Details of system? Furnace, condenser and thermostat make and model numbers? Some older thermostats have odd labelling.

2-stage heat or cooling?

The color of the wire isn't half as important as to where, how, or why it is connected.

If I buy a 2 wire --- there's red and white.

If I buy a 3 wire --- red, white & green

If I buy a four wire --- there's red, white, green, yellow.

If I buy a five wire --- there's red, white, green, yellow, blue.

If I buy a 8 wire ------- there's red, white, green, yellow, blue, orange, brown, and black.

If there is no reversing valve on the air conditioner: it's not a heat pump and you can not reverse the flow no matter what kind of concocted scheme of wire you attempt.

If you have a two stage AC --- what color of wire are you going to use to control second stage? pick one... if you don't have one to pick then you're pulling new wire.

Remember COLOR of wire is only differentiating the wire so you hopefully know that you are on the same wire at another terminal to connect it properly.

Every house is different. Some have B wires some don't. Some have B wires with no place to go. Some have B wires and don't care that they are B wires.

I can think of two possibilities. The condenser you have is a heat pump and it was never set up as dual fuel system. Or the house previously had a heat pump but at at some point a gas furnace was installed. Whoever installed the thermostat connected the B wire not thinking it would no longer be needed.

I'm hoping to get up there this afternoon to get the model, and look at the control unit.

This is a 1967 house. Oddly, it was built in Las Vegas without AC--we replaced the original furnace in 2009, and found a add-on AC unit bolted on, and a rafter cut to fit it. The ducts were not upgraded from 18" to 24". (and the incompetent of the installation of the replacement work is its own storyi, featuring the combination of a separate duct to each bedroom from the plenum, compounded by leaving the duct to the front half of the house turned sideways and blocked by the installation [put a bunch of those yellow guys screaming in horror here]).

Anyway, what has led to me wondering about this is the apparent failure of the control module (?) on the attic unit. We had been blaming the thermostat, but after replacing, further investigation found that shorting R to G will sometimes turn the fan on, and sometimes not. The new thermostat turns on the burner, but usually not the fan (it did once). The fan has also refused to shut off entirely three times. Pulling the thermostat didn't do it; I had to kill the breaker.

This is what led to my interest in B: do I really have four used wires, or five. If blue was only attached to B to not be loose, I can use it for C on a replacement thermostat.

(for that matter, why do they use C rather than just drawing from R, at least on units that don't have a separate Rc?

mm, another note: the reason that this was such a sudden discovery is that it had been a while since we used the AC--we have a large, central swamp cooler with a 1 HP motor. Since using a panel to change the flow so that it didn't all go into the family room, it's meen enough for most of a desert summer. We actually got as high as 117F while only 82 in the house on a 2% humidity day. (We switch to AC once it hits 83)

thanks

hawk

hmm, where did my later post go???

When I got to the attic and took the cover off, the brand wasn't promising:

But it answered the

question:

As seen within the black circle, the blue wire is clipped off and not attached to anything.

So it's usable for C if I get a different thermostat.

thanks to all.

I've started a separate thread at about fixing it.