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Such issues have particular relevance with regard to naturally ventilated buildings, where occupants are able to open windows, creating indoor conditions that are inherently more variable than buildings with centralized HVAC systems. In such settings, an alternative thermal comfort standard based on field measurements might be able to account for contextual and perceptual factors absent in the laboratory setting. Toward this end, the research began by focusing on three primary modes of adaptation: physiological, behavioral and psychological.
Physiological adaptation, also known as acclimatization, refers to biological responses that result from prolonged exposure to characteristic and relatively extreme thermal conditions. One example in hot climates is a fall in the set point body temperature at which sweating is triggered, leading to an increased tolerance for warmer temperatures. Laboratory evidence suggests, however, that such acclimatization does not play a strong role in subjective preferences across the moderate range of activities and thermal conditions present in most buildings.
Behavioral adaptation refers to any conscious or unconscious action a person might make to alter their body’s thermal balance. Examples include changing clothes or activity levels, turning on a fan or heater, or adjusting a diffuser or thermostat. Behavioral adjustments offer the best opportunity for people to participate in maintaining their own thermal comfort. Affording ample opportunities for people to interact with and control the indoor climate is an essential strategy in the design of naturally ventilated buildings
The psychological dimension of thermal adaptation refers to an altered perception of, and reaction to, physical conditions due to past experience and expectations. It is premised on the generalization, true across all sensory modalities (not just thermal), that repeated exposure to a new stimulus leads to a diminution of the evoked response. It also includes the idea that a person’s reaction to a temperature that is less than perfect will depend on expectations and on what that person is doing at the time.
The research described in this article involved assembling a quality controlled database containing 21,000 sets of raw data compiled from previous thermal comfort field experiments inside 160 different office buildings located on four continents and covering a broad spectrum of climatic zones. The gender and age distribution of the subjects was typical of office building populations. The large sample size reduced the risk of bias that might occur in relatively smaller samples used in climate chamber experiments. The data included a full range of both subjective and physical measurements, including thermal questionnaire responses, clothing and metabolic estimates, concurrent indoor climate measurements, a variety of calculated thermal indices and outdoor meteorological observations. Analysis of data was performed separately for buildings with centralized HVAC systems and naturally ventilated buildings (i.e., where occupants had access to operable windows). The analysis examined thermal comfort responses in terms of both thermal neutrality and preference, as functions of both in-door and outdoor temperatures. Observed responses also were compared to predictions of thermal sensation calculated using the heat-balance-based PMV model. The PMV model is the basis for ISO Standard 7730, and for the next version of Standard 55。
The following sections present select aspects of the research that directly relate to the proposal for an “adaptive” thermal comfort standard to be used as an alternate to PMV for naturally ventilated buildings in the next revision of Standard 55. A more detailed description of the research methods, statistical analysis techniques and results can be found in ASHRAE Transactions.
HVAC vs. Naturally Ventilated Buildings
To what extent do people behaviorally adapt in the two building types? Behavioral adaptation was analyzed by examining how changes in clothing, metabolic rate and air velocity varied as functions of indoor temperature. Mean metabolic rates in both building types stayed fairly constant at about 1.2 met units regardless of indoor temperature, ranging within a fairly tight cluster of 1.1–1.4 met units. In contrast, changes in clothing and air velocity were both significantly related to changes in mean indoor operative temperatures in all buildings