Indoor air quality affects working quality and efficiency particularly for those who work in the office for prolonged period. Poor air quality even compromises health of the staff in the office. For the air-conditioning system using package type or split type air conditioner, introducing the fresh air will improve the indoor air quality of the office. However, it will increase the energy consumption of the air-conditioning system. Attempting to improve indoor air quality with less power consumption, this study conducts the case study of a total heat exchanger retrofitted in an office building in hot and humid climate of Taiwan. Field measurement of indoor temperature, humidity, CO₂ concentration and energy consumption along with thermal sensation votes of staff in the office have been compared comprehensively after installing the total heat exchanger. Results indicated that the CO2 concentration decreases obviously and thermal comfort evaluation improves satisfactorily after the installation of total heat exchanger even though power consumption increased slightly. It reveals that it is feasible to improve the indoor air quality effectively with less power consumption approach in the office specifically for the air-conditioning system using package type or split type air conditioner under hot and humid weather condition.