216 Print Reading for HVACR
Copyright Goodheart-Willcox Co., Inc.
This wiring strategy prevents the two motor speed leads from becom-
ing energized at the same time. The coil of the IFR relay is controlled by the
G terminal on the space thermostat. If the fan switch on the thermostat is
set to the ON position, the R-G contacts on the thermostat close, and the
IFR coil becomes energized, Figure 9-23A. This causes the normally open
contacts on the relay to close, the normally closed contacts on the relay to
open, and the motor to operate at high speed, Figure 9-23B. The indoor fan
motor will also operate at high speed if the fan switch is set to the AUTO
position and the thermostat is set to COOL, as long as the temperature in
the conditioned space is above the thermostat’s set point.
If the R-G contacts on the thermostat are not
closed, the normally closed set of contacts on the IFR
relay remain closed and pass power to the low-speed
motor lead if the fan switch contacts are closed. The
fan switch is positioned in the air handler near the
heating system’s heat exchanger, so these contacts only
close when the heat exchanger is hot.
In operation, on a call for heat, the heat source
energizes, but the heat exchanger is still too cool to
allow the indoor blower to operate. After some time,
the temperature in the heat exchanger rises to a level
that causes the fan switch to close. This allows the
blower to cycle on in low speed, Figure 9-24. When
there is no longer a call for heating, the heat source de-
energizes. Because the heat exchanger is still warm, the
fan continues to operate and dissipate heat from the air
handler. Once the heat exchanger has cooled to the set
point on the fan switch, the fan switch’s contacts open
and the fan cycles off. If the equipment owner turns
the fan switch on the thermostat to the ON position
during the heating cycle, the R-G contacts on the ther-
mostat close, and the blower operates at high speed.
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR
A
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR
B
Goodheart-Willcox Publisher
Figure 9-23. A—When the G terminal on the thermostat is energized, the IFR relay coil is energized. B—The normally open set of
contacts on the IFR relay close to allow the fan motor to operate at high speed.
Goodheart-Willcox Publisher
Figure 9-24. In the heating mode, the increased temperature
of the heat exchanger closes the fan switch contacts and allows
the indoor fan motor to operate at low speed.
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR
Copyright Goodheart-Willcox Co., Inc.
This wiring strategy prevents the two motor speed leads from becom-
ing energized at the same time. The coil of the IFR relay is controlled by the
G terminal on the space thermostat. If the fan switch on the thermostat is
set to the ON position, the R-G contacts on the thermostat close, and the
IFR coil becomes energized, Figure 9-23A. This causes the normally open
contacts on the relay to close, the normally closed contacts on the relay to
open, and the motor to operate at high speed, Figure 9-23B. The indoor fan
motor will also operate at high speed if the fan switch is set to the AUTO
position and the thermostat is set to COOL, as long as the temperature in
the conditioned space is above the thermostat’s set point.
If the R-G contacts on the thermostat are not
closed, the normally closed set of contacts on the IFR
relay remain closed and pass power to the low-speed
motor lead if the fan switch contacts are closed. The
fan switch is positioned in the air handler near the
heating system’s heat exchanger, so these contacts only
close when the heat exchanger is hot.
In operation, on a call for heat, the heat source
energizes, but the heat exchanger is still too cool to
allow the indoor blower to operate. After some time,
the temperature in the heat exchanger rises to a level
that causes the fan switch to close. This allows the
blower to cycle on in low speed, Figure 9-24. When
there is no longer a call for heating, the heat source de-
energizes. Because the heat exchanger is still warm, the
fan continues to operate and dissipate heat from the air
handler. Once the heat exchanger has cooled to the set
point on the fan switch, the fan switch’s contacts open
and the fan cycles off. If the equipment owner turns
the fan switch on the thermostat to the ON position
during the heating cycle, the R-G contacts on the ther-
mostat close, and the blower operates at high speed.
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR
A
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR
B
Goodheart-Willcox Publisher
Figure 9-23. A—When the G terminal on the thermostat is energized, the IFR relay coil is energized. B—The normally open set of
contacts on the IFR relay close to allow the fan motor to operate at high speed.
Goodheart-Willcox Publisher
Figure 9-24. In the heating mode, the increased temperature
of the heat exchanger closes the fan switch contacts and allows
the indoor fan motor to operate at low speed.
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR