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Abbreviations
HVAC: Heating, Ventilation and Air
Conditioning system
HP: Heat Pump System. This type of HVAC
system uses a compressor with a reversing valve
to provide both heating and cooling capability.
HE: Electrically heated central heating system.
This system uses an electric heating element
along with a fan and ducting. Sometimes used as
AUX/Emergency heat with a heat pump system.
Electric baseboard heaters are not included in this
system type.
HG: Fuel-fired central heating system. This system
uses a gas or oil burner along with a fan and
ducting or a hot water baseboard system. Can be
used as a standalone heating system or as AUX/
Emergency heat with a heat pump system.
O/B: This option selects the type of reversing
valve for heat pump systems. An “O” reversing
valve is energized when cooling and a “B”
reversing valve is energized when heating. Most
systems use an O type reversing valve. Rheem
and Ruud typically use a B type reversing valve.
If you are unsure, contact your Heat Pump
manufacturer.
Terminal Assignments
For reference, standard wiring assignments for
typical HVAC systems are provided below.
Terminal
(HP)
Terminal
(Non-HP)
Heat Pump
System
(HP)
Conventional
System
(Non-HP)
C
C
24VAC common for control
circuit and thermostat power
input
L
L
System
monitor
Not used
G
G
Fan
Y1
Y1
1st stage
compressor
1st stage
cooling
Y2
Y2
2nd stage
compressor
2nd stage
cooling
RC
RC
24VAC power supply for cooling
side
RH
RH
24VAC power supply for heating
side
O/B
W1
Changeover
valve
1st stage
heating
E
W2
AUX/
Emergency
heat
2nd stage
heating
