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You need to request the output variables first.
To check for the total ventilation load, you can try requesting the following output variables :
Zone Ventilation Sensible Heat Gain Energy
Zone Ventilation Sensible Heat Loss Energy
Zone Ventilation Latent Heat Gain Energy
Zone Ventilation Latent Heat Loss Energy
and if you subtract the total(sensible+latent)gain - total(sensible+latent)loss, you should get the total ventilation load.