Abstract: A bypass system is incorporated into a roof curb to permit selective partial or complete deactivation of at least one section of a heat pipe of an air conditioning system thereby 1) to permit optimization of the sensible heat ratio of the air conditioning system for prevailing environmental conditions and, 2) to prevent moisture from condensing onto the evaporator or cooling section of the heat pipe and subsequently dripping into the return ducts of the air conditioning system. The bypass system is characterized by a bypass duct located adjacent one of the sections of the heat pipe and a bypass device which selectively channels at least some of the air which would otherwise flow through the controlled section of the heat pipe through the bypass duct instead. In its simplest form, the bypass device may comprise a single damper or the like positioned within the bypass duct.