Hello all,

I'm trying to install a new Honeywell RTH8500 digital thermostat but can't figure out the wiring from the old thermostat as there are no labels. It's an older White-Rodgers one with mercury. We have forced air heating and central air conditioning.

Basically there are 4 coloured wires:
- a white one that is attached to two screws (NOT attached to the mercury)
- a blue one (attached to the mercury)
- a tan one (attached to the mercury) (Note: At first I thought it was pink, but everyone else says it looks more tan... just a consideration/detail I thought I'd mention)
- a yellow one (attached to the mercury)

Interestingly (or maybe not for those who know what they're doing), the only label is an "L" on a screw that has no wire. :confused: Am I totally hooped?

At first I assumed that blue = B, pink = P, yellow = Y and white = W, but not all of the letters are accounted for on the new thermostat and the instruction manual specifically says not to presume on the colour of the wire.

Think you can suggest something that won't burn down the house?

Oodles of thanks!

-aferroyt-

The most common is as follows: w-white is for heating, b-blue for common, R-RED 24 VOLT power, y-yellow for a/c, G-Green for indoor fan. You need to make sure the indoor air handler/furnace etc has the exact match as the t-stat wiring It does not matter what color the wire is as long as they are the same all the way through to the indoor unit. You can look there by removig the door there and look at the wiring there,just make sure you don't remove any wires there unless you know what you are doing. You can wire your t-stat according to how theindoor unit is wired.

Thermostat wire color codes explained.
Please note. Not all installers use the proper color codes when they install or replace equipment. It may be necessary go go to the furnace or outdoor unit to verify what wires are for what purpose. Also be sure there are no splices in the wire that could change the wire colors between components.

R (red) or Rh (Red heat) 24 volts from equipment.
Rc (Red cooling) 24 volts from transformer in cooling equipment.
Note: If only furnace has a transformer and cooling equipment does not jumper Rc and Rh.
W (White) sends 24 volts to furnace control to start the heating cycle.
W2 (no standardized wire color, usually whatever wire color is available) controls second stage heat.
Note: W2 is most often used for heat pumps to control what is called emegency heat or Auxiliary heat, and most often will use the white wire.
Y (yellow) often (blue) is used, controls cooling unit (outdoor condensing unit) also is used for heat pump heat.
Y2 (no standardized wire color, usually whatever wire color is available) controls second stage cooling.
G (Green) controls the fan "on" operation of the furnace/air handler. Also often is used to start the blower for many electric furnaces.
C (common) most often brown but can also be black or other color available to installer.
O (orange) Energizes heat pump reversing valve for cooling (Trane and most other brands).
B (Blue) Energizes heat pump reversing valve for heating (Rheem, Ruud and Weatherking).