015

Figure 26.3 Hourly whole building electricity consumption at the SFA Building before and after night shut down of AHUs was initiated15.

than the base peak. The lower electricity peak indicates that some office equipment remained off during the daytime or the employees were more conscientious in turning off lights and equipment when they left the office. The annual energy cost savings, including electricity, heating, and cooling, were determined to be $100,000/yr using measured hourly data.

26.4.1.4 Slow Down Systems During Unoccupied/Lightly-Occupied Hours

Most large buildings are never completely unoccupied. It is not uncommon to find a few people working regardless of the time of day. The zones that may be used during the weekends or at night, are also unpredictable. System shut down often results in complaints. Substantial savings can be achieved while maintaining comfort conditions in a building by an appropriate combination of the following actions:

• Reset outside air intake to a lower level (0.05 cfm/ ft2) during these hours during hot summer and cold winter weather. Outside air can be reduced since there will be very few people in the building. Check outside and exhaust air balance to maintain positive building pressure

• Reset the minimum airflow to a lower value, possibly zero, for VAV terminal boxes

• Program constant volume terminal boxes as VAV boxes, and reset the minimum flow from the maximum to a lower value, possibly zero during unoccupied hours

• Reset AHU static pressure and water loop differential pressure to lower values

• Set supply air fan at a lower speed

Figure 26.3 Hourly whole building electricity consumption at the SFA Building before and after night shut down of AHUs was initiated15.

These measures maintain building comfort while minimizing energy consumption. The savings are often comparable with the shut down option. Figure 26.4 presents the measured hourly fan energy consumption in the Education Building at the University of Texas at Austin.

The Education Building has 251,000 ft2 of floor area with eight 50-hp AHUs that are operated on VFDs. Prior to the introduction of this measure, the motor control center (MCC) energy consumption was almost constant. The CC measure implemented was to set the fan speed at 30% at night and on weekends. The nighttime slow roll decreased the fan power from approximately 50 kW to approximately 25 kW while maintaining building comfort.

26.4.1.5 Limit Fan Speed During Warm Up and Cool Down Periods

If nighttime shut down is implemented, warm-up is necessary during the winter and cool down is required during the summer. During warm-up and cool down periods, fan systems are often run at maximum speed since all terminal boxes require either maximum heating or maximum cooling. A simple fan speed limit can reduce fan power significantly. This principle may also be used in other systems, such as pumps. The following actions should be taken to achieve the fan energy savings:

• Determine the optimal start up time using 80% (adjustable) fan capacity if automatic optimal start up is used

• Set the fan speed limit at 80% (adjustable) manually and extend the warm up or cool down period

Figure 26.4 Measured Post-CC hourly supply fan electricity consumption in the Education Building16.

Figure 26.4 Measured Post-CC hourly supply fan electricity consumption in the Education Building16.

by 25%. If the speed limit is set at another fractional value (x), determine the warm up period using the following equation:

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