Thermal comfort

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Definition

Human thermal comfort is defined by ASHRAE as the state of mind that expresses satisfaction with the surrounding environment (ASHRAE Standard 55). Maintaining thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

Thermal comfort is affected by heat conduction, convection, radiation and evaporative heat loss. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate thus maintaining thermal equilibrium with the surroundings. Any heat gain or loss beyond this generates a sensation of discomfort.^[1] It has been long recognised that the sensation of feeling hot or cold is not just dependent on air temperature alone.

Factors determining thermal comfort

Factors determining thermal comfort include:

• Air temperature
• Mean radiant temperature
• Air movement / velocity (see wind chill factor)
• Relative humidity (see also perspiration)
• Insulative clothing
• Activity levels.

Thermal stress

The concept of thermal comfort is closely related to thermal stress. This attempts to predict the impact of solar radiation, air movement and humidity for military personnel undergoing training exercises or athletes during competitive events. Values are expressed as the Wet Bulb Globe Temperature or Discomfort Index.^[2][3]

Operative temperature

The ideal standard for thermal comfort can be defined by the 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 more than 2 °C lower than 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.

In addition to environmental conditions, thermal comfort depends on the clothing and activity level of a person. The amount of clothing is measured against a standard amount that is roughly equivalent to a typical business suit, shirt, and undergarments. Activity level is compared to being seated quietly, such as in a classroom.

This standard amount of insulation required to keep a resting person warm in a windless room at 70 °F (21.1 °C) is equal to one Clo. Clo units can be converted to R-value in SI units (K/(W/m²) or RSI) by multiplying Clo by 0.155 (1 Clo = 0.155 RSI). (In imperial units 1 Clo corresponds to an R-value of 0.88 °F ft²hr/Btu.)

Research

These factors were explored experimentally in the 1970s. Many of these studies, led to the development and refinement of ASHRAE Standard 55, and were performed at Kansas State University by Ole Fanger and others. Perceived comfort was found to be a complex interaction of these variables. It was found that the majority of individuals would be satisfied by an ideal set of values. As the range of values continued to deviate from ideal, increasingly fewer people were satfisfied. This observation could be expressed statistically as the % of individual who expressed satisfaction by comfort conditions and the predicted mean vote (PMV)

See also

• P. Ole Fanger
• Room air distribution
• Psychrometrics
• Air conditioning
• Building insulation

References

• Thermal Comfort, P. O. Fanger, McGraw-Hill, New York, 1970.
• Thermal Comfort chapter, Fundamentals volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2005.
• Weiss, Hal (1998). Secrets of Warmth: For Comfort or Survival. Seattle, WA: Mountaineers Books. ISBN 0-89886-643-X. OCLC 40999076.