The purpose of this field study was to evaluate the ventilation practices of occupants and corresponding heating energy consumption in mechanically ventilated apartments, using airflow measurements, energy data collection, and occupant questionnaire. A sample of 139 apartments was selected for the field study from an apartment complex located in south metropolitan area of Seoul. About 68.3% (95 of 139) did not use mechanical fans for ventilation at all during the heating period and 58% of 95 responders cited elevated heating energy costs as the primary reason for not using the mechanical ventilation fan. Among the occupants who intermittently used the mechanical fans in their apartments (31.7%), the average operation time was less than 2 h per day. Total heating energy consumption is increased about 32% (P < 0.05) when the mechanical fan is operated during 4 h per day. Occupants' acceptability of the indoor air is also significantly improved (P < 0.01) in group IV that operated the mechanical fan during 4h per day. Averaged increase of the total heating energy consumption by the 4 h fan operation is 0.07 GJ/m(2) and its energy cost is estimated at 1.01 $/m(2) or 85-105 $ per household over whole heating period. (c) 2012 Elsevier B.V. All rights reserved.