Applications Guide Tracer Graphical Programming - Trane

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Chapter 7 Constant-volume AHU example
Figure 138: Determining whether to economize
Close the outdoor air damper completely when any of the following is
true:
• The unit is in unoccupied mode.
• The unit is in night heat/cool mode and heating.
• The unit is in night heat/cool mode and cooling, and economizing is
not allowed.
• The outdoor air temperature falls below the low outdoor air temperature
setpoint.
• The supply fan is off.
• The mixed air temperature sensor is failed.
Simplifytheprogrambyseparatingthedecisiontoeconomizefromthe
actual control of the damper. Add a second stage of decision making to
close the outdoor air damper in night heat/cool mode. Economizing is only
permitted in night heat/cool mode when the unit is cooling, the economizer
is enabled, and the outdoor air temperature is less than the economizer
changeover setpoint.
Add the OK to Economize binary variable to the same program for the
VAV air handler, as shown in Figure 138. Use this Variable block to pass
the result of the economize decision to other programs, such as the discharge
air control program.
Figure 139 on page 161 shows the modifying decision based on night
heat/cool mode.
160 CNT-APG001-EN