furnace and an AC unit. There are only four wires going into the existing thermostat - red, green, blue, white. On the back of the thermostat are a number of labeled contacts, only the following have wires connecting them:
W - white wire
Y - blue wire
R - red wire
G - green wire
Y is also jumpered to a contact labeled 1

4 Wire or 5 Wire Thermostat Wiring Problem with an Air Conditioner and Gas Furnace

4 Wire or 5 Wire Thermostat Wiring Problem

Wiring in the New Base Plate for the thermostat. This is a 2 wire thermostat for heating only.

We were given a new Honeywell VisionPRO Wi-FI Programmable Thermostat model TH8320WF and would like to install it. We do not have a heat pump, so I planned to use the conventional wiring. It just seems like one wire is missing and I suppose that is what the jumper was used for on the old thermostat.
I can upload pictures if it will be helpful.

Any suggestions on how to wire the new thermostat?

4 Wire or 5 Wire Thermostat Wiring Problem - The Answer

Answer: Thanks for the email Ed and I’m glad to help.

The first thing we need to do is to identify the wires and where they go using our comprehensive guide to thermostat wire colors and codes.

W - white wire - this wire goes to the gas controls to engage the heat on a call for heat from the thermostat when you have it set to heating mode. This closes the 24-volt control side of the gas furnace and begins the process of starting the gas furnace into its sequence of operation

Y - blue wire – This wire goes to your air conditioner and starts the air conditioner into its sequence of operation. Typically in most HVAC systems, this would be the yellow wire.
R - red wire – Red is the 24-volt hot wire which would power the relays or circuit boards for the air conditioner, the gas furnace, and the blower along with any accessories in the system controlled by the thermostat.
G - green wire – The G terminal or green wire goes to the blower fan inside the air handler or gas furnace to engage the blow when you turn the thermostat to fan only.
Y is also jumpered to a contact labeled 1 – This is a mystery and without further information, I can only assume it is un-used for anything………at least this jumper you are talking about. On your new thermostat, you will not have to worry about this anyhow as long as you follow the instructions that come with the new thermostat. That’s the good news……………now the not so good news. You did not mention if you had any additional wires there in thermostat bundle because with your new thermostat you are going to need one additional wire. Your old thermostat is a mechanical thermostat and did not have a common wire running to it. For the new thermostat to work, you need a 24 volt common to complete the circuit with the 24-volt hot wire to provide power for the thermostat because it is a state of the art digital programmable wi-fi thermostat it will need 24-volt power to work. That means you need a 24-volt hot and a 24-volt common wire. You already have the 24 volt hot with the red wire so now you need to either find another wire in that bundle or run all new thermostat wire so you will have the proper amount of wires for the job.

4 Wire or 5 Wire Thermostat Wiring Problem - Additional Resources

This article will help you – Running New Thermostat Wire along with this article - All About Thermostat Wire and another article will help you gain some understanding of basic control circuits in HVAC if you want to learn more and take it that far. Most people do not and they end up calling in a professional to get the job done.

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4 Wire or 5 Wire Thermostat Wiring Problem

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