Furnace Thermostat wiring help

Discussion in 'HVAC Heating & Cooling' started by plumber69, Oct 2, 2019.

  1. plumber69

    plumber69 In the Trades

    Joined:
    Jun 20, 2014
    Location:
    Prince Rupert, British Columbia
    Theres 4 wires hooked up to the furnace
    R
    W
    G
    COM 24

    Below is a picture of the thermostat.
    What wire goes where?
     

    Attached Files:

  2. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    And the model number for that thermostat is...?

    [​IMG]

    [​IMG]

    WiFi and some smart thermostats need the 24 AC common, most simpler thermostats don't.
     
    Last edited: Oct 3, 2019
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  4. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    C common would not be used by a conventional thermostat, but is used by new electronic thermostats. If you can run the wires now, you will be ready later if you go electronic on the thermostat.
     
  5. plumber69

    plumber69 In the Trades

    Joined:
    Jun 20, 2014
    Location:
    Prince Rupert, British Columbia
    Does the Com 24 go to the C on the thermostat.
     
  6. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Yes. I did not notice the C on your thermostat initially.

    The C and R have the 24 vac output of the transformer.
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    That's right.

    It's not clear without seeing the documentation of the thermostat model whether it's actually needed (some vendors use the same back plate for a range of thermostat models), but that's where it goes.
     
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Smart thermostats need some sort of power. Many get that from the use of batteries. Some of the newer ones tend to use more power than batteries can readily supply, at least with a decent life, without replacement. A thermostat is basically just a fairly simple switch, and just like your wall switch to turn a light on/off, doesn't need power to make it function - temperature makes the switch open/close in a really simple one (electronic ones may use a solid state temperature sensor, but that takes power). A WiFi enabled thermostat usually will need additional power than what a battery can provide. To have useable power at the thermostat, you need the common to complete the circuit. There are ways to siphon off a bit of power on some designs without the common, but you can't get a lot that way. For the HVAC functionality, power comes in, goes to the various switches, and when one or more of them turns on, it sends it back to the HVAC system where it then activates something, completing the circuit. Without common wire at the thermostat, there's no complete circuit to power the electronics of the thermostat unless it was designed with batteries or to parasitically get some while supplying power to something in the HVAC system.
     
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