Thermal comfort refers to the condition of mind that expresses satisfaction with the thermal environment and is evaluated by subjective evaluation. Over the years, a large amount of empirical data has been obtained on how these parameters are determined. This article describes the thermal comfort, particularly with regard to temperature and humidity.
Certain factors have significant influence on the thermal comfort of a occupant's space and thus can be divided into personal and environmental factors. Personal factors include clothing and personal activity and condition, while environmental factors comprise thermal radiation, temperature, air speed and humidity.
Conditions of Thermal Comfort
The feeling of comfort and awareness of the environmental conditions are associated with metabolic heat production and the ensuing adjustments of body temperature. In order to define a comfort zone of external parameters, it is vital to know the activity of an individual person. For instance, whether people are working or sitting in an office, doing exercise in a gym, or hiking outdoors.
Clothing is the second most important parameter. Its insulating properties is a vital parameter of body heat loss/conservation and thus for feeling thermal comfort. Seasonal weather conditions are influencing clothing to a great extent. Hence, the limitations for external parameters defining a comfort zone depend on the season.
Due to individual differences, it is not possible to specify a thermal environment, which will gratify everyone. This article provides three approaches for determining a comfort zone and defining limits: comfort zone according to ISO, comfort zone according to ASHRAE, and the concept of Heat Index.
Comfort Zone According to ISO7730
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Figure 1. RH/T diagram showing the comfort zone according to ISO7730.
The standard ISO773 is one approach for defining the comfort zone. However, this approach ignores the fact that higher temperatures can be tolerated at low humidity. Hence, its lower and upper temperature limits are vertical. This approach can be utilized in less complex applications for simpler execution of air-conditioning algorithms. Although temperature ranges are specified per season, the relative humidity is set between 70%RH and 30%RH (Figure 1) in summer and winter time, respectively. The limits are set to reduce the risk of eye irritation, dry or wet skin, microbial growth, and respiratory diseases.
Comfort Zone According to ASHRAE
Air speed and thermal radiation are predominantly outdoor effects that are difficult to measure and control. As a result, literature on thermal comfort concentrates on temperature and humidity.
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Figure 2. Relative humidity (RH)/temperature (T) diagram based on comfort zone according to ASHRAE 55-1992.
An individual’s thermal experience depends on both temperature and humidity: The greater the humidity the greater the temperature. This phenomenon is based on the fact that at increased humidity, the body’s cooling system on the skin is considerably reduced.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE’s) publication on thermal environmental conditions for human occupancy has defined a comfort zone for summer and winter season. Although this definition of comfort zone is rather complex, it can be altered by some estimation to a zone which only depends on relative temperature and humidity (Figure 2).
The effect of experiencing higher temperature together with high relative humidity is shown by slanting boundaries of the comfort zone for the lower and upper limits of temperature. This has an effect on heating, ventilation, and air conditioning (HVAC) control systems. When indoor spaces are monitored for both temperature and humidity, energy can be saved. This is because when there is low humidity, a higher temperature is tolerable and cooling is not required. In a temperature-controlled system, redundant cooling or heating can be done even if conditions are still within the comfort zone.
Heat Index
Heat Index specifies how the human body experiences temperature and is based on subjective measurements. However, it is only meaningful above 40%RH and 250°C. At high temperature and humidity conditions, the comfort factor can be controlled with the concept of Heat Index. The Heat Index in °C is given by
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Table 1. Coefficients for Heat Index formula
| C00 |
-8.7847 |
| C10 |
1.6114 |
| C01 |
2.3385 |
| C11 |
-0.1461 |
| C20 |
-0.0123 |
| C02 |
-0.0164 |
| C21 |
2.2117•10-3 |
| C12 |
7.2546•10-4 |
| C22 |
-3.5820•10-6 |
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Figure 3. Heat Index in °C.
The Heat Index as a virtue of relative temperature and humidity is shown in Figure 3. The meaning of the values is defined as follows:
- < 30°C: no discomfort
- 30 – 40°C: some discomfort
- 40 – 45°C: great discomfort
- > 45°C: dangerous
- > 54°C: heat stroke imminent
Conclusion
Thermal comfort is a vital concept for climate control systems. With a better understanding of comfort conditions, efficient climate controlling systems can be designed to suit the needs of the user. As described above, relative temperature and humidity play a vital role, and hence the well-being can be significantly improved by applying a temperature and humidity sensor.
This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.
For more information on this source, please visit Sensirion Inc.