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Building Manager Green Tip

September 2011

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Understanding Occupancy Sensor Features

Changing settings on occupancy sensors are easy!

Most people understand the basic concept behind occupancy sensors, but some may be unaware of the multiple programming settings that are built into many models. Some of these settings have the potential to increase the functionality of the sensors and further reduce energy consumption in a facility. Many common complaints by occupants, such as sensors turning lights on when not desired, can also be addressed through changes in the setup of these devices.

The way a sensor is programmed may also affect the incentives offered.  For switch mounted occupancy sensors, NSTAR offers a $35 per device incentive, but if the same device is set to vacancy mode--see below for details--the incentive rises to $40 as greater savings are expected using this feature.  In both instances, the sensor must be connected to at least 51 watts of load to be eligible for the incentive.

Programming Basics

Wattstopper PW-200 Programming Options

The image above is from the programming guide for the Wattstopper PW-100/200 series wall switch sensor, one of the most common devices deployed on campus. Many other manufacturers offer similar options on their products, but it's important to check the capabilities of a sensor before procuring them.  Though for many models changes can be made easily without the use of tools, it's vital that you consult building management staff and product specific installation instructions before making any changes.

Occupancy vs. Vacancy Modes - Green Building Services generally recommends that switch mounted sensors be set to the vacancy mode as opposed to the occupancy mode, particularly in areas that have access to natural light. In vacancy mode the load will only turn on when the switch is pressed regardless of whether the switch is detecting motion or not (Manual On, Automatic Off).  Occupancy mode will turn the load on whenever a person is detected regardless of whether or not they press the switch (Automatic On, Automatic Off).  Occupancy mode has a tendency to turn lighting on when it is not desired or needed, while vacancy settings give the occupant more control to set lighting levels. 

Time Delays - The default time setting that Wattstopper provides is 20 minutes, which is likely longer than is required for most spaces.  Sensors in hallways, bathrooms, and other spaces where occupants will be spending short periods of time should be set for small durations (5 minutes or less), whereas an open office space may justify a longer period to ensure no false-off situations occurs.

Light Level Controls - Though a bit more difficult to program, many sensors have a setting where the load will only turn on when motion is detected and a certain ambient lighting threshold is met, preventing lighting from turning on when high levels of natural light are present in the space.  This works in occupancy mode only, as in vacancy mode the lighting will still turn on whenever a person presses the switch. Generally the acceptable level is set manually, and details for adjusting the model above can be found on page 7 of this document.

Sensitivity and Shielding - Small spaces adjacent to highly trafficked areas (e.g. a break room next to a hallway) may require the use of shielding or sensitivity setbacks that will ensure the sensor does detect movement in areas outside of the space you're trying to control.  Shielding may be provided with the sensor, typically consisting of small stickers that are placed on the sensor lens.  Shielding will sometimes require adjustments to function properly and should be commissioned properly after installation.

Green Building Services provides consulting services to ensure that the design, construction and operation of Harvard's built environment has minimal environmental and human health impacts, maximizes occupant comfort and generates an awareness of sustainable design and building operations. To learn more about our work and services, visit http://green.harvard.edu/gbs.