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New Thermostat Wires Help

Hello, I am going to remove very old three wire thread woven thermostat wire and replace with modern three wire (heat only). Currently, furnace has newer two wire and they meet in the junction pictured (I assume this is not to code). The second picture is where the two wire connects to furnace. Question 1 Where will this third wire be connected here? Question 2 I would run a two wire but smart thermostats will need the third wire, correct? TIA


  • Jamie Hall
    Jamie Hall Member Posts: 23,322
    If you are running new wire (why?) you might as well follow the convention -- red and white go to the furnace control, with red to the powered side of the control (use a voltmeter to ground), and blue or sometimes yellow to the common terminal on whatever transformer is running your system. However, there may not be an obvious transformer, or an obvious common terminal -- so you may have to search around a bit to find a common terminal. That wire will be the common return for a smart thermostat. Or would could run the third wire and not worry about it until you do decide to get a smart thermostat...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    Yes, most new thermostats will need 3 wires. A 24 volt hot (the R terminal), 24 volt common (the C terminal) and heating signal (the W terminal).

    Replace it all the way to the furnace without breaking it.

    If you are going to go through the work pull 4 or more wires. That will give you room for the future or if one wire gets damaged you have spares.

    I will always pull an 8 wire if possible.
  • Zman
    Zman Member Posts: 7,569
    Ther really isn't "code" with low voltage wiring. If you go with new 3 wire, the Red and White go where they are presently. The new common wire goes to the common side of the transformer, probably where the yellow wires are.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • njwags95
    njwags95 Member Posts: 15
    Thank you all, guess I was overthinking it. I will run 3+ wire and only connect white and red until I’m ready for smart thermostat
  • Jim100Flower
    Jim100Flower Member Posts: 102
    I would go with 4 wires or more. I recently installed 3 smart thermostats and, on advise of the manufacturer, installed a new transformer in the basement to power the thermostats. I have 4 wires to each thermostat and needed the extra two to run the power up to the thermostat. (I'm not sure but I think the transformer was required because I did not have a C wire and could not use their power extender kit.)
    By the way, I absolutely love my smart thermostats. I got the ecobee 5 because it included a remote sensor.
    HVACNUT Member Posts: 5,841
    The wires at the transformer are tagged. Connect Red and White where the existing thermostat wires are spliced with the wire nuts.
    Your Common at the thermostat connects to the wire labeled "C" at the transformer.