The HVAC module consists of two main computational  engines: The Systems module that handles secondary systems and the Plant module that handles primary systems.

The Systems module simulates performance of the secondary HVAC equipment used to control the temperature and humidity of each zone within the building. The Systems module uses the output information from the Loads module and a list of user-defined system characteristics (e.g., air flow rates, thermostat settings, schedules of equipment operation, or temperature setback schedules) to calculate the hour-by-hour energy requirements of the secondary HVAC system. The Systems module calculates thermal loads based on variable temperature conditions for each zone.

The Plant module calculates the behavior of boilers, chillers, cooling towers, storage tanks, etc., in satisfying the secondary system's heating and cooling coil loads. It takes into account the part-load characteristics of the primary equipment in order to calculate the fuel and electrical demands of the building.