Patent ID: 7434413

Claim:
An HVAC controller for use in an HVAC system, the HVAC system including: a fan, cooling equipment, and fresh air ventilation components; a return air enthalpy sensor for sensing system return air enthalpy; and an outside air enthalpy sensor for sensing outside air enthalpy; a dry bulb sensor disposed to measure outside air temperature, wherein the dry bulb sensor is distinct from the outside air enthalpy sensor; the HVAC controller configured to receive signals from the return air enthalpy sensor, the outside air enthalpy sensor and the dry bulb sensor, the HVAC controller adapted to perform the following steps: determine whether the return air enthalpy sensor and the outside air enthalpy sensor are both functioning; if both the return air enthalpy sensor and the outside air enthalpy sensor are both functioning, operating the HVAC system using an economizer mode having a differential enthalpy economizer decision process; if the return air enthalpy sensor and the outside air enthalpy sensor are both not functioning, operating the HVAC system using a second mode that does not include a differential enthalpy economizer decision process; determine whether the dry bulb sensor is functioning; if the outside air enthalpy sensor is not functioning, and if the dry bulb sensor is functioning, operating the HVAC system using an economizer mode having an outside air dry bulb decision process; and wherein the HVAC controller is further adapted such that, if the outside air enthalpy sensor is functioning: if the dry bulb sensor is not functioning, operating the HVAC system using an economizer mode not having an outside air dry bulb override decision process; or if the dry bulb sensor is functioning, operating the HVAC system using an economizer mode including an outside air dry bulb override decision process.