Center for the Built Environment

We have reached a point in which lowering building energy consumption and thereby reducing greenhouse gas emissions is an ecological imperative. Rethinking how we mechanically condition our buildings is an important means for substantially lowering building energy consumption. Rather than using large amounts of energy for space heating and cooling to condition the full volume of interior air to be within a narrow temperature range, people can be comfortable with a wider room temperature range as long as they have personalized control over local conditioning, which is similar to task- ambient lighting design already well understood in theory and practice. This field study tested low-energy footwarmers as one way in which occupants can personally control their own thermal environment.

During a six month winter period, participants in an office in Berkeley, California, were given a low-energy footwarmer with an easily accessible knob for adjusting the heat provided by the footwarmer to their desired level. The room heating set point was gradually lowered from 70F to 66F. The building management system and independent data acquisition recorded data from which the energy consumption data of the mechanical system and the individual plug loads of each workstation could be determined. Three times per day, participants took an online survey about their thermal comfort. The results show that the added plug load from the low-energy footwarmers was substantially less than the heating energy saved by lowering the heating set point. There was a 38-75% overall reduction in heating energy, which depended on the setpoint and outdoor conditions. Occupant surveys showed equivalent thermal comfort ratings when comparing the existing conditions to when people had the footwarmer and a lower heating set point.

From low-cost retrofits of sealed VAV offices to the design of new buildings, low- energy personal thermal control systems can effectively provide thermal comfort when set points are expanded. Rethinking the fundamentals of set points can lead to scalable impact in reducing building energy consumption and ultimately in lowering our ecological footprint.