What is meant by thermal comfort?
Thermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment (ANSI/ASHRAE Standard 55). 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.
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. It has been long recognized that the sensation of feeling hot or cold is not just dependent on air temperature alone.
To have "thermal comfort" means that a person wearing a normal amount of clothing feels neither too cold nor too warm. Thermal comfort is important both for one's well-being and for productivity. It can be achieved only when the air temperature, humidity and air movement are within the specified range often referred to as the "comfort zone".
Where air movement is virtually absent and when relative humidity can be kept at about 50%, the ambient temperature becomes the most critical factor for maintaining thermal comfort indoors. However, temperature preferences vary greatly among individuals and there is no one temperature that can satisfy everyone. Nevertheless, an office which is too warm makes its occupants feel tired; on the other hand, one that is too cold causes the occupants' attention to drift, making them restless and easily distracted.
Maintaining constant thermal conditions in the offices is important. Even minor deviation from comfort may be stressful and affect performance and safety. Workers already under stress are less tolerant of uncomfortable conditions.
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What temperature should an office be?
A general recommendation is that the temperature be held constant in the range of 21-23°C (69-73°F). In summertime when outdoor temperatures are higher it is advisable to keep air-conditioned offices slightly warmer to minimize the temperature discrepancy between indoors and outdoors.
What humidity level and air velocity should an office be?
When relative humidity is kept at about 50%, office workers have fewer respiratory problems (specifically in the winter) and generally feel better. Higher humidity makes the office feel "stuffy". More important, it can contribute to the development of bacterial and fungal growth (especially in sealed buildings).
Humidity lower than 50% causes discomfort by drying out the mucous membranes, contributing to skin rashes. Dry conditions cause electrostatic charge on both office equipment and their users.
Air velocities below 0.25 metres/second (or about 50 feet/minute) does not create any significant distraction even in tasks requiring sustained attention.
Are there any standards on office temperatures?
The CSA Standard CAN/CSA Z412-00 (R2005) - "Office Ergonomics" gives acceptable ranges of temperature and relative humidity for offices in Canada. These values are the same as recommended by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard 55 - 2010 "Thermal Environmental Conditions for Human Occupancy". The recommended temperature ranges have been found to meet the needs of at least 80% of individuals. Some people may feel uncomfortable even if these values are met. Additional measures may be required.
|Temperature / Humidity Ranges for Comfort|
|Conditions||Relative Humidity||Acceptable Operating Temperatures|
|Summer (light clothing)||
If 30%, then
If 60%, then
24.5 - 28
23 - 25.5
76 - 82
74 - 78
|Winter (warm clothing)||
If 30%, then
If 60%, then
20.5 - 25.5
20 - 24
69 - 78
68 - 75
Source: Adapted from ASHRAE 55-2010.
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