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
(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
Figure 3-16. Typical ladder diagram.