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 wire jumpered to a contact labeled 1
4 Wire or 5 Wire Thermostat Wiring Problem with an Air Conditioner and Gas Furnace
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 for on the old thermostat.
I can upload pictures if it’s 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.
Understanding where the wires go and what they do help you. However, color is key in most thermostat installations provided the installer followed the conventional color code.
- 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. That 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 hotwire 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 the fan only.
- Y - wired to a contact labeled 1 - This is a mystery and without further information, I can’t tell you.
What Color Wires Go Where | 4 Wire or 5 Wire Thermostat Wiring Problem
I can only assume it is unused for anything, at least this jumper you are talking about here. On your new thermostat, you will not have to worry about this. Just follow the instructions that come with the new thermostat. That’s good news. And now, the not-so-good news. You did not mention if you had any additional wires there in the thermostat bundle because, with your new thermostat, you are going to need one extra 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.
Old Wiring - Additional Resources | 4 Wire or 5 Wire Thermostat Wiring Problem
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.
Wiring Chart for Basic Thermostat Wiring Terminals and Designations
|4 Wire or 5 Wire Thermostat Wiring Problem|
|Wire Color or Designation||Control Point/Function in Equipment|
|W-wire - 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-wire - yellow wire typical color||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-wire - red wire (can also be RH or RC - read thermostat manufacturer instructions)||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-wire - 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.|
|C-Wire - Various colors but could be blue or black||This goes to the 24 volt common side of the transoformer and is typically not used for mercusry type thermostat unless the thermostat requires power. See the thermostat manufacturers instructions.|
|O-wire - Orange wire||This wire is used only for heat pumps. It controls the reversing valve inside the heat pump condensing unit. Note: On Rheem and Ruud heat pumps this wire may be a different color.|
4 Wire or 5 Wire Thermostat Wiring Problem
4 Wire or 5 Wire Thermostat Wiring Problem - Additional reader frequently asked questions:
What are the 5 wire thermostats?
These were the older mercury switch thermostats that were mechanical. Mechanical versus newer thermostats that are digital. Some systems still have the older mechanical thermostats, which are typically 4-wire thermostats that control the same type of system described above.
How many wires does a thermostat need?
The answer to the question depends on the type of system you have and what kind of thermostat you have for control. Mechanical mercury switch thermostats with no special lights do not require the C-Wire. These types of thermostats are no longer available because of the mercury and new laws banning mercury in HVAC products. Here is a breakdown for mechanical thermostats and the types of HVAC systems they control:
- Air conditioner with a gas furnace - 4-wires for single-stage and 5-wires for two-stage
- Air conditioner with an electric furnace - 4-wires for single-stage and 5-wires for two-stage
- Stand-alone heating systems only (gas or electric) - 2 wires*
- Heat pump - this type of system can have up to 7 wires depending on setup
Digital thermostats normally require a power source unless they have a battery backup. For the same type of systems, here is a breakdown for the type of HVAC systems they control:
- Air conditioner with a gas furnace - 5-wires for single-stage and 6 wires for two-stage
- Air conditioner with an electric furnace - 5-wires and 6 wires for two-stage
- Stand-alone heating systems only (gas or electric) - 3 wires*
- Heat pump - this type of system can have up to 8 wires depending on setup
*Some boiler systems have multiple zones. Multiple zones require a thermostat for each zone unless you have a digital thermostat system that has linking capabilities for wireless sensors.
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