Issue link: https://hi.iaq.net/i/630729
Factors Affecting Indoor Air Quality 7 A properly designed and functioning HVAC system: ■ provides thermal comfort ■ distributes adequate amounts of outdoor air to meet ventilation needs of all building occupants ■ isolates and removes odors and con- taminants through pressure control, filtration, and exhaust fans Thermal Comfort A number of variables interact to deter- mine whether people are comfortable with the temperature of the indoor air. The activity level, age, and physiology of each person affect the thermal comfort require- ments of that individual. The American Society of Heating, Refrigerating, and Air- Conditioning Engineers (ASHRAE) Standard 55-1981 describes the tempera- ture and humidity ranges that are comfort- able for most people engaged in largely sedentary activities. That information is summarized on page 57. The ASHRAE standard assumes "normal" indoor clothing. Added layers of clothing reduce the rate of heat loss. Uniformity of temperature is important to comfort. When the heating and cooling needs of rooms within a single zone change at different rates, rooms that are served by a single thermostat may be at different temperatures. Temperature stratification is a common problem caused by convection, the tendency of light, warm air to rise and heavier, cooler air to sink. If air is not properly mixed by the ventilation system, the temperature near the ceiling can be several degrees warmer than at floor level. Even if air is properly mixed, uninsulated floors over unheated spaces can create discomfort in some climate zones. Large fluctuations of indoor temperature can also occur when controls have a wide "dead band" (a temperature range within which neither heating nor cooling takes place). A number of variables, including personal activity levels, uniformity of temperature, radiant heat gain or loss, and humidity, interact to determine whether people are comfortable with the temperature of the indoor air. Radiant heat transfer may cause people located near very hot or very cold surfaces to be uncomfortable even though the thermostat setting and the measured air temperature are within the comfort range. Buildings with large window areas some- times have acute problems of discomfort due to radiant heat gains and losses, with the locations of complaints shifting during the day as the sun angle changes. Large vertical surfaces can also produce a significant flow of naturally-convecting air, producing complaints of draftiness. Adding insulation to walls helps to moderate the temperature of interior wall surfaces. Closing curtains reduces heating from direct sunlight and isolates building occupants from exposure to window surfaces (which, lacking insulation, are likely to be much hotter or colder than the walls). Humidity is a factor in thermal comfort. Raising relative humidity reduces the ability to lose heat through perspiration and evaporation, so that the effect is similar to raising the temperature. Humidity ex- tremes can also create other IAQ problems. Excessively high or low relative humidities can produce discomfort, while high relative humidities can promote the growth of mold and mildew (see Appendix C). Ventilation to Meet Occupant Needs Most air handling units distribute a blend of outdoor air and recirculated indoor air. HVAC designs may also include units that introduce 100% outdoor air or that simply transfer air within the building. Uncon- trolled quantities of outdoor air enter buildings by infiltration through windows, doors, and gaps in the exterior construc- tion. Thermal comfort and ventilation needs are met by supplying "conditioned" air (a blend of outdoor and recirculated air that has been filtered, heated or cooled, and sometimes humidified or dehumidified).