ASHRAE-Standard-55 - Page 052
Extracted Content
(This appendix is not part of this standard. It is merely informative and does not contain requirements necessary for conformance to the standard. It has not been processed according to the ANSI require- ments for a standard and may contain material that has not been subject to public review or a consen- sus process. Unresolved objectors on informative material are not offered the right to appeal at ASHRAE or ANSI.)
INFORMATIVE APPENDIX I LOCAL DISCOMFORT AND VARIATIONS WITH TIME
I1. LOCAL THERMAL DISCOMFORT
Avoiding local thermal discomfort, whether caused by a vertical air temperature difference between the feet and the head, by an asymmetric radiant field, by local convective cooling (draft), or by contact with a hot or cold floor, is essential to providing satisfactory thermal comfort.
The requirements specified in Section 5.3.5 of this standard apply directly to a lightly clothed person (with clothing insulation between 0.5 and 0.7 clo) engaged in near-sedentary physical activity (with metabolic rates between 1.0 and 1.3 met). With higher metabolic rates and/or with more clothing insulation, people are less thermally sensitive and, consequently, the risk of local discomfort is lower. Thus, it is acceptable to use the requirements of Section 5.3.5 for metabolic rates greater than 1.3 met and with clothing insulation greater than 0.7 clo, as they will be conservative. People are more sensitive to local discomfort when the whole body is cooler than neutral and less sensitive to local discomfort when the whole body is warmer than neutral. The requirements of Section 5.3.5 of this standard are based on environmental temperatures near the center of the comfort zone. These requirements apply to the entire comfort zone, but they may be conservative for conditions near the upper temperature limits of the comfort zone and may underestimate discomfort at the lower temperature limits of the comfort zone.
Table I-1 shows the expected percent dissatisfied for each source of local thermal discomfort described in Sections 5.3.5.1 through 5.3.5.4. The criteria for all sources of local thermal discomfort should be met simultaneously at the levels specified for an environment to meet the requirements of Section 5.3 of this standard. The expected percent dissatisfied for each source of local thermal discomfort described in Sections 5.3.5.1 through 5.3.5.4 should be specified.
I2. 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 thermal satisfaction with 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. Figure I-1 gives the expected percentage of occupants dissatisfied due to radiant temperature asymmetry caused by a warm ceiling, a cool wall, a cool ceiling, or a warm wall.
The allowable radiant asymmetry limits are based on Figure I-1 and assume that a maximum of 5% of occupants are dissatisfied by radiant asymmetry.
I3. DRAFT
Draft is unwanted local cooling of the body caused by air movement. It is most prevalent when the wholebody thermal sensation is cool (below neutral). Draft sensation depends on whole-body thermal sensation, air speed, air temperature, activity, turbulence intensity, and clothing. Sensitivity to draft is greatest where the skin is not covered by clothing, especially the head region comprising the head, neck, and shoulders and the leg region comprising the ankles, feet, and legs.
Use of elevated air speed to extend the thermal comfort range is appropriate when otherwise occupants are slightly warm, as set forth in Section 5.3.4. When occupants are neutral or cooler, such as under certain combinations of met rate and clo value with operative temperatures to below 23°C (73.4°F), average air speeds within the comfort envelope of ±0.5 PMV should not exceed 0.20 m/s (40 fpm). This draft limit applies to air movement caused by the building, its fenestration, and its HVAC system and not to air movement produced by office equipment or occupants. This standard allows average air speed to exceed this draft limit if it is under the occupants’ local control and is within the elevated air-speed comfort envelope described in Section 5.3.4.
Draft at the lower-leg region may occur in buildings conditioned by thermally stratified systems, such as displacement ventilation and underfloor air distribution, or with cold-dropping airflow along external walls and/or windows. This problem could also occur in vehicles when the air is supplied at the floor level. Manufacturers of air diffusers intended for stratified systems often provide diffuser performance data that can assist designers in predicting Vankle . Various approaches are used by different manufacturers to derive the performance data. A standard method of test does not yet exist.
50 ANSI/ASHRAE Standard 55-2023