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My thermostat is a LuxPro LUX PSP511LCa. I looked behind the thermostat and I see the following:

  1. A red wire connected to RC
  2. A jumper connecting RH and RC
  3. A white wire connected to W
  4. A green wire connected to G

I see no Y or C wires.

Now, I have a new Honeywell RTH6580WF WiFi thermostat. The instructions tell me I need an R, W, G, Y, and C cables.

I have a few questions:

  1. Can I assume that my current RC cable is the same as the R cable, so I just plug it into the R slot?

  2. What do I do about the C and Y cables? I don't have either. I read that Y is for cooling. My house has no cooling, nor do I want it. Can I just ignore the instructions that tell me to connect the Y cable?

Attached is a picture of the thermostat wiring:

ThermostatWiring

HeaterWiring

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  • Can you post photos of both ends of the thermostat wiring?
    – ThreePhaseEel
    Oct 17, 2019 at 11:44
  • Are there any unused wires? You don't need the Y, but if the thermostat requires power (C wire) you will need that.
    – JPhi1618
    Oct 17, 2019 at 16:40
  • There are no unused wires. I just uploaded images.
    – NL3294
    Oct 17, 2019 at 18:28
  • Correction, I notice an unused blue wire when I look at the picture in my heater. Does that pretty much mean there's a blue wire hidden behind my wall behind my thermostat?
    – NL3294
    Oct 17, 2019 at 18:35
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    Yes! You are lucky. Connect that dangling blue wire to C and look behind the thermostat. (hint: turn the system off first)
    – gbronner
    Oct 17, 2019 at 19:20

2 Answers 2

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You have * power(R) * heat (W) * fan (G)

(note that on some furnaces R and W are reversed).

Standard answer is that either:

  • you have more wires in the thermostat cable that are not connected behind the thermostat one of which you can use as a C wire

  • you have some the willingness and ability to pull more cable to pull a c wire

  • you can get a 24v transformer near the thermostat and wire it to Rc and C, and wire your thermostat to Rh, W, and G.

You won't need to use the Y wire.

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  • You know, when I look at my image on the heater, I notice that blue wire hanging out. But when I look at my thermostat, I don't see the blue wire. Does that mean that it's probably hidden behind the wall?
    – NL3294
    Oct 17, 2019 at 18:29
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    Often it is. Take the thermostat off the wall and check. Or go down to your furnace and look carefully at the cable and see if it is poking out (though some installers trim them off) Tug on the group of three wires and see if it comes out.
    – gbronner
    Oct 17, 2019 at 19:16
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Blue Wire is typically for Y - cooling White wire -W - Heat Green Wire is for G - the fan Rh & Rc are also abbreviated as R. These are jumped. Thermostat call for heat or cooling.

I don't have a C wire at all and my Emerson Sensi Wifi thermostat works fine. (I've validated my setup with Sensi tech support)

Are you trying to control heating & cooling or just heating ?

attached pic of my wiringenter image description here

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2
  • 1
    yours works via 'power stealing', which doesn't work all that well with some thermostats (e.g. I have one that combines a thermostat with an alexa). You can use an 'add-a-wire' kit to add a c wire.
    – gbronner
    Oct 17, 2019 at 19:19
  • @gbronner - correct.
    – Vasuvius
    Oct 17, 2019 at 19:30

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