Most air-handling units in commercial buildings adopt an air-side economizer to use outdoor air for free cooling when the outdoor air is cool or cold. An economizer changeover is sequenced by high-limit controls for outdoor air. Several highlimit controls have been specified with consideration of climate zones and availability of humidity measurements. However, confusion regarding optimal outdoor air high-limit curves and the lack of accurate humidity measurements are major barriers to achieve maximum economizer energy savings. To obtain a comprehensive understanding of economizer high-limit controls and to identify an alternative for effective economizer operations, optimal high-limit curves are studied by simulating and comparing the energy consumption between minimum outdoor air and economizer modes of an air-handling unit. Investigation of impacts on optimal high-limit curves reveals that supply air temperature can replace return air humidity to attain optimal high-limit curves. A practical engineering approach for economizer control also is investigated by the use of internet weather data to substitute for local outdoor air humidity measurements.