CEMP-E
TI 810-11
30 November 1998
low-signal selector RHY. Low signal selection is performed in software.
Figure 3-13. Humidifier control loop.
10. THE TYPICAL SCHEMATIC.
a. The integration of standard control loops into a standard system starts with a schematic. A typical
schematic for single-loop controller applications is shown in figure 3-14 and for DDC applications is
shown in figure 3-15.
Figure 3-14. Typical single-loop controller schematic.
Figure 3-15. Typical DDC schematic.
b. Figure 3-14 shows the control loops arranged around an airflow diagram. When showing the
schematic, the designer will:
(1) Label all HVAC equipment.
(2) Label each control device with a unique identifier.
(3) Label the action (NC or NO) of all valves, dampers, and other appropriate devices.
(4) Label the action of all controllers as direct-acting DIR or reverse-acting REV.
(5) Label the input of all controllers (PV or CPA).
(6) For each device that operates contacts, show a line number on which each contact will
appear on a ladder diagram.
(7) For each relay contact, show the line number of a ladder diagram on which the relay
operating coil will appear.
(8) Show the location of all instruments not located in the flow stream or in the HVAC control
panel.
(9) Show a graphic representation of sequencing operations with open and closed positions
versus controller output and space temperature.
11. THE TYPICAL LADDER DIAGRAM.
a. When all the information necessary for a description of the system is not shown on the schematic,
a ladder diagram will be required for single-loop controller applications. As the logic is performed in
software, no ladder diagram is required for DDC applications.. A typical ladder diagram is shown in figure
3-16.
Figure 3-16. Typical ladder diagram.
3-13
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