A study of the energy requirements for conditioning ventilation air and the yearly performance of a membrane-based energy recovery ventilator (MERV) in Hong Kong is carried out. The weather data are classified into six process regions in the psychrometric chart and the percentage of annual hours in each region is determined to describe different energy requirements. The variations in the amount of required heating and cooling energy are calculated for different indoor temperature and humidity set points. It is found that the required annual energy is primarily used to remove moisture from fresh air, with only a small fraction used for sensible cooling. The energy for heating in cold weather is negligible. Energy recovery ventilators are employed to study the possible annual energy savings. Hour-by-hour calculations disclose that in hot and humid regions like Hong Kong, about 58% of the energy required for conditioning fresh air could be saved annually with an MERV, which recovers both latent and sensible energy, while only about 10% of the energy could be saved with a traditional sensible-only energy recovery ventilator (SERV). The more humid the weather, the more superior is an MERV in comparison with an SERV. (C) 2001 Elsevier Science Ltd. All rights reserved.