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Occupancy/Vacancy Sensor

Occupancy and vacancy sensors often provide the highest level of energy savings possible through the implementation of a single lighting energy control strategy in many different space types. This is because, in many buildings, spaces are unoccupied for a portion of the day while the electric lighting system is still operating. Differentiation is sometimes made between sensors that turn lights on and off in response to occupancy (hence the name occupancy sensor) and those that only turn them off when the space is unoccupied (a vacancy sensor). In many circles, however, both are still referred to as occupancy sensors, and many occupancy sensors permit the auto-on function to be either applied or disabled. Some energy codes, however, may only permit the installation of equipment that deactivates the lighting system since additional savings can be obtained, particularly in daylit areas, if the occupant elects not to energize the entire lighting system upon entering a space. Occupancy sensors are available in many different forms. They can be integrated into wall-switches, mounted on walls and ceilings, or they can be integrated into luminaires (usually of the pendant variety) in areas with office cubicles. The sensing technology they apply can be infrared (looking for movement of warm bodies within a space), ultrasonic (looking for reflection of high frequency sound waves by moving objects), acoustic (listening for occupant generated sounds), microwave (similar to ultrasonic in operation), or a combination of these (dual-technology sensors). Most technologies are best suited to certain types or configurations of spaces. See the table below.

Occupancy control has shown to provide significant energy savings in a variety of different types of spaces, with significant savings possible in spaces that have intermittent, shared occupancy. These space types include restrooms, break rooms, classrooms and others.

In existing buildings, light and occupancy loggers can be applied to monitor the number of hours that a space is unoccupied where lighting remains on over an extended time period (generally a week or more). The results obtained from such a study can then be used to estimate the anticipated savings and payback that might be obtained through the addition of an occupancy sensing control system in that space.

Manufacturers of occupancy sensor control systems typically provide recommended layouts of these systems for individual projects. Layouts must consider the coverage pattern of each sensor as well as space conditions such as obstructions, geometry and materials. The position of sensors relative to doors is also critical to avoid false readings from movement that occurs in adjacent spaces, such as in a corridor, or the obstruction that could be caused by placing a sensor behind a door.

Table 1. Basic guidelines for the application of occupancy sensors
(reprinted from Wattstopper reference below).
  • Use ultrasonic sensors in areas screened by partitions or furniture
  • Use PIR in enclosed spaces
  • Create zones controlled by different sensors to manage lighting in large areas
  • Use dual technology sensors for areas with very low activity levels
  • Install sensors on vibration-free, stable surfaces
  • Position sensors above or close to the main areas of activity in a space
  • Mask the sensor lens to define coverage of the controlled zone even more accurately
  • Integrate sensor use with other control methods (i.e. time scheduled control, daylighting)
  • Educate occupants about the new devices and what to expect
  • Use ultrasonic sensors in spaces with heavy air flow
  • Install ultrasonic sensors in spaces where the ceiling height exceeds 14 feet
  • Use PIR sensors in spaces where there are fixtures or furniture that obstruct a clear line of sight
  • Install PIR sensors so that their line of sight continues beyond doorways
  • Install sensors within 6-8 feet of HVAC outlets or heating blowers
  • Position a wall switch sensor behind an office door
  • Control emergency or exit lighting with sensors
  • Install PIR sensors in spaces where there are extremely low levels of occupant motion
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Lead Author(s): Rick Mistrick

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