A pneumatic controller for use in controlling airflow in a conditioned air distribution system is adjustable to accommodate different types of damper actuation and thermostats in different systems. The four-function controller can be manually reconfigured to be a direct acting/direct reset controller, a reverse acting/reverse reset controller, a direct action/reverse reset controller or a reverse acting/directing resent controller, all using a single diaphragm valve responsive to differential pressure between total air and static air inputs. A reset unit on one end of the controller can be mounted in four different positions, two upright and two inverted, to provide the four functions, with total air and static air inputs being reversed in position when the controller is in a reverse acting mode. A differential pressure responsive valve at the heart of the system is controlled by a spring which can act as either a tension spring, for direct acting, or as a compression spring, for reverse acting. Manual adjustments are provided for adjusting the spring pressure in the absence of reset, and for adjusting the degree of change in spring pressure when reset occurs. These two adjustments control minimum pressure differential threshold and maximum pressure differential threshold. The latter adjustment preferably includes a slidable fulcrum movement for shifting the position of a fulcrum about which a reset arm pivots during reset, to control the spring tension displacement during reset from zero, at one extreme, to a maximum displacement at the other extreme.