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Cx Monitor: ASHRAE Standard 209 – Energy Simulation Aided Design for Buildings

| Clark Denson

Building energy modeling has been used for many years to show compliance with local energy codes and to document energy performance-related points in beyond-code rating systems such as LEED® and Green Globes.  When used in this manner, energy modeling is typically seen as a necessary evil – just another box to check at the end of a project.  However, for just as long, a multitude of articles, white papers, and guidelines have been written highlighting the benefits of using energy modeling as a decision-making tool throughout the design process.  More recently, a growing number of building owners, utility incentive programs, and beyond-code rating systems have been looking to include a more structured, common-sense approach to energy modeling in their projects, but many have been looking and waiting for a central, guiding document to show the way.  Hopefully that wait will soon come to an end with the new ASHRAE Standard 209 – Energy Simulation Aided Design for Buildings.

While not published yet, Standard 209 is designed to provide a flexible framework through which an “adopting authority,” such as those mentioned above, can require a minimum level of energy modeling on a project.  For starters, the Standard requires that projects analyze the local climate and site to identify potential project-specific strategies, identify energy benchmarks against which to compare performance, set whole building energy goals, and conduct an energy-focused charrette or workshop.  During this charrette, all these tasks shall result in energy performance goals being written into an Owner’s Project Requirements (OPR) document.

The Standard then presents the concept of “modeling cycles,” or specific modeling tasks accompanied by common tasks related to gathering data, results reporting, and quality assurance.  There are seven modeling cycles identified for use during design, each with its own unique purpose and analytical steps for using energy modeling to help answer pertinent design questions.  These Design-Phase cycles include:

  • Simple Box Modeling
  • Conceptual Design Modeling
  • Load Reduction Modeling
  • HVAC System Selection Modeling
  • Design Refinement
  • Design Integration and Optimization
  • Energy-Simulation-Aided Value Engineering

After construction documents are issued, the Standard provides opportunities for four more modeling cycles during the Construction and Operations phases:

  • As-Designed Energy Performance
  • Contemplated Change Orders
  • As-Built Energy Performance
  • Post-occupancy Energy Performance Comparison

To be clear, completion of all the cycles is not required for compliance with the Standard.  Rather, the default path for compliance is completing 1) the Climate Analysis, Benchmarking, and Goalsetting tasks, 2) Modeling Cycle #3:  Load Reduction Modeling, and 3) one other Design-Phase modeling cycle.  If the adopting authority so desires, additional modeling cycles from the Standard can be required.

It was the goal of the ASHRAE Standard 209 committee to write a standard for energy modeling with enough flexibility that adopting authorities would include energy modeling early and often in more design projects, while providing a framework that can be applied consistently across the industry.