A screening methodology for climatic evaluation of the cooling potential of night ventilation in buildings

Night ventilation is a well known strategy for passive cooling of residences and small commercial buildings. The building's thermal mass can be cooled at night by ventilating the inside of the space with the relatively lower outdoor air temperatures, thereby lowering indoor temperatures during the warmer daytime period. Numerous experimental and theoretical studies have shown the effectiveness of the method to significantly reduce air conditioning loads or improve comfort levels in those climates where the night time ambient air temperature drops below that of the indoor air. One could develop/adapt computer programs with detailed mathematical component models to simulate and evaluate the performance of night ventilation strategies in a specific location for a particular building. A more basic problem is to develop a methodology whereby potential designers can screen various climatic locations and regions in order to perform a preliminary evaluation of which months of the year are good candidates for implementing such a scheme. Only after completion of such a phase is a detailed evaluation warranted for specific buildings. In this paper, effectiveness of night ventilation is quantified by a parameter called the Discomfort Reduction Factor (DRF) which is the index of reduction of occupant discomfort levels during the day time from night ventilation. Two different thermal network models which provide such insights are evaluated. Daily and monthly DRFs are calculated for two climate zones and three building heat capacities for the whole year. It is verified that night ventilation is effective in seasons and regions when day temperatures are between 30°C and 36°C and night temperatures are below 20°C. The accuracy of these preliminary screening models may be lower than using a detailed simulation program but the loss in accuracy in using such tools is more than compensated by the insights provided, along with better transparency in the analysis approach and results obtained.