Thermal comfort

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation (ANSI/ASHRAE Standard 55). The human body can be viewed as a heat engine where food is the input energy. The human body will generate excess heat into the environment, so the body can continue to operate. The heat transfer is proportional to temperature difference. In cold environments, the body loses more heat to the environment and in hot environments the body does not exert enough heat. Both the hot and cold scenarios lead to discomfort. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC (heating, ventilation, and air conditioning) design engineers. Most people will feel comfortable at room temperature, colloquially a range of temperatures around 20 to 22 °C (68 to 72 °F), but this may vary greatly between individuals and depending on factors such as activity level, clothing, and humidity.High humidity and low temperatures cause the air to feel chilly.Cold air with high relative humidity 'feels' colder than dry air of the same temperature because high humidity in cold weather increases the conduction of heat from the body.There has been controversy over why damp cold air feels colder than dry cold air. Some believe it is because when the humidity is high, our skin and clothing become moist and are better conductors of heat, so there is more cooling by conduction. Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation (ANSI/ASHRAE Standard 55). The human body can be viewed as a heat engine where food is the input energy. The human body will generate excess heat into the environment, so the body can continue to operate. The heat transfer is proportional to temperature difference. In cold environments, the body loses more heat to the environment and in hot environments the body does not exert enough heat. Both the hot and cold scenarios lead to discomfort. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC (heating, ventilation, and air conditioning) design engineers. Most people will feel comfortable at room temperature, colloquially a range of temperatures around 20 to 22 °C (68 to 72 °F), but this may vary greatly between individuals and depending on factors such as activity level, clothing, and humidity. Thermal neutrality is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. The main factors that influence thermal comfort are those that determine heat gain and loss, namely metabolic rate, clothing insulation, air temperature, mean radiant temperature, air speed and relative humidity. Psychological parameters, such as individual expectations, also affect thermal comfort. The Predicted Mean Vote (PMV) model stands among the most recognized thermal comfort models. It was developed using principles of heat balance and experimental data collected in a controlled climate chamber under steady state conditions. The adaptive model, on the other hand, was developed based on hundreds of field studies with the idea that occupants dynamically interact with their environment. Occupants control their thermal environment by means of clothing, operable windows, fans, personal heaters, and sun shades. The PMV model can be applied to air-conditioned buildings, while the adaptive model can be generally applied only to buildings where no mechanical systems have been installed. There is no consensus about which comfort model should be applied for buildings that are partially air-conditioned spatially or temporally. Thermal comfort calculations according to ANSI/ASHRAE Standard 55 can be freely performed with the CBE Thermal Comfort Tool for ASHRAE 55. Similar to ASHRAE Standard 55 there are other comfort standards like EN 15251 and the ISO 7730 standard. Satisfaction with the thermal environment is important because thermal conditions are potentially life-threatening for humans if the core body temperature reaches conditions of hyperthermia, above 37.5–38.3 °C (99.5–100.9 °F), or hypothermia below 35.0 °C (95.0 °F). Buildings modify the conditions of the external environment and reduce the effort that our body needs to do in order to stay stable at a normal human body temperature, important for the correct functioning of our physiological processes. The roman writer Vitruvius actually linked this purpose to the birth of Architecture. David Linden also suggests that the reason why we associate tropical beaches with paradise is because in those environments is where our bodies need to do less metabolic effort to maintain our core temperature. Temperature not only supports human life; coolness and warmth have also become in different cultures a symbol of protection, community and even the sacred. In building science studies, thermal comfort has been related to productivity and health. Office workers who are satisfied with their thermal environment are more productive. The combination of high temperature and high relative humidity reduces thermal comfort and indoor air quality.