Short-term mechanical ventilation of air-conditioned residential buildings: case study and general design framework

Many studies reported that there were insufficient ventilation and excessive CO2 concentration in air-conditioned residential buildings, but few solutions were provided. This study first investigated the performance of three possible ventilation strategies of air-conditioned residential buildings, including overnight natural ventilation, short-term natural ventilation, and short-term mechanical ventilation. On-site measurements were conducted in a typical occupied residential bedroom in Hong Kong in summer, where a number of environmental parameters including especially the CO2 concentration were monitored both inside and outside the room. Ventilation rates were calculated based on the time series of CO2 concentration. The results confirm that ventilation is needed in air-conditioned residential buildings. Overnight natural ventilation with even a small opening is associated with excessive energy consumption and deteriorated indoor thermal environment. Short-term natural ventilation strategies are inefficient and uncontrollable. Compared to the best short-term natural ventilation strategy, a reasonably designed short-term mechanical ventilation strategy requires only a 41% of ventilation period to complete one full replacement of indoor air and to reach a lower indoor CO2 concentration.  
This study second developed a general design framework of short-term mechanical ventilation strategy. This framework can determine appropriate design parameters, including ventilation period, ventilation frequency, and start concentration of ventilation, based on various combinations of indoor CO2 generation rate, net room volume, infiltration rate, and mechanical ventilation rate. A whole sleeping period of 8 h was divided into many repeated single V-shape ventilation periods; each single ventilation period is comprised of a short-term mechanical ventilation period and a follow-up CO2 build-up period. The single V-shape ventilation process was particularly investigated based on a criterion that the average indoor CO2 concentration is less than but close to 1000 ppm. A high efficient ventilation strategy, namely requiring a minimum total mechanical ventilation period, is a short single ventilation period and a high ventilation frequency. Although this study focused on air-conditioned residential buildings, the basic concepts and design framework should also be applicable for heated residential buildings.