214 Print Reading for HVACR
Copyright Goodheart-Willcox Co., Inc.
9.4.1 Thermostat
The thermostat is the operational control interface
between the air-conditioning system and the system
operator. For most applications, the space thermostat
is a low-voltage device, which controls the operation
of the main system components. The operator sets
the thermostat to the desired mode of operation and
the desired temperature set point. The system then,
if properly wired and confi gured, operates based on
these settings.
On a simple cooling-only system, the thermostat
has three terminals: R, G, and Y. The R terminal is
powered by the transformer, the G terminal ener-
gizes and de-energizes the indoor fan relay coil, IFR,
and the Y terminal energizes and de-energizes the
compressor contactor coil, CC, Figure 9-17. A typi-
cal wiring diagram for the low-voltage control cir-
cuit on a split air-conditioning system is provided in
Figure 9-18.
The main selector switch has two mode settings:
COOL and OFF. The operator can also select between
two fan settings: ON and AUTO. When in the ON
position, the evaporator blower motor remains on
regardless of the position of the main selector switch.
In the AUTO position, the evaporator blower motor
cycles on and off with the compressor. The thermo-
stat must also be set to the desired space temperature.
When the system is set to COOL, the system cycles on
and off to maintain the proper temperature setting on
the thermostat. When the indoor blower is in cooling
mode, the G terminal on the thermostat is powered,
and the IFR coil in the indoor unit is energized. Vari-
ous thermostat positions are shown in Figure 9-19.
In a heating-cooling system, the thermostat has an additional termi-
nal that controls the heating mode of operation, called the W terminal.
Goodheart-Willcox Publisher
Figure 9-17. Typical thermostat terminals on a simple cooling-
only air-conditioning system.
24 V
R
G
Y
IFR
CC
T-STAT
Goodheart-Willcox Publisher
Figure 9-18. Low-voltage control wiring for a simple, cooling-
only split air-conditioning system.
Indoor unit Outdoor unit
Thermostat
G Y R
IFR
CC
R
G
Y
C
Compressor circuit
Fan circuit
AUTO
A B C D E F
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
Goodheart-Willcox Publisher
Figure 9-19. Various internal thermostat connections. A—System: OFF, Fan: ON. B—System: OFF, Fan: AUTO. C—System: COOL,
Fan: ON, thermostat calling for cooling. D—System: COOL, Fan: AUTO, thermostat calling for cooling. E—System: COOL, Fan: ON,
thermostat not calling for cooling. F—System: COOL, Fan: AUTO, thermostat not calling for cooling.
Copyright Goodheart-Willcox Co., Inc.
9.4.1 Thermostat
The thermostat is the operational control interface
between the air-conditioning system and the system
operator. For most applications, the space thermostat
is a low-voltage device, which controls the operation
of the main system components. The operator sets
the thermostat to the desired mode of operation and
the desired temperature set point. The system then,
if properly wired and confi gured, operates based on
these settings.
On a simple cooling-only system, the thermostat
has three terminals: R, G, and Y. The R terminal is
powered by the transformer, the G terminal ener-
gizes and de-energizes the indoor fan relay coil, IFR,
and the Y terminal energizes and de-energizes the
compressor contactor coil, CC, Figure 9-17. A typi-
cal wiring diagram for the low-voltage control cir-
cuit on a split air-conditioning system is provided in
Figure 9-18.
The main selector switch has two mode settings:
COOL and OFF. The operator can also select between
two fan settings: ON and AUTO. When in the ON
position, the evaporator blower motor remains on
regardless of the position of the main selector switch.
In the AUTO position, the evaporator blower motor
cycles on and off with the compressor. The thermo-
stat must also be set to the desired space temperature.
When the system is set to COOL, the system cycles on
and off to maintain the proper temperature setting on
the thermostat. When the indoor blower is in cooling
mode, the G terminal on the thermostat is powered,
and the IFR coil in the indoor unit is energized. Vari-
ous thermostat positions are shown in Figure 9-19.
In a heating-cooling system, the thermostat has an additional termi-
nal that controls the heating mode of operation, called the W terminal.
Goodheart-Willcox Publisher
Figure 9-17. Typical thermostat terminals on a simple cooling-
only air-conditioning system.
24 V
R
G
Y
IFR
CC
T-STAT
Goodheart-Willcox Publisher
Figure 9-18. Low-voltage control wiring for a simple, cooling-
only split air-conditioning system.
Indoor unit Outdoor unit
Thermostat
G Y R
IFR
CC
R
G
Y
C
Compressor circuit
Fan circuit
AUTO
A B C D E F
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
AUTO
R
Y
OFF
COOL
G ON
Goodheart-Willcox Publisher
Figure 9-19. Various internal thermostat connections. A—System: OFF, Fan: ON. B—System: OFF, Fan: AUTO. C—System: COOL,
Fan: ON, thermostat calling for cooling. D—System: COOL, Fan: AUTO, thermostat calling for cooling. E—System: COOL, Fan: ON,
thermostat not calling for cooling. F—System: COOL, Fan: AUTO, thermostat not calling for cooling.