Elsevier

Energy and Buildings

Volume 104, 1 October 2015, Pages 233-243
Energy and Buildings

Using footwarmers in offices for thermal comfort and energy savings

https://doi.org/10.1016/j.enbuild.2015.06.086Get rights and content

Highlights

  • 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.

Abstract

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.

Introduction

Roughly 10% of the world's energy is spent to heat and cool the interiors of commercial buildings. This is done to create a range of indoor temperatures that assure comfortable and productive occupants. The temperature range has a significant impact on the amount of energy used. A narrow range requires more heating and cooling, often simultaneous. For each degree Celsius that the thermostat heating setpoint is lowered or the cooling setpoint raised, a building's total (heating + cooling) HVAC energy consumption is reduced 10%, and savings of 40–50% are possible [1]. If it were possible to have equivalent comfort outside the conventional interior temperature range, substantial amounts of HVAC energy might be saved with no loss in performance.

Personal comfort system (PCS) devices have been developed that cool or heat thermally sensitive parts of the human body to provide thermal comfort in a wider range of ambient conditions. They also improve comfort psychologically by making thermal control personally available [2], [3], [4], [5]. PCS cooling mostly takes place through convective cooling by fans [6], [7], [8], [9], [10], [11], [12] or through cooled chairs [13], [14], [15], [16]. PCS heating has been provided by heated chairs [17], [18], [19], [20], [21], a lower body warming enclosure [22], radiant panels in the kneehole [8] and footwarmers [7], [8], [21]. The literature on personal comfort systems has been recently reviewed and the comfort effectiveness of numerous systems compared [23].

Most PCS testing has been done in laboratory thermal chambers on student subjects. There have been relatively few field studies of PCS in actual buildings, and these only during warm seasons [10], [24], [25]. One of these found 100% thermal satisfaction in a building when the occupants had been given PCS in their workstation [26]. Though the study was limited in scope, finding 100% of the occupancy satisfied is a unique result in comfort field studies. The study also found that in summer occupants’ comfort was better at slightly higher temperatures with PCS than without PCS at lower ambient temperature. To date, there has been no field study in which PCS was heating occupants, or which determined HVAC energy savings associated with using PCS.

The PCS device evaluated in this study originated from a fundamental laboratory study [6] that determined how cool feet dictate the discomfort of the entire body in cool environments [27]. Not only are warm feet essential for the perception of comfort in cool environments, but feet warming is also very effective at restoring comfort once one is cool. Based on this, the authors developed footwarmers that focus radiant heat on the feet and ankles. Emphasis was placed on making them energy-efficient. They were laboratory tested in realistic workstations in which subjects performed tasks representing office work [7].

The encouraging findings from the laboratory study suggested that an actual office with footwarmers should be tested for an extended period in winter. Office workers would be repeatedly surveyed about their comfort as the interior temperature is systematically varied. At the same time, the HVAC energy savings associated with lowered office temperatures would be quantified. The field study would provide evidence about potential benefits and drawbacks of footwarming in an actual functioning office.

Section snippets

Methods

Footwarmer description: Because there were no efficient footwarmers available in the market, the authors fabricated 100 for use in field studies (Fig. 1). In order to create a rapid warming effect, the design uses four incandescent reflector bulbs as heat sources. The filament and bulb heat up almost instantly, and the radiation is focused on the top of the feet and ankles where shoe and clothing insulation is least. Non-absorbed radiation is retained to the extent possible within a reflective

Environment

Outdoor temperature: The winter 2012–2013 was atypically warm. Table 1 presents the monthly mean, averages of all the daily maximum and minimum temperatures for each month of the study. The monthly mean temperature ranged between 9.7 (January 2013) and 17.1 (October 2012).

Indoor temperature: Fig. 5 shows the perimeter temperature, core temperature, setpoint, and outside temperature every fifteen minutes during occupied hours (8 AM–6 PM) throughout the study.

The black dots are the setpoint

Discussion

Corrective power: Throughout the study (including the base-case reference condition in Period 1), whole-body thermal sensation remained between ‘neutral’ and ‘slightly cool’ (Fig. 8), and the feet thermal sensation was neutral (Fig. 10). The occupants’ thermal acceptability remained essentially constant, and well above the 80% threshold prescribed in ASHRAE Standard 55 (Fig. 6, Fig. 7). This establishes that the footwarmers’ ability to offset the effects of cooling the ambient surroundings

Conclusions

This appears to be the first study of a personal heating system in an office building, in which both comfort and energy savings were quantified. The occupancy had a representative age distribution and was performing skilled knowledge work.

Occupants were provided with low-power footwarmers during a six-month winter test period in which their room setpoint was gradually reduced from 21.1 °C to 18.9 °C and then back up to 21.1 °C. Footwarmer use increased with lowered setpoints. Including the 25% of

Acknowledgements

This research was funded by the California Institute for Energy and Environment (CIEE) through grant POSP01 from its State Partnership for Energy Efficient Demonstrations (SPEED) program. Karl Brown managed the program and helped access the field study building. The authors would also like to thank Prof. Jorn Toftum at Technology University of Denmark for his assistant at managing survey website, Alan Daly at Taylor Engineering for HVAC system adjustments, the 16 participants who continued to

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