Abstract: Systems and methods are described for a HVAC control system. An apparatus, includes: a microcontroller; a digital temperature sensor coupled to the microcontroller; a liquid crystal display coupled to the microcontroller; a set of cursor buttons coupled to the microcontroller; a first data structure of personal identification numbers coupled to the microcontroller; a second data structure of historical system usage data coupled to the microprocessor; an upload software thread coupled to the microprocessor and to the first data structure; a billing computer coupled to the second data structure; a real time clock coupled to the microcontroller; and a back up power supply coupled to the real time clock.

Abstract: Systems and methods are described for a usage monitoring HVAC control system. A method, includes: providing a usage monitoring heating ventilation and air conditioning control system, the usage monitoring heating ventilation and air conditioning control system including a programmable digital thermostat with an on board memory, issuing personal identification numbers to each of a plurality of system users; associating each of the plurality of system users with at least one of a plurality of user types; storing the personal identification numbers in a first data structure in the on board memory; and linking each one of a plurality of entries in the first data structure by reference to at least one of a plurality of entries in a second data structure in the on board memory, the second data structure including a list of user types.

Abstract: Systems and methods are described for synchronizing an HVAC control system. A method, includes: a synchronization sequence including: reading a base time from an internal clock at a first time and saving the base time; measuring an elapsed time interval, from the first time to a second time, by counting an external clock using a frequency counter; and then resetting the internal clock to the base time plus the elapsed time regulating user access to an interactive user interface of a programmable thermostat, each user identified by a personal identification number associated with a user type, wherein each personal identification number is recognized by the programmable thermostat as belonging to an access level and users identified as building owners or maintenance personnel are granted further system access.