Chapter 9 Electrical Wiring Diagram Basics 215
Copyright Goodheart-Willcox Co., Inc.
This terminal controls the coil of the heat relay, HR,
Figure 9-20. The thermostat’s internal temperature-sens-
ing switch, in series with the W terminal, closes on a drop
in temperature, initiating the heating cycle, Figure 9-21A.
The thermostat’s temperature-sensing switch, wired in
series with the Y terminal, closes upon a rise in tempera-
ture, thus initiating the cooling cycle.
Like a cooling-only thermostat, the fan switch on
a heating-cooling thermostat can be set to either the
ON or AUTO position. When in the ON position, the
indoor blower remains on regardless of the position of
the contacts between terminals R and W, Figure 9-21B.
When in the AUTO position, the indoor blower motor
circuit is isolated from the thermostat because the
blower is controlled by the temperature in the appli-
ance’s heat exchanger, Figure 9-21C.
9.4.2 Indoor Fan Controls on a
Ducted Heating and Cooling System
On a ducted heating and cooling system, there is one indoor blower
located in the air handler. The air handler is responsible for delivering air
in both the heating and cooling modes of system operation. Most often,
the blower operates at high speed in the cooling mode, and low speed in
the heating mode. The indoor blower should be operational in the heat-
ing mode only when the heat exchanger is warm. This is to prevent the
occupants of the conditioned space from experiencing cold drafts from
the duct system at the start and end of the heating cycle.
The strategy commonly used to control the motor under this set of circum-
stances is shown in Figure 9-22. In this wiring diagram, the speed of the motor
is determined by an indoor fan motor relay, IFR. The high-speed motor lead is
wired through a normally open set of contacts on the relay, and the low-speed
motor lead is wired through a set of normally closed contacts on the same relay.
Goodheart-Willcox Publisher
Figure 9-20. The thermostat of a heating-cooling system uses
the W terminal to control the heating mode of operation.
R
G IFR
Y CC
W HR
24 V
T-STAT
Goodheart-Willcox Publisher
Figure 9-21. A—Simplifi ed wiring diagram of a heating
and cooling thermostat. B—Fan switch in the ON position.
C—The thermostat does not control the indoor fan motor when
the system is in the heating mode and the fan switch is set to
AUTO.
A B C
R
Y
OFF
COOL
G ON
AUTO
HEAT W
R
Y
OFF
COOL
G ON
AUTO
HEAT W
R
Y
OFF
COOL
G ON
AUTO
HEAT W
Goodheart-Willcox Publisher
Figure 9-22. Control wiring for the indoor fan motor on a
heating and cooling system.
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 terminal controls the coil of the heat relay, HR,
Figure 9-20. The thermostat’s internal temperature-sens-
ing switch, in series with the W terminal, closes on a drop
in temperature, initiating the heating cycle, Figure 9-21A.
The thermostat’s temperature-sensing switch, wired in
series with the Y terminal, closes upon a rise in tempera-
ture, thus initiating the cooling cycle.
Like a cooling-only thermostat, the fan switch on
a heating-cooling thermostat can be set to either the
ON or AUTO position. When in the ON position, the
indoor blower remains on regardless of the position of
the contacts between terminals R and W, Figure 9-21B.
When in the AUTO position, the indoor blower motor
circuit is isolated from the thermostat because the
blower is controlled by the temperature in the appli-
ance’s heat exchanger, Figure 9-21C.
9.4.2 Indoor Fan Controls on a
Ducted Heating and Cooling System
On a ducted heating and cooling system, there is one indoor blower
located in the air handler. The air handler is responsible for delivering air
in both the heating and cooling modes of system operation. Most often,
the blower operates at high speed in the cooling mode, and low speed in
the heating mode. The indoor blower should be operational in the heat-
ing mode only when the heat exchanger is warm. This is to prevent the
occupants of the conditioned space from experiencing cold drafts from
the duct system at the start and end of the heating cycle.
The strategy commonly used to control the motor under this set of circum-
stances is shown in Figure 9-22. In this wiring diagram, the speed of the motor
is determined by an indoor fan motor relay, IFR. The high-speed motor lead is
wired through a normally open set of contacts on the relay, and the low-speed
motor lead is wired through a set of normally closed contacts on the same relay.
Goodheart-Willcox Publisher
Figure 9-20. The thermostat of a heating-cooling system uses
the W terminal to control the heating mode of operation.
R
G IFR
Y CC
W HR
24 V
T-STAT
Goodheart-Willcox Publisher
Figure 9-21. A—Simplifi ed wiring diagram of a heating
and cooling thermostat. B—Fan switch in the ON position.
C—The thermostat does not control the indoor fan motor when
the system is in the heating mode and the fan switch is set to
AUTO.
A B C
R
Y
OFF
COOL
G ON
AUTO
HEAT W
R
Y
OFF
COOL
G ON
AUTO
HEAT W
R
Y
OFF
COOL
G ON
AUTO
HEAT W
Goodheart-Willcox Publisher
Figure 9-22. Control wiring for the indoor fan motor on a
heating and cooling system.
L1 N
HIGH
LOW
RC
IFR
Indoor fan motor
Fan switch
R G
Space
thermostat
24 V
X-FRMR