Methodology for the Evaluation of Natural Ventilation in ... - Cham

evaluating the air exchange rate within the buildings. The methods used to measure theventilation performance of the building are described in this section.Building location and siting can influence the ventilation for a naturally ventilated building, butthe focus of the assessment was on the building itself and the internal conditions. The windowgeometry proved to be a challenge. The effect of window geometry has been studied to a limitedextent (Heiselberg 1999, 2001) but not specifically for the top hung awning-type windows thatare used in the prototype building. The awning-type windows are hinged at the top, and are keptopen by friction at the hinge. These window types are used both for the smaller, upper windowsas well as for the lower, large occupant controlled windows. A method for determining theairflow rate of incoming air and exhaust air had to be developed because natural ventilation inbuildings relies on external conditions to provide fresh air and remove internal heat gains, andwind speed and direction can change quickly. This characteristic increases the complexity ofevaluating ventilation effectiveness. Part of the complexity lies in the effective opening area ofthe window; it has both a horizontal area, as well as two vertical pieces, that can all affect thetotal airflow rate. Initially velocity measurements were taken in the horizontal plane using handheldhot-wire anemometers, as that dimension was determined to be the largest contributor toincoming and outgoing airflow for the window.The stack vents were a key design characteristic that had to be considered when evaluatingventilation performance. The fans integrated in the stack vents were included in the assessment.Hot-wire anemometers were used to measure the air velocity just outside the louvers from theexterior and just below the fan in the interior. Measurements were taken under two conditions;with the fans on and off. In addition, smoke pencils were used when taking measurements at theexterior to determine the direction of the airflow.Another method employed to determine the ventilation rate of the prototype building was themonitoring of carbon dioxide (CO 2 ) levels within the occupied spaces using a Tel-Aire carbondioxide sensor, combined with a HOBO® H8 series data recorder. Measuring CO 2 can be used todetermine air exchange rates, and to evaluate indoor air quality. Several groups have definedmaximum acceptable levels of CO 2 for office spaces. Levels above 1,000 ppm can lead tolethargy and headaches (ems, 2004). However, both the United States OSHA (OccupationalSafety and Health Association) and the United Kingdome BSRIA (Building Services Researchand Information Association) have defined maximum exposure limits to be 800 ppm over aneight-hour period for office areas. The CO 2 level is dependent on the ventilation distribution,occupant density, and amount of outside air being introduced into the space (ASHRAE 2001).When evaluating the indoor environment with respect to occupant health, ASHRAE suggeststhat an indoor level of CO 2 650 ppm above the outside level is representative of an air exchangerate of 20 cubic feet per minute, with an occupant density of 100 ft 2 per person (ASHRAE 1997).Occupant comfort is also affected by higher CO 2 levels, with 20 percent of people dissatisfied atCO 2 concentrations of 650 ppm above the outdoor level (Liddament, 1996). In offices, carbondioxide levels are primarily due to the respiration of the occupants. Initially the CO 2 andtemperature monitor was place outside, away from the building in order to record the externalconditions as a baseline. Then the CO 2 sensor was placed at desk level, away from directexposure from an occupant, in the second floor office area and data recorded every fifteenminutes over the twelve-month monitoring period. On site visits, the number of people in eachoffice area was logged over the period of the day and compared to the data recorded for that day.50