Different results in Ideal Loads if I calculate the ventilation + infiltration load all together at the zone level than if I calculate the infiltration at the zone level and the ventilation at the system level.

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With the same building I have simulated the building by placing all the outside air in SpaceInfiltrationdesignflowrate and with openstudio it gives me this results in End Use Use Consumption (kWh) Heating 2,186 Cooling 319.

Then with the same building I have simulated the building separating the outside air in SpaceInfiltrationdesignflowrate and DesignSpecificationOutdoorAir and with openstudio it gives me this results in End Use Use Consumption (kWh) Heating 1,789 Cooling 333.

I have used OpenStudio together with the Ideal Air Loads HVAC measure.

Make sure you defined the DesignSpecification:OutdoorAir object into the Ideal Air Loads object. Check the idf file.

It is defined in the idf file. Energyplus must calculate differently when the air enters the thermal zone through infiltration than when the air enters through mechanical ventilation. The strange thing is that there is such a difference.