Handbook of air conditioning and refrigeration / Shan K

23.28 CHAPTER TWENTY-THREE
Diagnostics
FIGURE 23.10 Dew point control.
The system diagnostics automatically monitors the operation of all workstations, modems, LAN
connections, control panels, and controllers. The failure of any device should be displayed in a diagnostic
display module for which a legend is provided. In a recent model by a manufacturer, 49
different diagnostics can be read at the display module, and the last 20 diagnostics can be held in an
active history buffer log in the module. The diagnostic module is also capable of displaying outputs
of microprocessor-controlled run tests to verify operation of every thermistor, potentiometer, fan,
and compressor before the air system is started.
A smoke detector is usually located in the return air passage. Once the smoke detector detects smoke
in the return airstream, it sends a signal to the DDC controller and energizes the fire alarm system, as
shown in Fig. 21.3b. Smoke controls and stairwell pressurization systems are discussed in Chap. 22.
A pressure sensor is usually used to sense the pressure drop across the filter. As soon as the pressure
drop exceeds a predetermined limit, an alarm signal appears at the display module to indicate
the need for replacement or maintenance.
Safety controls are often part of the system components in HVAC&R equipment, as discussed
previously.
23.10 RECOMMENDATIONS AND INTERACTION
BETWEEN CONTROLS
Recommendations for VAV Controls
● Equipment, control valves, and air dampers should be properly selected and sized. Oversized
equipment and components degrade the control quality.
● Proportional-integral (PI) control mode is the widely used control mode in HVAC&R applications.
PI control eliminates the offset, resulting in a better indoor environment and a lower energy