ASHRAE Journal - May 2009 - 46

Use for demand-controlled ventilation in certain space types. Demand-controlled ventilation (DCV) is a strategy that attempts to dynamically reset the outdoor airflow delivered to a zone based on changing population within that zone. Commonly used technologies include CO2 sensors and occupancy sensors (motion detectors), but time-of-day schedules are an often overlooked option. A time-of-day schedule can be created in the BAS to predict the number of people that occupy a zone throughout the day. This variation in population can then be used to reset the required ventilation airflow for the zone. This approach is best suited for spaces where the occupancy pattern is predictable, such as cafeterias, gymnasiums, and some classrooms. If the facility contains a BAS, it probably includes a time-of-day scheduling function, the only additional cost is programming time to set up the schedules. And, unlike a CO2 sensor, this technology does not need to be periodically cleaned and calibrated, so it is more reliable. However, there are spaces where CO2 sensors are worth the added cost and risk. But, for any space with a predictable occupancy pattern, a time-of-day schedule can reliably deliver significant energy savings without additional sensor cost.3 Combine with occupancy sensors to enable an occupied standby mode. When an occupancy sensor is used in combination with a time-of-day schedule, the sensor can be used to indicate if the zone is unoccupied although the BAS has scheduled it as occupied. This combination is used to switch the zone to an “occupied standby” mode. In this mode, all or some of the lights in that zone can be shut off, the temperature setpoints can be raised or lowered by 1°F to 2°F (0.5°C to 1°C), and the ventilation delivered to that zone can be reduced, typically to the building-related ventilation rate, Ra, required by ANSI/ASHRAE Standard 62.1.4 In addition, for a multizone variable-air-volume (VAV) system, the minimum primary airflow setting of the VAV terminal serving that zone can be lowered to avoid or reduce the need for reheat. These minimum primary airflow settings are typically selected to ensure proper ventilation or to increase air circulation for occupant comfort. However, with no occupants and a reduced ventilation requirement, the minimum primary airflow setting can be lowered significantly. When the occupancy sensor indicates that the zone is occupied, the zone is switched back to occupied mode. Optimize HVAC system operation. When a building automation system is used for time-of-day scheduling, the next logical step is to use it to optimize the control of the system and reduce energy use.5 Optimal start. When night setback is used, the HVAC system must start prior to occupancy and operate long enough for the indoor temperature to reach the desired occupied setpoint by the time people occupy the building. In many buildings, the timeof-day schedule is programmed to start the system early enough so that the building will warm up or cool down fast enough on the worst-case morning. As a result, for all other days of the year, the system starts earlier than needed. This increases the number of operating hours and increases energy use. 46 ASHRAE Journal An alternative approach is a strategy called optimal start. The BAS is used to determine the length of time required to bring each zone from its current temperature to the occupied setpoint temperature. Then, the controller waits as long as possible before starting the system, so that the temperature in each zone reaches the occupied setpoint just in time for occupancy. The optimal starting time is determined using the difference between the actual zone temperature and the occupied setpoint temperature. It compares this difference with the historical performance of how quickly the zone has been able to warm up or cool down. Some systems also compensate for the current outdoor temperature. This strategy reduces the number of system operating hours and saves energy by avoiding the need to maintain the indoor temperature at occupied setpoint prior to the beginning of the scheduled occupied period. Optimal stop. A related strategy is optimal stop. As mentioned previously, at the end of the scheduled occupied period, the HVAC system is shut off and the indoor temperature is allowed to drift away from the occupied setpoint. However, the building occupants may not mind if the indoor temperature drifts just a few degrees before they leave at the end of the day. Optimal stop uses the BAS to determine how early heating and cooling can be shut off for each zone, so that the indoor temperature drifts only a few degrees from the occupied setpoint. Only cooling and heating are shut off; the supply fan continues to operate and the outdoor-air damper remains open to continue ventilating the building. Consider disabling optimal stop during humid weather to avoid introducing unconditioned outdoor air into the building. The optimal stop strategy also reduces the number of system operating hours, saving energy by allowing indoor temperatures to drift prior to the end of the scheduled occupied period. Unoccupied economizing. During the unoccupied mode, the indoor temperature can sometimes drift warmer than the occupied cooling setpoint temperature. This requires the HVAC system to start and cool down the building prior to scheduled occupancy. However, if the outdoor dry-bulb temperature is cooler than the indoor temperature during some part of the unoccupied period, it is possible to use the outdoor air for “free” cooling. This strategy, often called unoccupied (or nighttime) economizing, can save energy by precooling the building prior to the morning cool-down period. Unoccupied economizing uses the BAS to determine if the condition of the outdoor air is suitable for free cooling, and determines which zones could benefit from precooling. For example, unoccupied economizing may be allowed whenever the outdoor dry-bulb temperature is cooler than the zone temperature by a defined margin, 15°F (8°C) for example. Then, the BAS starts the fan and opens the outdoor-air damper, precooling the building to a preset temperature before turning the system off. In addition, a humidity sensor can be added to prevent unoccupied economizing when the outdoor air is cool, but humid. Integrate with other subsystems to optimize HVAC energy use. When a facility has other control systems installed, there may be opportunities to integrate with them to further reduce HVAC energy use. As discussed previously, the same time-ofashrae.org May 2009

ASHRAE Journal - May 2009

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