A method of controlling an existing heating, ventilation, and air conditioning system that is configured to condition at least one zone and includes at least one terminal box having a terminal box interface and plurality of control components. The method comprises integrating a control device into said system such that the control device is in communication with the terminal box, terminal box interface, and control components of said system. The control device collects a plurality of zone characteristics, airflow measurements, and occupancy variables, calculates for a primary air freshness based on the plurality of airflow measurements, an outside air requirement based on some of the plurality of zone characteristics and occupancy variables, and a minimum airflow set point by dividing the calculated outside air requirement by the primary air freshness. The control components of said system are modulated based on the calculated minimum airflow set point.