Measuring Building Occupancy
Who is in your building? How long are they there? Obtaining this information has its benefits.
July 1, 2013
There are many potential benefits to knowing not only how many people are in a building, but ideally where they are located. Why would we want to know? Well, with better information on occupancy, we could make smarter choices in several areas.
- Demand controlled ventilation. Under ASHRAE 62, ventilation levels for the ventilation portion for occupants (the People Outdoor Air Rate or Rp) can be adjusted based on occupancy. The general way to do this is to measure and control based on CO2 levels. While this works well, CO2 sensing is expensive to install and properly maintain. Good data on occupancy can be used as an alternative to utilizing CO2. When areas are not at all occupied, then the ventilation requirement for the area outdoor air rate (Ra) can also be moved to zero.
- Occupancy control. Being able to see when an area is occupied could be a valuable tool for turning off lights as well as HVAC.
- Building utilization. Good data about occupancy can be a valuable tool to show how a building is actually utilized. Imagine being able to see how many people are in an office area and when or how classrooms are actually used. This information may help owners make decisions about space planning and better utilization.
- Security. Accurate people counts can be invaluable for security. It can also help with emergency evacuation and safety.
On past projects, we have used several proxies for occupancy including the use of CO2, manual operator input, and data from card access systems. While all of these are good proxies, none is highly accurate. For example, not all building occupants use their cards to enter a building, and in many buildings, occupants do not use their cards when exiting the building.
There are now a series of new technologies available for people-counting. Some of these are coming from security systems, and others have evolved from retailing. Retailing? Yes, it turns out that retailers needed an accurate way to measure how long customers have been standing in line so that they can decide if they should open another register. This same technology can be readily used across a commercial building and integrated in as part of an intelligent building. The technologies include infrared sensors, cameras, and thermal imagers. When looking at a solution some considerations include cost, flexibility, support, and of course the ability to deal with the issue of occupant privacy.
The movement toward a better-integrated, inclusive, and accurate occupant measuring system is something to consider. While these systems add to project cost and complexity, they offer benefits and could reduce the need for existing tools such as occupancy and CO2 sensors.