Center for the Built Environment

Personal comfort systems provide comfort by targeting heating or cooling to important parts of the human body, making tight ambient temperature control less important. This paper provides evidence that comfort is possible under cooler-than-normal ambient temperatures when occupants have personal control over a very local thermal condition—the warmth of their feet. During a six-month winter period in Berkeley California, office workers were given low-energy adjustable footwarmers and the room heating set point was gradually lowered from 21.1C (70F) to 18.9C (66F). Occupant surveys showed statistically equivalent thermal comfort for the original ‘higher heating setpoint no-footwarmer’ condition and the ‘lower heating set point plus occupant-controllable footwarmer’ condition. The overall reduction in heating energy varied between 38% and 75% depending on the setpoint reduction and outdoor conditions. The added plug load energy from the low-energy footwarmers was substantially less than the heating energy saved by lowering the heating set point (11-21W vs 500-700W average power per occupant during occupied hours).  A few subjects had ergonomic issues with the particular footwarmers used, so usage was not universal.  Additional designs or options will be needed in future applications.