Compliments of Harold Hydonicnetworks

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from a post last winter:
R), Red, hot side of transformer.
(C) Common side of transformer (See B)
(Y), Yellow, Compressor activity (cooling or cooling and heating on a heat pump).
(W), White, Heat (gas burner, oil burner , electric heat, (auxiliary heat on a heat pump including defrost output from the outdoor unit to activate electric heat and turn on the AUX. heat lamp).
(G), Green, furnace blower fan. (needed for air conditioning, heat pumps and some electric furnaces).
(O), Orange , Energize to cool (used for reversing valve on heat pumps)
(B), Blue or Orange, Energize to heat (used on some systems, Rheem/Ruud is notorious for this).
(B) or (X), Blue, brown or Black, common side of transformer. Needed on some electronic thermostats or if you have indicator lamps. Do not confuse with (B) Reversing valve (energize to heat) above. York and Trane like to use (B) as common.
(E), blue, pink, gray or tan, emergency heat relay on a heat pump. Active all the time when selected, usually not used.
(T), Tan or Gray, outdoor anticipator reset.
Used on GE/Trane/American Standard and some Carrier Products.
(W2), Pink or other color, second stage of heat (may be same as (W) on heat pump or fossil fuel system). Note: some thermostats require a jumper from W1 to Y for heat pump operation.
(Y2), Blue or Pink, second compressor stage.
(L), Blue, brown, tan or gray service indicator lamp.
Numbers, see manufacture's diagrams but can be stages of electric heat especially on York/Borg Warner/Coleman-Evcon, Frasier Johnson/ Air-pro.
(X2), Second stage of Electric Heat on GE/Trane/American Standard. See link above. Also used as indicator lamp or misc. contacts on other systems

Old Label_________New Label__Description
R, V-VR or VR-R ______ R_____ 24 VAC, Return
Y,Y1 or M____________ Y_____ Stage 1 Cooling/Heating Circuit
O or R ______________ 0______Reversing Valve, (Cooling Mode)
B __________________ B_____ Reversing Valve (Heating Mode)
F or G ______________ G_____ Fan control relay
Y2 _________________ Y2____ 2nd Stage Cooling Circuit
W1 or W2 or W-U _____ W2___ 2nd Stage Heating Control
L or X _______________ L _____ System Monitor LED
E __________________ E_____ Emergency Heating Circuit
C,X or B ____________ C_____ 24 VAC, Transformer Common Side
Luck, Kizmet