This article applies to the EMS Si Thermostat.
Thermostat to HVAC Equipment
18 AWG 20 AWG 22 AWG
128 ft/380 m 80 ft/240 m 50 ft/150 m
Terminal Description and Electrical Ratings
Terminal Description Voltage Current Max
N/C Unused N/A N/A
Y 1st stage compressor 24VAC 2A
W (O/B) 1st stage heating 24VAC 2A
or reversing valve
G Fan 24VAC 2A
RC Cool transformer * 24VAC 2A
RH Heat transformer * 24VAC 2A
C Common 24VAC 2A
Y2 2nd stage compressor 24VAC 2A
W2 (AUX) 2nd stage heating or 24VAC 2A
1st stage auxiliary heat
R1+ Input 1+ Temperature/dry contact only
R1- Input 1- Temperature/dry contact only
R2+ Input 2+ Temperature/dry contact only
R2- Input 2- Temperature/dry contact only
* Factory jumper installed between RC and RH. Remove jumper for 2-transformer applications. See wiring diagrams for additional information.
1-stage Furnace with 1-stage AC
1-stage heating & cooling system.
1-stage Furnace with 2-stage AC
Connection diagram for 1-stage furnace with 2-stage cooling.
1-stage Heat Pump with 1-stage Aux Heat
Connection diagram for 1-stage heat pump with 1-stage auxiliary heat
The wiring diagram illustrates the connections for a 2-stage heat pump with 1-stage auxiliary heat.
2-stage Boiler with 1-stage AC
Connection diagram for a 2-stage boiler with 1-stage AC.
2-stage Boiler with 2-stage AC
A connection diagram for a 2-stage heating boiler with 2-stages of AC.
1-stage Boiler with 1-stage AC
The diagram illustrates the connections for a 1-stage heating boiler with 1-stage A/C and air handler.
2-stage Furnace with 1-stage AC
Wiring diagram for 2-stage furnace with 1-stage AC
2-stage Furnace with 2-stage AC
Wiring connections for a 2-stage furnace with 2-stages of AC