Abstract: A compact indoor environmental control system which has cooling, heating, and ventilation components mounted and positioned to provide a slim design for in-the-wall installation. A cabinet is provided with cooling and heating components mounted to leave plenums at opposite ends of the cabinet so that air outlets can be selected on any one of five sides of the plenums. The heating, cooling and control sections are of modular design to permit full accessibility for maintenance, repair or replacement. A variety of configurations for intake and exhaust of inside and outside air are possible because of the versatility provided by the modular construction.