Remove the old thermostat’s front cover from the wall base. Some covers pull off easily, while others
may need to be released by using a screwdriver.
CAUTION
our old thermostat may have a sealed glass tube containing mercury. Be careful not to damage the
tube or dispose of the tube in your trash. For safe disposal information, please see Mercury Notice on
page 36.
Label wires
Tip: Taking a picture with a camera or smartphone can help you not only remember how wires are
connected to the terminals, but can also ensure that you label your wires correctly. Mislabeling the
wires may result in a high energy bill or damage to your heating and cooling system.
For each wire that comes out of wall, follow these 4 steps.
Note that this step is very important for configuring your system in section 4.5. Please follow
instruction carefully.
1. Locate the terminal letter that the wire is connected to in the first column of the table
2. Write down the old thermostat terminal letter in the second column
3. Label that wire with the corresponding letter in the third column
4. Disconnect the labeled wire carefully, and proceed to the next wire until all wires are labelled
Please note that not all terminals may be used and that there’s no standard color code for thermostat
wires, so your wire colours may vary. For your reference, we’ve included a terminal label reference
chart below to help you connect the wires from your old thermostat to your new thermostat in case
you get stuck.
Terminal labelling reference chart
If your current terminal has the following letter: Write down that letter here: Label the wires
with the following letters: Terminal function
RH, R, R5, 5: — : RH: 24V Power (Heating)
RC: — : RC 24V: Power (Cooling)
C, X, B*: — : C: Common Wire
W, W1, W/E, Aux/E, E: — : W/E: 1st Stage Heat (for conventional system); 1st Stage Auxiliary Heat
(for heat pump system)
Terminal labelling reference chart, contʼd.
If your current terminal has the following letter: Write down that letter here: Label the wires
with the following letters: Terminal function
W2**: — : W2: 2nd Stage Heat (Conventional) 2nd Stage Auxiliary
Heat (Heat Pump)
Y, Y1: — : Y: 1st Stage Compressor
Y2: —: Y2: 2nd Stage compressor
G: — : G: Fan Relay
O, B*, O/B: — : O/B: Reversing Valve (for heat pump systems)
L: — : L: Heat Pump Diagnostic
Do you have a “B” wire?
Labeling the “B” wire can be tricky. It could be labeled as “C” or “O/B” depending on whether you
have an “O” wire connected to your old thermostat. Please pay attention and follow the asterisk (*)
comments below while labeling.
* Label “B” as “C” only if the old thermostat also had a wire in the “O” terminal block. If there was no
wire in the “O” terminal block, label the wire as “O/B”
** On heat pump systems with separate W2 and E wires, label both wires W/E (2 wires on one
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