ASHRAE Journal - May 2009 - 43

the system must still prevent the building from getting too cold or too hot. In addition, when unoccupied, the building typically does not require outdoor air for ventilation, so the outdoor-air dampers can be closed. Allowing the indoor temperature to drift during unoccupied periods, often called night setback, saves energy by avoiding the need to operate heating, cooling, and ventilating equipment. Figure 2 shows the potential energy savings of using night setback in an example office building equipped with a traditional water-source heat pump system.2 If the indoor temperature is allowed to drift when unoccupied, 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 are expected to enter the building. Use zone temperature sensors with a BAS instead of programmable thermostats. To lower installed costs, some buildings use simple, residential-style, programmable thermostats with no building automation system. A programmable thermostat can vary the temperature setpoint based on time of day and day of the week, allowing for night setback. But these devices also allow the building occupants to override setpoints or ignore the schedule (by using the hold feature of the thermostat), undermining the potential for energy savings. A more sustainable approach is to use zone temperature sensors and a building automation system that coordinates the operation of all components of the HVAC system. Operating schedules are adjusted through the BAS, and cannot be overridden by the occupants using the hold feature on a programmable thermostat. Avoid overly conservative scheduling through the use of timed override buttons. Determining the times to start and stop the HVAC system is typically based on assumptions regarding building occupancy. Most building managers or operators want to avoid complaints from the occupants and the time required to respond to those complaints. For this reason, many take a conservative approach, starting the system early and stopping it late. This practice can be costly from an energy perspective, since the entire building may be operating to maintain occupied temperature setpoints, even though only a few spaces are occupied. A relatively simple solution to minimize comfort complaints and avoid wasting energy is to equip the zone temperature sensor with a timed override button. If a person wants to use a space during a time when it has been scheduled as unoccupied, they simply press this button and the BAS switches that zone into the occupied mode. This returns the temperature to the occupied setpoint and delivers ventilation air to that zone. Typically, the BAS automatically returns this zone to the unoccupied mode after a defined fixed period of time (two hours, for example). Using the timed override feature allows the opportunity to be more aggressive with time-of-day operating schedules. This also helps avoid wasting energy by starting and stopping the HVAC system based on typical use, not worst-case or once-a-year use. Once occupants are educated about using the timed override feature, energy savings and minimal complaints can coexist. May 2009 85 80 Zone Temperature (°F) 75 70 65 60 55 50 6 a.m. Noon 6 p.m. Unoccupied Setpoint System On System Off Occupied Setpoint Occupied Hours Figure 1: Night setback. 100 HVAC Energy Consumption (% of Base) 80 60 40 20 0 Atlanta Louisville Without Night Setback Minneapolis With Night Setback Figure 2: Energy saving potential of night setback. Use separate time-of-day schedules for areas with differing usage patterns. For simplicity, many building managers or operators define only one (or a few) time-of-day schedules to operate the entire building. However, if areas of the building have significantly different usage patterns, this approach wastes energy because the entire building may be operating to maintain occupied temperature setpoints, although only part of the building is in use. A more energy-efficient approach is to create separate time-of-day operating schedules for areas of the building with significantly different usage patterns. If the facility has a building automation system, it probably includes a time-of-day scheduling function, the only additional cost is the operator’s time to set up the schedules. Results from Actual Energy Retrofit Projects Time-of-day operating schedules are often a significant opportunity to save energy in existing buildings. The following are the results of two energy retrofit projects. The first example is a small school district located in Michigan. This project included implementing time-of-day scheduling and night setback in three school buildings, totalASHRAE Journal 43

ASHRAE Journal - May 2009

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