User:7alberto7/Local thermal discomfort

Local Thermal Discomfort
Even though the comfort models based on the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) usually describe compliance to thermal comfort for the body as a whole, thermal dissatisfaction may also occur just for a particular part of the body, due to local sources of unwanted heating, cooling or air movement. According to the ASHRAE 55-2010 standard, there are four main causes of thermal discomfort to be considered. A section of the standard specifies the requirements for these factors, that apply to a lightly clothed person engaged in near sedentary physical activity. This is because people with higher metabolic rates and/or more clothing insulation are less thermally sensitive, and consequently have less risk of thermal discomfort.

Draft
Draft is unwanted local cooling of the body caused by air movement, most prevalent when the thermal sensation of the whole body is cool (below neutral). Draft sensation depends on the air speed, air temperature, activity, and clothing. Sensitivity to draft is greatest where the skin is not covered by clothing, especially the head, neck, shoulders, ankles, feet, and legs.

Radiant Temperature Asymmetry
The thermal radiation field about the body may be nonuniform due to hot and cold surfaces and direct sunlight. This asymmetry may cause local discomfort and reduce the thermal acceptability of the space. In general, people are more sensitive to asymmetric radiation caused by a warm ceiling than that caused by hot and cold vertical surfaces. ASHRAE standard gives the predicted percentage of dissatisfied occupants (PPD) as a function of the radiant temperature asymmetry and specifies the acceptable limits.

Floor Surface Temperature
Occupants may feel uncomfortable due to contact with floor surfaces that are too warm or too cool. The temperature of the floor, rather than the material of the floor covering, is the most important factor for foot thermal comfort for people wearing shoes. ASHRAE standard 55 specifies the allowable range of surface temperatures of the floor for people wearing lightweight shoes.

Vertical Air Temperature Difference
Thermal stratification that results in the air temperature at the head level being higher than at the ankle level may cause thermal discomfort. ASHRAE standard 55 gives the predicted percentage of dissatisfied occupants as a function of the air temperature difference between the head level and ankle level. Thermal stratification in the opposite direction is rare and perceived more favorably by occupants.