Why occupancy-responsive adaptive thermostats do not always save - and the limits for when they should

So-called ‘smart thermostats’ are beginning to fill the gap left in efficiency programs after researchers and policy makers discovered that in practice, simple programmable thermostats do not guarantee energy savings. As a result, EPA ended EnergyStar certification of programmable thermostats in 2010. Many recent pilots for communicating thermostats, occupancy-responsive thermostats, and adaptive control schemes have shown significant annual HVAC savings on the order of 10-20%. However, the form and function for technologies in this space vary widely. Some controls merely allow for remote management (e.g., web-based setpoint scheduling or smart-phone interface and control), while other devices monitor occupancy and automatically adjust set-points when a space is vacant. Still other technologies automatically adapt to user behaviors and preferences in order to anticipate changes and adjust HVAC operation. These differences have different savings implications. Further, the application into which any of these technologies is installed also impacts savings potential. The study focuses particularly on a series of pilot evaluations conducted with one occupancy-responsive adaptive thermostat system that resulted in very little energy savings during normal operation in university residence halls. These results came as a great surprise to the research team, especially since the HVAC system run-time for vacant zones was reduced to nearly zero in the buildings. The detailed evaluation of this case forms a conceptual basis for explanation of the limitations for smart thermostat devices. The research shows that considerable savings can be had in certain instances, but that the impact is sensitive to technology and application. The study also reviews previous research on the technology and recommends methodological improvements for future studies