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RESNET Forms HERS® Software Consistency Task Group to Enhance the Consistency of the Calculation of HERS® Index Scores

Jan 11, 2018

A critical element in ensuring the consistency of HERS® Ratings is enhancing the consistency which RESNET accredited HERS® Software Programs calculate the HERS® Index Score. There are currently situations where a home could have a different HERS® Index Score based upon which software program is being used.

At the 2017 Fall RESNET Board of Directors Meeting, the RESNET Board adopted the following actions in terms of enhancing the consistency of how HERS® software calculates HERS® Index Scores:

  • Set a Timeline Where All Accredited HERS® Rating Software Simulation Engines Must be Based on Hourly Calculations – Hourly energy use simulation is the current industry standard modeling of the energy performance. RESNET has one software provider that is based on seasonal simulation. Previously the RESNET Board adopted a policy that all accredited HERS® software simulation engines must be hourly based. A deadline for this being required was never adopted.
  • Develop a Common Schema for HERS® Software Programs – A common schema is a file format with a standardized set of building input fields that facilitates the exchange of data/information between software tools. A common schema will provide:
    • Uniformity in data inputs
    • Ability to map data between software platforms
    • Consistency in how data is uploaded to/downloaded from the RESNET National Registry
    • Streamlined analytical and data processing capabilities via the RESNET Registry
  • Enter into a Collaborative Modeling Process to Resolve Differences Among Software Programs – The collaborative modeling approach will identify example homes where software tools produce HERS® Indices that are significantly different from one another and probe into the details to identify the source(s) of the difference(s). Some differences originate in modeling details where RESNET does not give sufficient guidance to software tools as to which calculation method to use. The collaborative approach will incrementally solve issues with stricter modeling guidelines as issues arise. It would also involve discussions about timing of updates so as to not be disruptive to the market.
    • HERS® software tool developers then would update their modeling engines / assumptions. Releases of software updates with significant HERS® impacts would be coordinated with RESNET to minimize industry disruptions.
    • An example of an area already identified as needing further guidance is modeling foundation wall heat transfer. The Standard does not specify which calculation method to use, and software tools are accordingly using very different methods. In a case such as this, RESNET would specify a specific modeling procedure, e.g. the Winkelmann method.
  • Develop a Draft Work Scope and Budget for a RFP for a Common HERS® Index Platform – A common HERS® Index platform would consist of utilizing an existing simulation engine, and implementing an interpretative layer to input data into the engine and produce calculations of the HERS® Index and determination of ENERGY STAR compliance. This approach would virtually eliminate HERS® Index inconsistencies by the various HERS® software programs.

On December 14, 2017, the RESNET Board adopted the following timeline where all accredited HERS® Rating simulation must be based on hourly calculations:

All accredited HERS® software programs shall demonstrate that their simulation engines are hourly based as part of the 2020 accreditation process.

RESNET has formed a HERS® Software Consistency Task Group to oversee the work on the common schema, collaborative modeling and adoption of a single calculation engine.

The task group members are:

  • Kelly Stephens, SunRiver St. George, Chairman
  • Jacob Atalla, KB Home
  • Martha Brook, California Energy Commission
  • Brian Christensen, NORESCO
  • Ethan Croteau, WrightSoft
  • Philip Fairey, Florida Solar Energy Center
  • Cy Kilbourn, Ekotrope
  • Amir Nosrat, ICF International

The task group would have two subcommittees:

  • Schema Subcommittee – Will be responsible for initially setting up a work plan proposal to the RESNET Board and then develop the schema. It is recommended that Philip Fairey be appointed to chair the subcommittee.
  • Collaborative Modeling Subcommittee – Will oversee the collaborative process. The first task for the subcommittee will be to develop a work plan for the board to approve. It is recommended that Cy Kilbourn be appointed to chair the subcommittee.

In terms of the single engine development, RESNET staff will develop a work plan and proposed budget for Board approval then draft the RFP for the Board’s consideration.

RESNET will continue to offer updates as the work of the task group progresses, including a session ” HERS® Consistency – A Discussion from a Software Perspective” during the 2018 RESNET Building Performance Conference. For more information go to 2018 RESNET Conference.