Revit Systems Analysis

A. Use Case 1: Application of the PRM measure to a BIM model - Revit Systems Analysis Workflow

This use case showcases how an energy modeler can utilize a BIM (Building Information Modeling) model created in Revit by an architect/engineer and can translate it to a BEM (Building Energy Modeling) model to apply the PRM measure in OpenStudio. The goal is to automatically generate a proposed and baseline model(s) according to ASHRAE 90.1 2019 Performance Rating Method (Appendix G) to streamline the energy modeling process.

Revit is a building information modeling software used for planning, designing, constructing and managing buildings and infrastructure. The following workflow is based on the capabilities present in Revit 2024 at the time of writing and is likely applicable to other versions. Changes and updates to the software may lead to modifications in these steps over time.

The steps to apply the PRM measure to a BIM model created in Revit are as follows:

  1. Create a design model
  2. Define assemblies, internal loads, HVAC
  3. Export the model
  4. Apply 90.1 PRM measure
1. Create a design model

A layout of a 3 storey medium office building was created using the Revit elements like walls, roofs, floors, and windows. The building is rectangular in shape with a footprint measuring 161 ft by 108 ft, where the longer perimeter sides have a north-south orientation and contains 17,420 ft2 per floor. The window to wall ratio (WWR) was around 32%. Spaces were separated as conference rooms, private as well as open office spaces based on their function.

Plan 3D Model
userdata userdata
2. Define assemblies, internal loads, HVAC

  • Envelope Construction: Envelope assemblies were assigned using schematic construction types present in the Revit built-in library. Generic or actual construction types can also be used based on the amount of information available at hand. Envelope Construction was assigned using the Energy Settings tab on Energy Optimization under the Analyze menu.

  • Internal Loads: Internal loads were assigned based on the default loads present in Revit. The defaults could be based on the building typology or on space by space method according to various standards and databases. These assumptions can be accessed at Revit Help. In this example, the occupancy, lighting and power definitions, and schedules were assigned to spaces based on space-by-space method.

  • Thermal Zones: Thermal zones were assigned based on the core/perimeter layout.

  • HVAC: A gas furnace inside a Packaged Air Conditioning Unit (PACU) with Variable Air Volume (VAV) reheat was modeled with one system serving each floor.

HVAC

  • Location: Location was set up as Denver, Colorado (Climate Zone 5B) using the Location option under Energy Optimization under the Analyze menu.
3. Export the model

Then, the Revit model was exported as an OpenStudio model using Revit Systems Analysis feature since it offers a streamlined workflow and effectively translates most of the gbXML content into BEM using OpenStudio measures. “Rooms and Spaces” mode was used which allows the BEM model to inherit all rooms and incorporate the pre-assigned thermal zoning from the BIM model. The .osm file generated was used to export the model to OpenStudio.

3D Model in OpenStudio after Export HVAC Layout in OpenStudio after Export
Export HVAC

Other import alternatives could be used such as the gbXML export. Some of the options include generating a gbXML file using Revit native gbXML export, Systems Analysis or Revit Insight. Any information not translated properly during the export process or missing must be addressed before running the PRM measure. Note that the basic import feature of the OpenStudio application (i.e. the user interface) only translates a limited set of gbXML contents including geometry, constructions, thermal zones, and schedules. The translation of the remaining contents can be achieved using OpenStudio measures.

4. Apply 90.1 PRM measure

Finally, the 90.1 PRM measure was applied in OpenStudio by following the steps mentioned in the sections Use with OS App, Use with OS SDK using CLI or Use with OS SDK API.

Key information required to run the PRM measure includes space types for lighting power density, building area type for window to wall ratio, HVAC system selection and water heater system, and process loads. Thus, before running the measure, it is important to check the exported BEM model for any missing information and addressing them by following the steps provided in Best Practices section to ensure a successful measure run. Weather file must be uploaded in OpenStudio before running the simulation.

Once the PRM measure was applied on the BEM model, a proposed as well as baseline model(s) following the set of PRM rules were generated, some key details of which are provided below.

3D Baseline Model in OpenStudio rendered by surface type HVAC Layout in OpenStudio for the Baseline model
Export HVAC
  • The generated baseline model had a WWR of 31%.
  • The model showed the correct boundary conditions for all elements.
  • HVAC system transformed to include one hot water loop, 3 systems of Type System 5.
  • Opaque assemblies were transformed to the following:
Category Dependent Variable Standard Requirement Baseline Value
Exterior Wall Climate Zone 5B, nonresidential U-0.084 (R-11.9) PRM Steel Framed Exterior Wall R-11.9
Roof Climate Zone 5B, nonresidential U-0.063 (R-15.87) PRM IEAD Roof R-15.87
Window Climate Zone 5B, nonresidential, 30-40% WWR U-0.57, SHGC-0.39, VT-0.43 PRM U 0.57 SHGC 0.39 VT 0.4 Simple Glazing Window
  • Lighting Power Density assigned based on the space types.
Category Dependent Variable Standard Requirement Baseline Value
LPD Office, Open plan 1.10 W/ft2 1.10 W/ft2
Office, Enclosed 1.10 W/ft2 1.10 W/ft2
Office, Conference 1.30 W/ft2 1.30 W/ft2
  • Dependent Variable: The variables that determine the changes from a user model to a baseline model
  • Standard Requirement: The requirements mentioned in the ASHRAE 90.1 2019 PRM method for a baseline model
  • Baseline Value: The values assigned to the generated baseline model to match the Standard Requirements
Best Practices

  • Standard Space or Building Type Mapping

    • Cross validate different building area types. In PRM there are three building type categories used to determine: the baseline window-to-wall-ratio (WWR), HVAC system type, and service hot water (SHW) system type that needs to have the same type assigned to them. For example, if the section of a building model is categorized as an Office (<=5000 ft^2), it must not be classified as a Multifamily building type to determine the baseline model SHW system.
    • If required, manually map space types from Revit to those listed in PRM to align them accurately.

  • HVAC System Integration

    One of the key data elements often missing in the initial stage of design is related to the HVAC systems. For example, ideal load air system commonly defined for earlier design phases since a complete HVAC system is not typically necessary. Additionally, various export methodologies might not be able to translate the HVAC system design into energy models. Following steps must be used to ensure a smooth PRM run.

    • Address missing HVAC system data including details like fan power distribution ratios, crucial for a fully automated solution.
    • A solution that can be adopted is generating the export using Revit Systems Analysis’s built-in functionality. This feature generates a complete OpenStudio model using a gbXML based schema that includes a comprehensive HVAC system description, making it ready for the OSSTD-PRM generation process.

  • Other building service systems

    • Ensure data related to other building service systems such as SHW, heating, renewable and system controls, is included in the BEM model to avoid inaccuracies in the baseline model.
    • Consider default settings or templates provided by OpenStudio for creating these systems if necessary.