There are 5 methods to set the infiltration rate for the SpaceInfiltrationDesignFlowRate object. I believe that setting a values will change the 'designFlowRateCalculationMethod' to the appropriate value. Try Trying looking at the your resulting value in your resulting model, or use an info statement in the measure to report the calculation method after you set the ACH.

• setDesignFlowRate()
• setFlowperSpaceFloorArea()
• setFlowperExteriorSurfaceArea()
• setFlowperExteriorWallArea()
• setAirChangesperHour()

Note that in the GUI ExteriorSurfaceArea and ExteriorWallArea display the value in the same field, but have unique calculation methods.