This paper addresses an optimal design problem of ERV (Energy Recovery Ventilator) system that seeks for proper sizing and location of supply diffusers/exhaust registers. A typical of a residential unit consisting of three bedrooms, a living room, and a kitchen was selected. The elements in a cost function includes energy use, thermal comfort, and $CO_2$ concentration, leading to multi-criteria optimization problem. In this study, the aforementioned optimization problem was solved using Analytic Hierarchy Process (AHP), Chaos Genetic Algorithm (CGA) and EnergyPlus 8.0. AHP was employed to calculate relative weighting factors between the aforementioned different three performance aspects. Since the given model-based optimization problem is discontinuous and nonlinear, the authors integrated CGA to EnergyPlus. It has been elaborated in the paper that the CGA approach is superior to a traditional Genetic Algorithm (GA) in terms of simulation time and searching for global optima.