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Key Concepts

The following sections highlight some key concepts in EnergyPlus

Everything Included

One principal that EnergyPlus uses is that (almost) everything is specified in the input file. This means that instead of referencing an external library for materials, schedules, equipment performance, etc., the input objects that fully describe those items should be included directly in the input file. In addition, each input object contains a list of values for every field that needs one. The DataSets folder distributed with EnergyPlus contains these kinds of details and to use them, the input objects should be copied into the input file that you are developing. This approach does make the file include more specification than you might be used to, and typically results in a large input file, but you will have the assurance of knowing that all the inputs related to your building are in the input file you have developed. There are a few exceptions where external data is referenced such as with Schedule:File input objects.

Wall Thickness

Exterior and interior walls in real buildings have a thickness as specified on building plans by detailed cross-sections. For EnergyPlus, the Construction input object is made up of a list of names for the Material input objects that make up the wall or roof or floor. Each material input object has a thickness along with the conductivity, density, specific heat and other factors. These thicknesses should match the thicknesses shown in the detailed cross-sections. But when it comes to specifying the walls themselves in three-dimensional space, the walls should be entered assuming zero thickness. Once each surface has been placed, changing the material thickness will have no impact on zone volume, ceiling height, floor area, shading, or daylighting. For most modern buildings the choice of where to locate the wall: inside vs. outside vs. centerline should have little impact on results, so many modelers just pick one and let the volumes be slightly off. Using centerlines throughout the model splits the difference. Or some modelers use outer edges for exterior walls and then use centerlines for interior walls. If you are modeling a very thick wall, such as an old stone building, then you also have thermal mass considerations. If you use the outside edges there will be too much mass, inside will be too little. Again, centerline will split the difference and will be very close to the correct amount of thermal mass (possibly losing some corner mass).

Zones Are Not the Same as Rooms

A zone, sometimes called a "thermal zone," is a theoretical construct that usually describes a group of rooms that can be treated as a single thermal entity. A zone typically includes multiple rooms. Often a zone can be seen as a group of rooms that share a single thermostat. In the case of many similar rooms with the same thermal and operating characteristics, even if many different thermostats are used, they may still be grouped into a single zone in EnergyPlus.

One Construction Per Surface

Only one type of construction can be associated with each surface so if the top half of a wall is made up of a different construction than the bottom half of the wall, the top half and the bottom half each need to be represented as separate surfaces.

Available Outputs Created by First Simulation

Since EnergyPlus creates a custom list of possible output variables during each simulation, you need to perform a simulation first before you can see them. To create the list use the Output:VariableDictionary input object and then check the .RDD and .MDD files that are created. Select the outputs you want and specify them in the input file.

Always Plan Ahead

Some preliminary steps will facilitate the construction of your input file. EnergyPlus requires some information in specified, externally available formats; other information may require some lead time to obtain. The following checklist should be completed before you start to construct your input file.

  • Obtain location and design climate information for the city in which your building is located. If possible, use one of the weather files available for your weather period run.
  • Obtain sufficient building construction information to allow specification of overall building geometry and surface constructions (including exterior walls, interior walls, partitions, floors, ceilings, roofs, windows, and doors).
  • Obtain sufficient building use information to allow specification of the lighting and other equipment (e.g., electric, gas, etc.) and the number of people in each area of the building.
  • Obtain sufficient building thermostatic control information to allow specification of the temperature control strategy for each area of the building.
  • Obtain sufficient HVAC operation information to allow specification and scheduling of the fan systems.
  • Obtain sufficient central plant information to allow specification and scheduling of the boilers, chillers and other plant equipment.
  • Obtain utility tariff information when expressing the results as costs.
  • Obtain component cost information when performing life-cycle costs.