Keeping building occupants comfortable while minimizing energy use is a balancing act for engineers who design HVAC&R systems and buildings. One way they can achieve this balance is through requirements in a standard from an international technical association.
ASHRAE‘s Standard 55, Thermal Environmental Conditions for Human Occupancy, specifies the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.
Earlier this week, research that looks at the method used to determine thermal comfort in Standard 55 was published via an article, “Energy Consumption in Buildings and Female Thermal Demand,” in Nature Climate Change. The research looks at the method used to determine thermal comfort in ANSI/ASHRAE Standard 55.
“The interpretation of the authors regarding the basis for Standard 55 is not correct,” says Bjarne Olesen, Ph.D., a member of the ASHRAE Board of Directors, internationally renowned thermal comfort research and former chair of the Standard 55 committee. “The part of the standard they are referring to is the use of the PMV/PPD index. This method is taken from an ISO/EN standard 7730, which has existed since 1982. The basic research for establishing comfort criteria for the indoor environment was made with more than 1,000 subjects with equal amount of women and men.
“In the main studies, where they did the same sedentary work and wore the same type of clothing, there were no differences between the preferred temperature for men and women. So the researchers’ finding of a lower metabolic rate for females will not influence the recommended temperatures in the existing standards. Also their study is not conclusive. They only studied 16 females at a sedentary activity. They should also have studied 16 men at the same activity to be able to compare. The reason why we, in some field studies, find that women prefer higher room temperature than men is attributed to the level of clothing. Women adapt better their clothing to summer conditions while men are still wearing suit and tie. So if the thermostat is set to satisfy the men, the women will complain about being too cold. In the standard, this adaption of clothing to summer is taken into account so if the standard is followed the women would be satisfied; but maybe not the men.”
ASHRAE President David Underwood notes that the standard has been continually refined and updated since it was first published in 1966, reflecting changes in the industry and new research as it becomes available. Standard 55 is based on an earlier document developed in 1938 by two predecessor societies of ASHRAE, titled Code for Minimum Requirements for Comfort Air Conditioning.
“The standard continues to focus on defining the range of indoor thermal environmental conditions acceptable to a majority of occupants, while also accommodating an ever increasing variety of design solutions intended to provide comfort and to respect today’s imperative for sustainable buildings,” Underwood says.
Olesen notes the researchers should have consulted other studies and technical guidance.
“They should also have looked at the ASHRAE Handbook, Fundamentals, which explains the background for the standard and addresses differences between men and women, young and elderly, and more with literature references,” he says.
In their paper, researchers Boris Kingma and Wouter van Marken Lichtenbelt state, “Energy consumption of residential buildings and offices adds up to about 30 percent of total carbon dioxide emissions; and occupant behavior contributes to 80 percent of the variation in energy consumption. Indoor climate regulations are based on an empirical thermal comfort model that was developed in the 1960s. Standard values for one of its primary variables—metabolic rate—are based on an average male, and may overestimate female metabolic rate by up to 35 percent. This may cause buildings to be intrinsically non energy-efficient in providing comfort to females. Therefore, we make a case to use actual metabolic rates. Moreover, with a biophysical analysis we illustrate the effect of miscalculating metabolic rate on female thermal demand. The approach is fundamentally different from current empirical thermal comfort models and builds up predictions from the physical and physiological constraints, rather than statistical association to thermal comfort. It provides a substantiation of the thermal comfort standard on the population level and adds flexibility to predict thermal demand of subpopulations and individuals. Ultimately, an accurate representation of thermal demand of all occupants leads to actual energy consumption predictions and real energy savings of buildings that are designed and operated by the buildings services community.”