An air conditioner with coolant pumped from a chiller (11) firstly through a heat exchange conduit (23) of an outside air heat exchanger (10) to provide a minimum wetted surface temperature when treating moist outside air at its (maximum) temperature which is supplied through this heat exchanger (10), providing maximum dehumidification. The leaving coolant from the outside air heat exchanger (10) is the source of the coolant which then passes to the return air heat exchanger (12) which cools and dehumidifies the return air. The leaving air from the return and outside air heat exchangers (12, 10) are mixed to become the supply air for a conditioned room which is economically dehumidified without need to overcool and reheat, nor to curtail the design flow rate of ventilation air and hence without contributing to "sick building syndrome" conditions developing. As a consequence is a design with performance to standards over its full range of operation.
