Using footwarmers in offices for thermal comfort and energy savings

• Personal foot warmers were tested for energy and comfort in an office building.
• Footwarmers enabled cooler rooms in winter without affecting thermal comfort.
• Energy saved by lowering setpoints greatly exceeded electricity used by footwarmers.
• Efficient footwarmer design used only 20 W per person to offset 2 K (4 °F) cooler room.

An office equipped with personal footwarmers was maintained at cooler-than-normal indoor temperatures in the winter, producing great energy savings. The occupants’ thermal comfort was not affected. The footwarmers provide individual heating control over a segment of the body that most strongly influences comfort perception when one is cool overall. If cooler ambient indoor temperatures could be made comfortable, savings in central heating energy would be possible. During a six-month winter period in Berkeley California, knowledge workers with low-energy footwarmers experienced a lowering of room heating set point from 21.1 °C (70 °F) to 18.9 °C (66 °F). Surveys showed equal thermal comfort in the original ‘higher heating setpoint no-footwarmer’ condition and the ‘lower heating set point plus occupant-controllable footwarmer’ condition. Heating energy was closely monitored throughout. It dropped 38–75% depending on the setpoint reduction and outdoor conditions. The added plug load energy from the low-energy footwarmers was much less than the central heating energy saved by lowering the heating set point (3–21 W vs 500–700 W average power per occupant during occupied hours). A few subjects had ergonomic issues with the particular footwarmers used, so usage was not universal. Additional foot- and leg-warmer design options would help.