CETP Instructor PowerPoint Template 2.0

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Reading Electrical Control Circuit Diagrams
In the course of repairing a gas appliance,
technicians must routinely refer to electrical
circuit diagrams. In this module, you will learn
to

• Identify basic circuit diagrams
• Identify a timer bar chart (timer cycle)
• Apply your knowledge to reading circuit diagrams

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Identifying basic circuit diagrams
The Heating, Ventilation and Air Conditioning
(HVAC) industry applies four basic rules when
drawing circuit diagrams.
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Connecting or Component Diagram
Connection diagrams, sometimes called component
location diagrams, use symbols for various parts
of the control system. The parts of the control
system are located on a piece of paper in much
the same relative position as they are used in
the equipment.
Connection diagrams

• give component wiring instructions
• aid in identifying components

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Ladder Diagram
Ladder diagrams use symbols for the component
parts. Wires connect the symbols to their
control units as simply as possible. Wiring is
arranged as follows The leads from the power
source are drawn vertically on the paper much as
the "side rails" of a ladder are located. The
neutral wire in a ladder diagram is indicated by
the notation L2. The hot wire is indicated by the
notation L1. Then each individual circuit is
placed crosswise as the "rungs" of a ladder would
be.
Figure 2. Ladder Diagram
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Unlike the component diagram, the ladder diagram
does not show the actual location of components.
Ladder diagrams are useful in determining
sequence of operation and how loads are activated
in a circuit. Notice that in HVAC ladder
diagrams, the step down transformer is the
central component and divider between high
voltage load circuits and lower voltage control
circuits. Ladder diagrams graphically
illustrate the requirements for safe current
flow.
Figure 4. Simple Circuit Ladder
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Identifying a timer bar chart (timer cycle)
Time bar charts are true sequence-of-operation
control diagrams. Also caller timer cycle
charts, these diagrams are commonly used to
communicate sequence of operations for clothes
washers, dryers and dishwasher. Unlike circuit
diagrams which provide wiring and wiring
connection information, time bar charts give
technicians the means to troubleshoot a series of
equipment systems that are controlled through a
timed sequence.
Figure 4. Time Bar Chart (Timer Cycle)
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Applying your knowledge to reading circuit
diagrams
Figure 5. Standing Pilot Furnace Parts Diagram
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Figure 6. Standing Pilot Furnace Connection and
Ladder Diagrams
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Circuit Diagram Worksheet-Condensing Furnace
Figure 7. Condensing Furnace Parts Diagram
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Figure 9. Condensing Furnace Connection and
Ladder Diagrams
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Circuit Diagram Worksheet I-Heating/Cooling
Packaged Unit
Figure 10. Unit Heating Section
Figure 13. Gas Valve Burner
Figure 11. Unit Side View
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Figure 14. Heating/Cooling Packaged Unit
Connection and Ladder Diagrams
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Figure 15. Heating/Cooling Packaged Unit
Connection and Ladder Diagrams
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Time to See If You Got the Key Points of This
Module

• Complete the Review on page 21.
• See if you are ready for the Certification Exam
  by checking off the performance criteria on page
  22.