Last month’s trend graph was 24 hrs in the life of a large museum AHU in a northern climate. The constant volume system delivered constant 55°F supply air to terminal reheat coils serving multiple temperature control zones. These temperature control zones were primarily art galleries open to the public during the day. The AHU consisted of a mixing box with airside economizer, heating hot water preheat coil, chilled water coil, humidifier, and supply fan.
The trend graph showed two distinct operating modes: the left side of the graph was in occupied mode when the fan was on, and the right side of the graph was in unoccupied mode when the fan was off. The system clearly went into a shutdown mode between 8:30 p.m. and 6:30 a.m., with the outside air dampers fully closed and the return air, mixed air, and discharge air temperatures essentially equal to each other. Both the heating and cooling coils were commanded closed. All of that was very good from an energy perspective, although the practice of turning off air-handling equipment at any time in a museum is suspect from a collections preservation perspective.