The present disclosure provides a method for controlling an economizer of an HVAC system. The economizer has an outside air stream of incoming outside air, a return air stream of return air, and a mixed air stream of mixed incoming outside air and return air that is provided to the cooling unit of the HVAC system. The economizer also includes one or more controllable dampers for controlling a mixing ratio of incoming outside air to return air in the mixed air stream. The control method includes positioning the one or more controllable dampers in first and second configurations such that the mixed air stream has first and second mixing ratios of incoming outside air to return air in the mixed air stream. The method also includes recording first and second measures related to the temperature of the mixed air stream when the dampers are in each of the first and second configurations. First and second measures related to other parameters of the mixed air stream may be recorded as well. Based on the recorded first and second measures related to the temperature of the mixed air stream and possibly other recorded measures related to mixed air stream parameters, the method determines whether and/or how much of the incoming outside air to admit into the economizer via the one or more controllable dampers during subsequent operation of the HVAC system.