HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New MillenniumThe modification of a setpoint may counteract the control program.An unreasonable change to a cooling setpoint in a 65°F zone, could resultin wasted energy and a higher cost of operations. A useful featureapplied with most setpoints is to have a parameter that limits theamount of change allowed for the setpoint.Another use of setpoints for equipment and general-purpose controlis alarming. First, the setpoint will be established for a warning orcritical alarm. These setpoints may initiate a local notification and perhapsa telephone dial out.Alarm setpoints can be used to program different alarm conditionsfor the time of day. This can be done to eliminate most nuisance alarms.SchedulesSchedules are a type of controller software that is essential toHVAC control. A fundamental requirement for cost reduction is reducingthe number of hours of operation. In addition to reducing energycost, this also extends the life of equipment since there are fewer runhours.Schedules make savings possible in most equipment control applications,excluding hospitals and some specialized facilities. There areusually control sequences for both occupied and unoccupied modes.The schedule defines when each mode begins and ends. The setpointsare keyed to these schedules that customize the desired conditions to thetime of day. This could be used from a chiller or unit ventilator application.Scheduling is usually integrated with the occupied and unoccupiedmodes and time of day based alarms. Another example of integratingscheduling with other sequences is demand limiting withtime of peak and time of use rates. Electric utility rates will vary, butit is common to charge more for electricity that is consumed duringpeak times of the day or season. The scheduling functions are integratedwith the demand schedules to identify the beginning and endof such periods and to change the setpoints accordingly. In certaintypes of buildings such as schools and churches, schedules changedaily and seasonally so quick access is important. A variable air volume(VAV) air handling unit (AHU) might implement control sequenceswith the priority of Table 2-2.©2001 by The Fairmont Press, Inc. All rights reserved.