206 Print Reading for HVACR
Copyright Goodheart-Willcox Co., Inc.
The wires are also color-coded in the circuit, which is helpful to the
fi eld technician. It is much easier for a technician to try to locate a red
wire in a bundle of multicolored wires than trace out a black wire in a sea
of other black wires. Because every wire is accounted for in the schematic
diagram, a technician can fi nd wiring errors more easily.
9.2.3 Ladder Diagrams
When HVACR systems become more complex and
include many wires, it is often diffi cult to read sche-
matic diagrams. To simplify the diagram, a ladder
diagram is used. Ladder diagrams represent each indi-
vidual circuit in the system on a separate line. These
diagrams allow technicians to easily locate a com-
ponent on the diagram and determine all the system
control or switching devices that control its operation.
This is useful for troubleshooting when a system com-
ponent is not operating correctly. Most manufacturers
provide both a schematic and a ladder diagram for
equipment.
An example of a ladder diagram is shown in
Figure 9-8. The power supply, in this case L1 and N,
is represented as the vertical rails of the ladder. The
control devices are located on the horizontal rungs
on the ladder, as are the power-consuming devices.
Each power-consuming device is typically located
on its own line or rung. As more circuits are added
to the system, more rungs are added to the ladder
diagram.
In the ladder diagram shown, several observations can be made:
■ The indicator light, IL, is controlled by only the manual switch, MS.
■ The heater, HTR, is controlled by only the heating thermostat,
STAT.
■ The pump, P, is controlled by a set of normally open contacts, A-B.
■ The blower motor, BM, is controlled by both the pressure switch,
PS, and the limit switch, LIM. These switches are wired in series
with each other. For the blower motor to be energized, both the
pressure switch and the limit switch must be in the closed position.
9.2.4 Sequence of Operations
Ladder diagrams are used to troubleshoot a system device because they
allow the technician to easily determine the system’s sequence of opera-
tions. The sequence of operations is the order of events that occur to
activate a device. Understanding the sequence of operations for a system
helps a technician effectively evaluate and troubleshoot a system.
Goodheart-Willcox Publisher
Figure 9-8. An example of a ladder diagram.
A B
BM
IL
HTR
P
STAT
LIM
MS
PS
L1 N 120 VAC, single-phase
Copyright Goodheart-Willcox Co., Inc.
The wires are also color-coded in the circuit, which is helpful to the
fi eld technician. It is much easier for a technician to try to locate a red
wire in a bundle of multicolored wires than trace out a black wire in a sea
of other black wires. Because every wire is accounted for in the schematic
diagram, a technician can fi nd wiring errors more easily.
9.2.3 Ladder Diagrams
When HVACR systems become more complex and
include many wires, it is often diffi cult to read sche-
matic diagrams. To simplify the diagram, a ladder
diagram is used. Ladder diagrams represent each indi-
vidual circuit in the system on a separate line. These
diagrams allow technicians to easily locate a com-
ponent on the diagram and determine all the system
control or switching devices that control its operation.
This is useful for troubleshooting when a system com-
ponent is not operating correctly. Most manufacturers
provide both a schematic and a ladder diagram for
equipment.
An example of a ladder diagram is shown in
Figure 9-8. The power supply, in this case L1 and N,
is represented as the vertical rails of the ladder. The
control devices are located on the horizontal rungs
on the ladder, as are the power-consuming devices.
Each power-consuming device is typically located
on its own line or rung. As more circuits are added
to the system, more rungs are added to the ladder
diagram.
In the ladder diagram shown, several observations can be made:
■ The indicator light, IL, is controlled by only the manual switch, MS.
■ The heater, HTR, is controlled by only the heating thermostat,
STAT.
■ The pump, P, is controlled by a set of normally open contacts, A-B.
■ The blower motor, BM, is controlled by both the pressure switch,
PS, and the limit switch, LIM. These switches are wired in series
with each other. For the blower motor to be energized, both the
pressure switch and the limit switch must be in the closed position.
9.2.4 Sequence of Operations
Ladder diagrams are used to troubleshoot a system device because they
allow the technician to easily determine the system’s sequence of opera-
tions. The sequence of operations is the order of events that occur to
activate a device. Understanding the sequence of operations for a system
helps a technician effectively evaluate and troubleshoot a system.
Goodheart-Willcox Publisher
Figure 9-8. An example of a ladder diagram.
A B
BM
IL
HTR
P
STAT
LIM
MS
PS
L1 N 120 VAC, single-phase