Twenty five cellular offices in the Wilkinson Building at the University of Sydney, Australia are ventilated through operable windows and doors and have been retrofitted with a supplementary reverse cycle cooling/heating system with an occupant controlled fancoil unit in each room. Energy consumption and occupancy and temperature status of rooms have been monitored since the system was commissioned at the end of 1997. Occupant surveys have shown that perceptions of air quality and thermal comfort have improved considerably since the system was commissioned and are now significantly above whole population averages. Regression analysis suggests significant relationships between perceived air quality, thermal comfort and overall comfort with a very strong relationship between overall comfort and perceived effect on performance of work. Estimation by dynamic simulation indicates that measured energy consumption by the supplementary system is less than a quarter of what would be expected if the same spaces were air-conditioned. Mean temperatures in occupied rooms are found to have a linear association with outdoor temperatures until the outdoor daily minimum reaches about 17oC corresponding to a mean indoor temperature of about 25oC. As outdoor temperature continues to rise mean indoor temperature remains constant indicating mechanical intervention by a considerable proportion of occupants. As would be expected, the number of rooms using the equipment displays a strong binomial relationship with outdoor daily temperatures with a minimum in mild mid season weather. Monthly energy consumption displays a similar relationship with outdoor conditions.