ASHRAE 55 provides two different methods for evaluating thermal comfort: Predicted Percentage of Dissatisfied (PPD) and Predicted Mean Vote (PMV).
PPD is a statistical index that estimates the percentage of people in a given space who may feel uncomfortable due to thermal conditions. The PPD index is calculated by considering the percentage of occupants who would feel too cold or too warm based on their individual comfort preferences. A PPD value of 10% means that 10% of occupants are predicted to be dissatisfied with the thermal conditions.
PMV, on the other hand, is a single numerical value that represents the predicted average thermal sensation of a group of people in a given space. It takes into account the same factors as PPD, including air temperature, humidity, air velocity, and radiant temperature. A PMV value of 0 indicates that the predicted thermal sensation is neutral or comfortable.
Both PPD and PMV are useful tools for evaluating thermal comfort in indoor environments. PPD is useful for identifying the percentage of occupants who may be dissatisfied with thermal conditions and can be used to identify potential comfort problems in space. PMV, on the other hand, provides a single numerical value that can be used to set temperature and humidity setpoints for HVAC systems in order to achieve an acceptable level of thermal comfort for most occupants.
In general, ASHRAE 55 recommends that the PPD be kept below 10% and the PMV be kept between -0.5 and +0.5 for optimal thermal comfort. However, it is important to note that these values may vary depending on the specific needs and preferences of occupants in a given space.