State machine controller for operating variable air volume terminal units of an environmental control system

An apparatus for operating a system which regulates environmental conditions in a building, the system having sensors for sensing the environmental conditions and having system elements for altering the environmental conditions, the apparatus comprising:

• a state machine controller connected to operate the system elements, and having operational states comprising a plurality of automatic states in which the system elements are operated to alter the environmental conditions in response to the sensors and a plurality of command states in which operating parameters of the system are predefined, the state machine controller including a memory which contains data defining conditions for transition between the operational states wherein the state machine controller changes from one operational state to another in response to occurrence of conditions defined by the data;
• and
• a diagnostic element which detects when one of the sensors has malfunctioned, wherein the state machine controller enables the diagnostic element only when the state machine controller is operating in one of the automatic states;
• wherein the state machine responds to the diagnostic element by altering operation of the system elements when one of the sensors has malfunctioned.