Thermal comfort

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Human thermal comfort is the state of mind that expresses satisfaction with the surrounding environment, according to ASHRAE Standard 55. Achieving thermal comfort for most occupants of buildings or other enclosures is a goal of HVAC design engineers. Indoor air quality is not part of thermal comfort, but is also a key concern of HVAC designers.

Fundamental studies of thermal comfort, such as acceptable ranges of dry-bulb temperatures, relative humidities, and activity levels were completed in the 1970s. Many of these studies, which led to the development and refinement of ASHRAE Standard 55, were performed at Kansas State University by Ole Fanger and others. Some key findings were that not everyone will be satisfied by a particular set of indoor environmental conditions, but in ranges of conditions about 80% express satisfaction. If very good conditions are in place, a maximum of 95% of all persons might be satisfied. Statistical methods were used to evaluate the thermal comfort opinions of the many test subjects to yield what are known as the comfort conditions; the predicted mean vote (PMV) was one of the measures used.

Most important for thermal comfort is the so called operative temperature. This is the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. In addition, there should be low air velocities and no 'drafts', little variation in the radiant temperatures from different directions in the room, the humidity has to be in a comfortable range, and the air temperatures in a height of 0.1 m above the floor should not be lower than 2°C than the temperature at the place of the occupant's head. Also the temperatures should not change too rapidly - neither across the space nor with time.

The conditions for thermal comfort depend on the clothing and on the activity level of a person. The unit 'clo' describes the level of clothing, which insulates the body, and the unit 'met' describes the activity level. A clo equal 1.0 is typical of a business suit, and a met of 1.0 is seated quietly, such as in a classroom.