A new concept in control system design and installation is developing as a result of application-specific modules. The initial impact of these modules was to shift the installation of intelligent controls from the field to the factory, application-specific controllers are being installed in VAV boxes and other HVAC equipment in ever increasing numbers. These units can operate independently or be connected with a communications link into a network. Now these same modules are providing a very cost effective means of retrofitting older control systems. Also, because of their ability to perform independently, they are providing a means by which the system designer can again assume real responsibility for the design of the control system.