Home thermostat wiring

Thread Starter


Joined Sep 9, 2010
Is it normal for system power to be connected to yellow instead of red?

Today I'm installing a new-fangled smart thermostat that my local power company sold me at a big discount. So I'm having to deal with the fact that there are all sorts of permutations of wiring systems. I'm sure @#12 has seen them all.

Starting at my old thermostat, I see I have a 4-wire system and that my "yellow" wire is actually blue. I can handle that.

My new thermostat requires a "C" wire for power and comes with a power extender to deal with a 4-wire system like mine. So down inside my furnace I find that my 4 wires are attached in the logical spots, color-to-color with blue acting as yellow. Still good.

But then I notice that power from the little transformer, white and red, is wired white to Blue/Common – makes sense – and red to Yellow at the furnace board. Huh? It's that Red-to-Yellow connection I don't get. Why isn't it Red-to-Red? I suppose I can continue without knowing, but it bugs me. I don't want to zap my "power extender" with a non-standard hookup that it doesn't expect.



Joined Nov 30, 2010
In air conditioning language:
Green is the interior fan command. If 24 VAC, then fan.
Yellow is the compressor command.
White is the first stage heat command. That's W1. If you have a second stage, that's W2.
Red is the 24 VAC supply.
C is, "common" of the 24 VAC supply, not, "cool".
Rc means, "Red/cool" and Rh means "Red/heat" for when you have two separate systems for heat and cool. If you only have one system, you jumper Rh to Rc and supply them both from one 24 VAC transformer.

Orange and blue (as labels on the equipment) are there to run the reversing valve on the Freon system. If you have a reverse cycle air conditioner, you reverse the Freon flow in the winter. If you have a heat pump, you reverse the Freon flow in the summer. They are rare enough around here that I never memorized whether it's blue or orange for which one. I just hook 'em up. If it runs backwards, it must be the other one.:D

The colors of the wires in the wall are OFTEN whatever was on the truck that day. The labels on the machine are God. If Joe Jaggoff used blue for "common" at the thermostat, believe the label, not the wire color. The first thing I do on a job with control problems is check to see what colors the installer used and memorize them.

Sorry about the 6 day delay. I was out with surgery this week.


Joined Nov 30, 2010
Welcome back, presume that surgery was successful
Home free now, but I was a total bitch for about a week.:(:mad::(
Nothing like pacing the floor in pain to make me grumpy!
That was a good post. I think I'll copy it to my blog because everybody knows I'm the A/C man.:)