The unique system dynamics of five different HVAC designs including the AHU (recirculation air unit) system, the MAU (make-up air unit) / RCU system, the MAU / RCU / FDCU (Dry fan coil unit) system, the MAU / axial fan system, and the MAU / FFU (fan-filter unit) system for a typical 200 mm DRAM wafer fabrication plant are discussed and compared. The MAU / FFU system reveals the most efficient in energy performances. The MAU / axial fan system exhibits slight less efficient in energy performance, compared with the MAU / FFU system. The most influential factor on energy performance between these two systems is the design of the noise abatement system, especially the pressure drop of the silencer, which dominates almost half of the internal static pressure of axial fan. Bypassing a certain portion of fab return air to mix with make-up air can maximally reduce 50% of energy consumption of the AHU system. However, it is limited by location bypass damper installation The energy requirements to cool, dehumidify, preheat, and/or humidify outdoor air are significant in the make-up air unit (MAU) of clean room air-conditioning systems, and can represent 30% to 65% of the total thermal energy required to maintain a clean room environment. Because of these high-energy requirements, cost-effective means to reduce energy costs can influence unit production costs. Reducing or displacing mechanical cooling or electrical heating requirements can achieve the greatest opportunity for significant energy savings. This paper, therefore, aims to improve the energy performance of the MAU system by properly arranging compositions of components of a typical MAU applied in a semiconductor cleanroom. Explicitly, we investigated the influence of various factors including the fan location (draft-through type vs. push-through type), chilled water system (single chilled water temperature system vs. two chilled water temperature system), and reheating scheme (electrical heating vs. hot water provides by heat recovery chiller). The result shows that the draw-through type accompanied by two chilled water temperature system with heat-recovery function exhibits the lowest electrical power consumption. This study also aims to heat recovery methods i.e. run-around coil, heat pipe and dry cooling coil (DCC) water return, and cascade cooling, and reheating/humidification schemes including wet media, direct water atomized, steam and two phase flow. The result shows that the energy recovery by DCC water return method performs best energy utilization. Wet media scheme is the best humidification scheme in winter time. General exhaust system of clean room for Hi-tech FABs contains great deal of energy, and it can be utilized for make-up air handling unit (MAUs) for pre-cooling and pre-heating use since its temperature is lower (or higher) relative to the atmosphere. In the past, the most of similar studies focus on “percentage of transform” from exhaust system. That is, these researches are all based on first law of thermodynamics analysis and only consider in “Quantity”. However, these studies can not reflect the “Quality” of energy to be transformed. In this study, we use concept of secondary law of thermodynamics analysis to optimize a design of compact heat exchanger install inside MAUs based on specified outline dimension, fixed inlet temperature and mass flow rate for hot side and cold side. Result indicate a “appropriate” compact heat exchanger can be sized based on secondary law of thermodynamics analysis within minimum entropy production (or irreversibility) for cause by temperature difference and pressure drop both. In comparison with the most widely use “Run-around coil “ heat recovery system, the compact heat exchanger via optimization based on this study performs higher heat recovery efficiency than “Run-around coil “ heat recovery system. Beside, the optimized compact heat exchanger in this study can save 5% of energy for pre-cooling and 12% energy for pre-heating for MAUs