ASABE Technical Library - Abstract

Abstract. Indirect methods are widely used for determining air exchange rates (AER) in naturally ventilated buildings because they are relatively easier and cheaper than direct methods. The main goal of this study was to evaluate two common indirect methods (CO₂ and H₂O mass balances) against a direct method, and identify factors influencing these indirect methods. Indirect methods with 24-h data averaging yielded more reliable AER than with shorter averaging times (i.e., 1, 2, and 12 h). The mean AER based on 24-h averaging ranged from 11 to 39 h⁻¹ across all study periods and methods. The CO₂-balance method tended to overestimate AER, while the H₂O-balance method tilted towards underestimation of AER. The cows’ CO₂ production rate was estimated at 0.178 m³ h⁻¹ hpu⁻¹, the correction factor Ks was estimated at 0.65, based on 24-h data averaging for our study barn. Barn AER increased with wind speeds. For specific wind directions, barn AER was not significantly different amongst the three methods. However, wind directions significantly affected barn AER regardless of the method. The differences between CO₂-balance and direct method varied significantly across wind directions. Indirect methods were unreliable during milking times, and when indoor-outdoor temperature, absolute humidity, and CO₂ concentration differences were less than 1.0°C, 0.3 g m⁻³ and 100 ppm, respectively.