Psychrometric calculations use thermodynamic properties to analyze conditions and processes involving moist air. By calculating these various saturations we can determine the necessary airflows, entering and leaving air temperatures, and equipment loads of the zone. The various equations themselves are beyond the scope this paper but a brief overview of the process follows.
Psychrometric conditions are controlled by three variables: supply air temperature, set point, and humidity (which Revit allows to float). In Revit, the user must set the first two variables to a specified value, and the third he has the option to either set or allow to float (i.e. let the engine determine an optimal humidity). If the humidity is allowed to float, then the engine will attempt to find a humidity that minimizes additional load gains. If the humidity is specified, there may be cases where the psychrometric conditions cannot be attained (the engine allows a 5% tolerance). In this case, an error is displayed for the user in the loads report.
Assuming that the conditions can be attained, the airflow for the zone is the first component calculated, followed by the required ventilation. Once these are known, the engine determines the psychrometric characteristics for the outside air conditions. This allows for the calculation of the entering air and the mixing air temperatures. After these temperatures and airflows are known, the engine can determine the remaining values including ventilation loads, equipment loads, and reheat values.