The present invention is related to the field of heating, ventilation and 
air conditioning (
HVAC). More particularly, the present invention is related to methods and systems for controlled heating and cooling in order to reduce costs and the 
carbon footprint of said heating and cooling by optimizing the use of 
fresh air ventilation. The present invention is directed to mathematical algorithms incorporated into a controller and a method of determining control signals that are dependent on said mathematical algorithms and user 
programming that integrates information from multiple sensors, thermostats as well as weather information. Used in any home or building, the controller controls heating, cooling and ventilation systems in order to reduce costs and the 
carbon footprint of said heating and cooling by optimizing the use of 
fresh air ventilation. The controller works with typical 
HVAC systems generally in buildings and homes. The Smart-Stat algorithms are programmed into the controller and enable the controller to identify user-determined set-points alongside data from one or multiple 
internal temperature sensors. The user-determined set-points are also linked to 
time of day and day of week in a manner typical for typical 
thermostat devices available today. In such typical 
thermostat devices the controller will call for cooling or heating depending on the set points and conditions determined by the sensors in the building. The present invention is capable of interrupting the call for cooling or heating depending on whether the mathematical algorithms identify suitable outside weather conditions that permit the use of outside 
air cooling or outside 
air heating. Thus the call for heating or cooling can be redirected to call for ventilation instead of heating or cooling.