Indoor air quality and economizer control methods and controllers

An HVAC controller for use in an HVAC system, the HVAC system including:

• a fan, cooling equipment, and fresh air ventilation components;
• first means for sensing enthalpy disposed to sense system return air enthalpy;
• and
• second means for sensing enthalpy disposed to sense outside air enthalpy;
• the HVAC controller configured to receive signals from both the first and second means for sensing enthalpy, the HVAC controller adapted to perform the following steps:
• determine whether the first and second means for sensing enthalpy are functioning;
• if both the first and the second means for sensing enthalpy are functioning, operating the HVAC system using an economizer mode having a differential enthalpy economizer decision process;
• and
• if the first means for sensing enthalpy is not functioning, operating the HVAC system using a second mode that does not include a differential enthalpy economizer decision process.

A method of operating an HVAC system comprising:

• determining whether an enthalpy sensor disposed for sensing return air enthalpy is operational;
• determining whether an enthalpy sensor disposed for sensing outdoor air enthalpy is operational;
• if both sensors are operational, operating the HVAC system using an economizer mode using dual enthalpy analysis;
• and,
• if the sensor disposed for sensing outdoor air enthalpy is operational and the sensor disposed for sensing return air enthalpy is not operational, operating the HVAC system using an economizer mode using a single enthalpy analysis.

A system for controlling an HVAC system including number of sensors and HVAC operation devices including a fan, dampers, and a temperature modification apparatus, the system comprising:

• a first controller configured for providing a specialized HVAC system method;
• a second controller having inputs and outputs for coupling to the first controller, the sensors, and the HVAC devices, the second controller adapted to perform the steps of:
• observing which sensors are functioning;
• determining whether the specialized HVAC system method is appropriate;
• and
• controlling interactions between the first controller and the HVAC devices.