Human Thermal Comfort Levels

Human Thermal Comfort Level usually refers to the question of how comfortable would human being felt itself in the certain room conditions. All humans are different in their perception or sensation of the environmental changes. A thermal room perfect for each and everybody does not exist, some people would still be unhappy.

The environment most close to perfect is ISO 7730, and it is usually called an acceptable thermal room climate. Around eighty percent of people staying there are satisfied with the thermal conditions.

The understanding of comfort of humans depends on how close thermal conditions are to the balance. It is influenced by the factors like: air temperature, relative humidity, temperature of the surrounding surfaces, air movement in the vicinity of the body, heat resistance of the clothes. Some of the physiologic factors are important as well; they are weight and the level of body heat, metabolism or heat production of the body.

The heat that is made by the body metabolism is transported by all available physical structures of heat transfer, such as conduction, radiation, evaporation and convection. The heat balance dependent on all the above factors would look like this:

H - Ediff - El - Es - Epe = R + C

Where H is internal heat production of the body,

  • Ediff is the latent hear transfer through the skin,
  • El is heat transfer by breathing,
  • Es is the sensible heat transferred by breathing,
  • Epe is latent heat transfer by perspiration evaporation,
  • R – the radioactive heat transfer from the surface of clothing and C is convective heat transfer through clothing.

The body has to always maintain the balance it uses blood circulation, perspiration and skin temperature, breathing to do so. Comfort or discomfort, respectively, is rated according to [1, 2] with the index PMV (predicted mean vote) in a range of values between -3 and +3 (cold, cool, slightly cool, neutral, slightly warm, warm, hot). From the index PMV, the predicted percentage of dissatisfied (PPD) with the given comfort can be calculated.

The human organism keeps the temperature of the body constant at thirty-seven degrees, no matter what are the outer or inner conditions. To achieve this, the way and frequency of the heat transfer from skin to the inside organs and back have to be properly modified. Under increased metabolic heat production, the blood circulation in the skin is intensified, which causes a higher surface temperature, and therewith-higher heats transfer. If this is not sufficient, perspiration sets in. Through its evaporation the body is cooled, which is supported by movement of the ambient air. Since the perspiration evaporation is highly dependent on the pressure of air against skin, the humidity of the air has a direct influence on the thermal comfort.

The movement of air and air velocity in wherever the human being is situated also regulates the thermal balance of humans. The h higher is the air velocity the higher would be heat transfer in the areas or skin not covered with clothes. For example at normal temperatures, and acceptable air velocity I s 0.1 up to 0.2 m/s. Besides the mean air velocity also the amplitude and frequency of air velocity influence the balance.

The parameters that influence the thermal comfort are also the building properties. Some of them are: an efficient shading of windows, a reasonable heat capacity of the building, which is evident for inner building elements with a floor area specific mass of at least three hundred kg/m2; heat insulation of opaque building elements, with an U-value of less the ½ w/(m).

Leave a Reply