# 5. Usage of EnergyPlus as an FMU¶

The following items need to be observed when importing an FMU that contains EnergyPlus:

1. A tool that imports the FMU needs to make sure that the version of EnergyPlus which has been used to export the FMU is (a) installed and (b) added to the system path environment variable. Otherwise, the simulation will fail with an error. If EnergyPlus has been properly added to the system path environment variable, then it can be started from any DOS prompt on Windows, command shell console on Linux, or Terminal window on Mac OS by typing RunEPlus.bat on Windows and runenergyplus on Linux or Mac OS. [1]

2. The Number of Timesteps per Hour in EnergyPlus must be equal to the sampling time of the FMU. For example, consider the following EnergyPlus IDF snippet:

Timestep,
6;        !- Number of Timesteps per Hour


Then, a tool that imports the FMU must synchronize it every 10 minutes. Otherwise, the simulation will stop with an error. [2]

3. EnergyPlus contains the object RunPeriod. The start and end day of this object is ignored. [3] However, the entry Day of Week for Start Day will be used. For example, consider the following IDF snippet:

RunPeriod,         ! Winter Simulation
Winter Simulation, !- Name
1,                 !- Begin Month
2,                 !- Begin Day of Month
3,                 !- End Month
31,                !- End Day of Month
Monday,            !- Day of Week for Start Day
Yes,               !- Use Weather File Holidays and Special Days
Yes,               !- Use Weather File Daylight Saving Period
No,                !- Apply Weekend Holiday Rule
Yes,               !- Use Weather File Rain Indicators
Yes;               !- Use Weather File Snow Indicators


This IDF snippet declares January 2 to be a Monday. Hence, if an FMU is simulated with start time equal to 3 days, then the first day of the simulation will be Tuesday. There should only be one instance of RunPeriod in the IDF input file.

4. A tool that imports the FMU must specify the start and stop time in seconds at initialization. The start and stop time cannot have the value “infinity”.

5. The weather file which comes along with an FMU is used to determine if the year is a leap year. If no weather file is included in the FMU, then the assumption is that the year is not a leap year.

6. During the warm-up period and the autosizing of EnergyPlus, no data exchange occurs between the FMU and the master program. Thus, inputs of EnergyPlus remain constant during these times and are equal to the initial values specified in the IDF input file.

7. The simulation results are saved in a result folder which is created in the current working directory. The name of the result folder is Output_EPExport_xxx, where xxx is the FMU model instanceName as defined in the FMI specifications.

Footnotes

 [1] This is because the FMU implements the FMI for co-simulation in the Tool Coupling scenario.
 [2] This is because the External Interface in EnergyPlus synchronizes the data at the zone time step which is constant throughout the simulation.
 [3] This is because a tool that imports an FMU has its own definition of start time and stop time.