I've recently moved into my first house and am looking to replace the existing basic thermostat in the home. Here is a picture of how I found it wired:

enter image description here

Researching the wires, I've read that commonly W2 is suppose to be white and blue wire is usually C (Common); and that W2 is used in 2 stage heating systems. So now I'm concerned if this old thermostat was not wired properly? Or if my wire colors are just uncommon...

I'm unsure if my furnace (Amana GUIS070CA35) is actually two stage or not. When I get the new thermostat should I just plug the blue wire into C and see what happens? The thermostat I'm looking at would use the 24V common wire to power the display. So I'd guess if it didn't power up that the blue wire was actually for W2.

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    I'd start by looking at your furnace and seeing where the wires are plugged into - take a picture of that end as well. It's usually fairly easily accessible, and the terminals should be labeled on there as well. It wouldn't hurt to turn off the breaker while doing so. – Aaron Oct 3 '12 at 16:16
  • Pics of the furnace wiring: imgur.com/a/YzJZg So I think I'm just an idiot, as the plate says "two stage heating". So this must mean the thermostat is currently wired correctly and the W2 is indeed for the two-stage heating. – WhiteZero Oct 3 '12 at 22:44
  • It sounds like it is 2 stage, but these aren't the control board - it should have printed on it the same letters, and should be where the thermostat wire actually terminates... – Aaron Oct 4 '12 at 1:09

Almost every thermostat installation manual I've read always says something along the lines of "Make sure to mark you wires based on the currently connected thermostat. Do not rely on the wire coloring".

The colors are really just a guideline, there is no requirement to adhere to them.

Looking at the controller board of your furnace should confirm how they are connected. It's possible that the blue wire isn't even connected to anything on the furnace side. Perhaps the furnace was replaced at some point and the thermostat wasn't re-wired.

It's not uncommon to not have a C wire as older thermostats were either unpowered or powered by batteries. New thermostats (especially the ones with LCD screens, Wifi, etc.) have larger power requirements and rely on the C wire to provide a return path for power coming from the R terminal.


Upon further inspection of my furnace, it does indeed appear that it's two-stage heating. This should mean that the blue wire connected to W2 is indeed normal.

Furnace picture

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