Methodology has been elaborated in the Promotion Work for the calculation of energy consumption for various sets of ventilation systems in the air treatment processes using systems optimization. The operation of various air treatment devices is analysed by the variations in the enthalpy of the outdoor air, the inlet air and the outlet air treatment processes. When working out the calculation methodology, I used the climatological data of Latvia about the repetition frequency of the combinations of the outdoor air parameters in a 5-years’ interval, recalculating them to the enthalpy of the outdoor air parameters. Thus it was determined how much energy is required for a heating season in order to ensure a microclimate which complies with the animal welfare norms.