Cushing Library located on Texas A&M University campus is a special building, which needs precise temperature and relative humidity control, because it stores a number of rare collections and memorial books. This paper will concentrate the unit, which serves the book stacks. This AHU is a multiple zone, constant air volume (MZCAV) system, with reheat and direct digital control (DDC). It has a standard cooling coil, glycol cooling coil, steam humidifier, and heat recovery. Because of programming problems and hardware problems, the relative humidity was not controlling properly. In this paper, the new control program for temperature and relative humidity control is implemented and the energy savings from the new control program is estimated.