Occupancy vs. Vacancy Sensors for Commercial Buildings

Occupancy vs. Vacancy Sensors for Commercial Buildings

Lighting controls, including occupancy and vacancy sensors, can go a long way toward reducing lighting energy use in commercial applications, from new construction to existing buildings. Sensors are also required in most settings by ASHRAE/IES 90.1.

To make the most of energy savings from using less lighting, it’s important to install a sensor with the ideal type of sensing technology for the space.

Not sure which sensor or technology is the best to use for the job at hand? Check out this overview of what you need to know.

Functionality differences

Both occupancy and vacancy sensors automatically turn lights off after a space has been unoccupied for a set amount of time. They differ in that occupancy sensors automatically turn lights on when they detect a person in a room while someone needs to manually turn on lights associated with a vacancy sensor.

Some occupancy sensors are available with automatic partial-on, which turn lights on to a reduced level — and the occupant can then raise the lighting level from a wall keypad or other device.

You’ll also find sensors that can switch between occupancy sensing and vacancy sensing.

Technological differences

Occupancy and vacancy sensors primarily use one of two technologies: passive infrared (PIR) or ultrasonic, although other technologies such as acoustic are available.

• PIR sensors

PIR sensors identify heat energy and detect its movement within their line of sight, which is how these sensors know when to turn on the lights.

They’re engineered to detect major motion; they can’t sense through obstacles like walls or furniture. They perform best in smaller spaces with few obstructions and high levels of occupant movement, such as storage rooms, small restrooms, private offices, and hallways.

PIR sensors also use very little power, making them common in wireless (battery-powered) models.

• Ultrasonic sensors

The technology in ultrasonic sensors detects when someone is in a room by bouncing ultrasonic waves off objects in the space and measuring changes in the reflected sound, which the sensor interprets as motion.

These sensors don’t require an unobstructed line of sight because they can “see” around obstacles and are sensitive enough to detect minor motion, such as typing.

However, because they measure changes in sound, these sensors can cause lights to inadvertently turn on (called false tripping) in certain situations, such as when vents rattle or moving air from the HVAC system rustles window blinds or papers on a desk.

• Acoustic sensors

Acoustic sensors detect room occupancy by listening for sound patterns such as conversation or chair movement. They’re most often paired with PIR sensors for dual coverage.

When paired with a PIR sensor, the PIR sensor turns the lights on and off when the sensor detects heat or movement. The acoustic sensor keeps the lights on if someone is sitting still for a relatively long period of time and only making minimal noise, such as typing.

• Dual-technology sensors

Dual technology sensors combine two sensing technologies (such as acoustic and PIR, mentioned above), which helps these sensors better avoid false-on’s and false-off’s.

Both technologies must detect someone in a room to turn the lights on, but only one of the technologies needs to continually sense the person for the lights to stay on.

Dual-technology sensors are ideal for spaces such as classrooms and conference rooms, where people tend to sit still for a while.

Where each works best

Occupancy sensors are ideal for spaces where people enter briefly, come and go throughout the day, or enter with their hands full, such as restrooms, stairwells, breakrooms, storage rooms, warehouse aisles, and utility spaces.

Vacancy sensors are often used in areas where occupants can decide whether they need overhead lighting, such as private offices, conference rooms, meeting spaces, and training rooms. (Keep code requirements in mind: vacancy sensors may not be appropriate in spaces, such as electrical rooms, where there’s a code requirement for a minimum light level at all times.)

Wired vs wireless

Sensors included in a building’s electrical system, especially in new construction or major renovations, are often hardwired as part of a lighting control system.

If you’re working on a retrofit project and want a faster, easier install, consider wireless sensors. They’re also a good choice for phased lighting upgrades or for spaces that require more flexibility, since you can easily move them if a room layout changes.

Installation best practices

Depending on the spaces and the type of sensor, you may be able to mount the sensor on the ceiling, wall, or in a corner. You might also be able to install it directly into a compatible light fixture — a good option for industrial applications and warehouses.

When deciding where to mount a sensor, avoid placing it near HVAC vents and ceiling fans and avoid installing wall-mounted sensors behind doors. (A good rule of thumb is to keep sensors more than 8 feet from HVAC vents.)

You may need to use multiple sensors depending on the size of the space, the sensor’s coverage area, and any partitions in the space.

Also consider the sensor’s time delay, which determines how long lights stay on after no movement is detected. If the delay is too short, lights may turn off while a room is still in use. If it's too long, lights may stay on longer than necessary. (Energy codes, including ASHRAE 90.1 and California Title 24, have specific standards for shutoffs in commercial spaces.)