What is in this article?:
Besides dry-bulb air temperature, mean radiant temperature, and air velocity, factors affecting thermal comfort are:
- Metabolic rate.
- Clothing insulation.
Determining Thermal-Comfort Levels
ASHRAE offers standards, guides, and tools to aid assessment of thermal comfort: ANSI/ASHRAE Standard 55 includes charts and equations that help determine the level of thermal comfort for particular situations, while “Performance Measurement Protocols for Commercial Buildings: Best Practices Guide”1 outlines methods for measuring and evaluating thermal comfort, and Version 2 of ASHRAE Thermal Comfort Tool software calculates a couple of important values related to thermal comfort.
One of the values ASHRAE Thermal Comfort Tool calculates is MRT, factoring the temperatures of surrounding surfaces, the distance between those surfaces and occupants, and the emissivity of the surfaces. Figure 2 shows a snapshot of the tool with some example input values. In this example, the surface temperatures range from 65°F to 72°F, and MRT is calculated at 69.1°F.
ASHRAE Thermal Comfort Tool also can calculate percent of persons dissatisfied (PPD), the percentage of occupants that would find certain thermal conditions unsatisfactory. It calculates this value based on user inputs for the six main factors affecting occupant comfort. ASHRAE recommends a PPD of less than 10 for acceptable thermal comfort.
Following is an example of a thermally comfortable space according to ANSI/ASHRAE Standard 55:
• Air temperature: 70°F.
• MRT: 70°F.
• Humidity ratio: 0.010 lb of water per pound of dry air.
• Air speed: 19.7 fpm.
• Metabolic rate: 1.1 met (e.g., office work).
• Clothing level: 0.90 (typical winter indoor clothing).
Plugging those values into ASHRAE Thermal Comfort Tool results in a PPD of 8, which falls within the range of acceptable thermal comfort (Figure 3).
Now, let’s say it is Monday morning, and the interior surfaces are cold. Lowering the MRT from 70°F to 65°F and keeping the other values the same results in an unacceptable PPD of 16 (Figure 4). Not even raising the air temperature to 73°F results in acceptable conditions (PPD of 11) when the MRT is lowered to 65°F (Figure 5).
When determining the expected level of thermal comfort, you may need to work within the confines of current facility requirements. I have seen owner requirements that include acceptable dry-bulb temperature and humidity, but I have yet to see owner requirements addressing MRT or air speed in relation to occupant thermal comfort. Also, remember that despite all of the research and literature on thermal comfort and the charts, equations, and tools ASHRAE provides, not all occupants experience thermal conditions the same way.