Hi. I'm trying to install a Honeywell RTH7600 programmable thermostat. I've detached my old thermostat, and I'm trying to figure out how to match up the wires. It's heat and air conditioning, and the old thermostat was a pretty basic, older round variety (also honeywell).

Problem is that I have a red, a white, a green and a blue wire. I can't figure out where the blue wire goes. Does it attach to C (I saw that on another post on this site).

Any help would be MUCH appreciated. I'm in Chicago and it is FREEZING here, and I don't think I'll be able to get the old thermostat reattached (the wire stubs are too short and my screwdriver doesn't fit the screws to take off the plate!)

Wires from wall

R -> R
W -> W
G -> G
B -> ??

New thermostat:

R (jumpered to RC)
W
G
C
Y

Thanks in advance!

The blue wire will connect to the "Y" terminal on the new thermostat.

Hmmm. Well, I attached it to the C terminal and the heat at least seems to be working. What does the Blue and Y terminals do? Are you sure that it's Y and not C?

Here (http://wiki.xtronics.com/index.php/Thermostat_signals_and_wiring) is the generally accepted convention for colors and terminals. It would be common sense for the previous person to wire according to that standard, but common sense isn't as common as you'd think.

Trace down that blue wire then you'll have your answer or repost with what you found.

So I'm not quite sure what I am supposed to look for on that page (or "trace") that would give me my answer. That site does not suggest anything for a blue wire except to go to Y2 for a heat pump, which I don't think I have (although I am guessing).

I am getting more confused now. Maybe if someone could explain to me what each of the 4 wires does?

So I'm not quite sure what I am supposed to look for on that page (or "trace") that would give me my answer. That site does not suggest anything for a blue wire except to go to Y2 for a heat pump, which I don't think I have (although I am guessing).

I am getting more confused now. Maybe if someone could explain to me what each of the 4 wires does?

It also lists blue as Sometimes common side of transformer. Try to follow that blue thru the wall or and on to wherever it goes, or go open your air handler or furnace and see if the blue appears there.

It would also be helpful to know what we are controlling: furnace only, furnace and A/C, boiler, etc. Make and model too.

So since the blue wires are only listed under heat pump, does that mean I have a heat pump? How would I figure that out?

I mentioned that it is heating and air conditioning, but I don't know much more than that (I'm not at home right now to check out a model #).

It's gas, and it heats a large single-person apartment (my landlord is a deadbeat - that's why i am installing a thermostat myself without even knowing what kind of furnace I have). The old thermostat looked somewhat like this one:

http://www.inspect-ny.com/heat/Thermostat_Honeywell537-DFs.jpg

It had Auto/Fan On and Heat/Cool/Off settings. Mercury tube inside.

The furnace itself was replaced recently, but I am pretty sure the thermostat is fairly old. I wouldn't trust that whoever installed the new furnace used proper techniques or standard wire colors (again, the landlords don't really care and have pretty sloppy, slap-dash repairmen on staff).

I can't trace the wires into the wall very far, but it's possible I'd be able to see where they connect to the furnace when I get home.

Does any of that help?

Thanks a lot!

Understood, and because this is not your property I would suggest you find out where that blue wire goes. Just so you know if you connect it up wrong you could blow the furnace transformer. Usually not an expensive fix but not something you want to get into, especially since its not your equipment.

Let us know what you find.

Hook the blue wire to the "Y" terminal and be done. Years ago they made a 4 wire cable that contained red,white,green and blue wires

Red-24 volts (power)
Green-Blower
Blue- air conditioning
white-heat.

Great - thanks guys! I'll connect it to the Y. So what is the C terminal that I have it connected to currently?

C is common. Between C and R there will be 24 V all the time. "C" allows a thermostat to not relay on batteries and just use the batteries for backup. Mercury thermometers did not need the "C" terminal.

If you go back to the page I posted you'll see a diagram. At the top is a transformer to drive the thermostat. C is the common/neutral from a 24VAC transformer and R is usually the hot side. As mentioned some thermostats need power to drive their electronics, but your tstat use 24VAC, battery or both. Since you have no common you must power it with batteries.