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Run-Check-Edit Repeat

For most building energy modeling projects, whether assisting in early design, refining a design, selecting a control method, or calibrating an existing building, the use of EnergyPlus will be part of a repeating process. The process will probably be in the form of:

  • Running an EnergyPlus input file
  • Checking error and other output files
  • Fixing the input file
  • Repeat

Don't expect that an initial model is ever correct; it is probably not. Initially, errors are likely to exist. The .ERR file should be the first file checked each time EnergyPlus is run. The .ERR file has several levels of messages:

  • Warning
  • Severe
  • Fatal

A Fatal error means that EnergyPlus has stopped during the simulation and the input file needs to be fixed before the simulation can be run to completion. Fatal errors should be the first thing fixed. Some Fatal messages reference previous Severe messages so in that case those should be fixed. Since the entire simulation was not performed, it is likely that once the fatal errors are fixed that new Severe and Warning messages will be shown. After all Fatal messages are eliminated, you should work on Severe messages; they should also be fixed. Finally, Warning messages should be reviewed. Often Warning messages are informative and point out unusual configurations, conditions, or choices. If what is being described by the Warning message is as intended, then the Warning message can be ignored. More often, the Warning message points out something that is not as intended and should be fixed or addressed. Since the .ERR file is a text file; you can usually keep it open in a text editor program. Many (but not all) text editor programs will detect that the .ERR file has been updated after each EnergyPlus simulation and lets you load the most recent version.

The next files to be examined are ones that show output results from the simulation. Either the tabular output file (usually an HTML file see Output:Table:SummaryReports and OutputControl:Table:Style) or CSV file (see Output:Variable and Output:Meter) should be examined depending on what you want to look at. Upon examination of the output results, it is very likely that an aspect of the building and its systems are not behaving as expected. For example, the Annual Building Utility Performance Summary report contains a subtable titled Comfort and Setpoint Not Met Summary. If an annual simulation has 100s or 1000s of hours of setpoint not met, then the HVAC system is undersized, or the controls are not working as expected. With an input file representing many thousands of assumptions, some assumptions made by you or as a default of EnergyPlus are likely to be incorrect. Revising the EnergyPlus input file to address this may cause new issues to be shown in the .ERR file so it should always be examined after each change.

To speed the process of running the simulations, you may want only to run a design day (see SimulationControl and SizingPeriod:DesignDay) or a subset of the year (see RunPeriod) while developing and debugging the inputs. This approach speeds up the simulation time itself, and if used, please remember to recheck the .ERR file when running an annual simulation for the first time.