RESNET Embarks on Effort to Enhance Consistency of HERS Software Programs Calculating the HERS Index

Since 1996, The Residential Energy Services Network (RESNET) has been the national standard setting body of the national home energy rating (HERS) industry, charged with developing national standards and growing the market.  During the early days of RESNET and home energy ratings, very few homes were being rated because the market was so young and many builders and municipalities were unaware of or unfamiliar with home energy ratings.  As the HERS industry grew and the number of homes rated in the US increased, there was increased scrutiny of home energy ratings.  Many studies have been conducted on HERS Rating Software on the issue of accuracy and repeatability.  With more builders rating their homes and more jurisdictions adopting the HERS Index as a code requirement, the HERS Index has come under a microscope.  Currently in the crosshairs of builders and the rating industry is the issue of HERS Index consistency.

Over the past three years (Since October of 2013), RESNET has embarked on a mission to address the issues of quality and consistency in the Home Energy Rating (HERS) Index scores to ensure credibility and a successful future.  There has been growing concern among national production builders that a single home could show differing HERS Index scores from different RESNET Accredited Providers.  This concern could create mistrust in the HERS Index and reduce the value to builders and the industry. The question at hand is: Where does the issue with inconsistency exist – with the Rating Software or the Providers/Raters utilizing the software?  To address the concerns from the builders, RESNET convened a HERS Index Score Consistency Task Force composed of builder representatives, HERS Software developers and Rating Providers. The task force identified three areas generating inconsistency and gave the following recommendations.

  1. Enhanced Quality Assurance of Ratings
  2. Enhanced Training and Testing of Home Energy Raters
  3. Home Energy Rating Software Program Improvements to Ensure Greater Consistency in the calculation of HERS Index Scores

The HERS Index Score Consistency Task Force appointed a group composed of rating software developers, quality assurance designees, rating providers, and homebuilders to develop specific software recommendations for each the following areas:

  1. Establish limits on input variables for whole-house ventilations systems in the RESNET Standards
  2. Determine bounds checks that can be incorporated into software to limit or warn users when input values are beyond reasonable limits
  3. Enhance rating software tools to enable QAD flags to be set for internal inconsistencies that should be checked prior to entering a building file into RESNET registry
  4. Modify RESNET registry XML schema to include reporting of QAD flags

HERS Rating Software vendors were required to implement these recommendations in order to remain RESNET accredited.  Right around this timeframe, the RESNET Board approved the RESNET/ANSI 301 Standard for adoption into the HERS Rating Software.  Given the amount of development time required to implement the Task Force recommendations and the ANSI/RESNET/ICC 301 Standard, RESNET mandated that HERS Rating Software shall be fully compliant by July of 2015.  However, due to the inability to consistently implement the 301 standard , the deadline has been extended a few times, and currently stands at July of 2016. With the recommendations by the Software Consistency Group in addition to the continued extension of the RESNET/ANSI 301 Standard deadline, it appears that HERS Index consistency remains elusive.

Since the recommendations of the HERS Index Score Consistency Task Force, other concerns about the HERS Index have arisen.  There is growing trepidation about the lack of transparency in the HERS Software algorithms, modeling engines and the interpretations of the RESNET Standards.  There is also concern over the slow response of the HERS Rating Software tools to incorporate new building or HVAC technologies into modeling engines.  RESNET is looking for a better answer to the HERS Consistency issue that addresses rating software tools, transparency and technological advances.

The RESNET Board of Directors has been focusing on a motion that would sponsor the creation of a single simulation engine for calculating Home Energy Ratings.  The motion involves developing an open-source Energy Rating Index (ERI) Software Module that provides a HERS Index and associated data per RESNET/ANSI/ICC Standard 301.  RESNET will be the financial backer and steward of the ERI Software Module and manage it going forward.  The ERI Module will take input from third party HERS software providers and return results; it will not include an input interface or formatted reports that compete with third-party software providers.  Simply put, any third party RESNET-accredited software provider could send building data to the ERI Module to calculate the HERS Index.  Third party software providers would be alleviated of developing and maintaining code implementing RESNET/ANSI/ICC Standard 301 as well as a building energy simulation engine. This allows them to focus on business support as well as the user experience. The ERI module will leverage the publically available, and broadly supported, EnergyPlus Building Energy Simulation Software.

As part of this RESNET motion, RESNET will update Procedures for Verification of RESNET Accredited HERS Software Tools (RESNET Publication No. 002-15) based on EnergyPlus, providing an accreditation pathway for new or existing HERS Software vendors.  RESNET accredits HERS Software tools based on accuracy tests outlined in RESNET Publication No. 002-15.   The accreditation tests currently utilize accuracy criteria based on DOE-2, BLAST and SUNREL simulation engines and would be updated using EnergyPlus simulation results.  Any HERS Software tool that passes the updated RESNET accreditation test suite will be accredited to calculate the HERS Index without using the RESNET ERI Software Module.

A RESNET funded ERI Software Module will:

  • Improve consistency within the HERS Index scores.
  • Provide transparency with algorithms and rulesets.
  • Increase the speed with which new, emerging efficiency technologies are recognized in HERS.

Consistency is challenged by lack of robust tool input QA, differences in how the standards are interpreted and implemented, differences in how a limited number of inputs characterizing Rated Features are utilized to drive robust simulation engines[1], and differences in simulation engines.  Transparency will foster broad community collaboration and acceptance; it provides an opportunity for a wide range of stakeholders to contribute to the effort (e.g., HERS software developers, manufacturers, trade organizations, etc.), thus improving accuracy and increasing capabilities.  The ability to incorporate new technologies into HERS Ratings has been stymied by reliance on a small group of volunteers to develop calculation guidance and develop standards, and private software companies who redundantly implement the standards and develop the capability to model the new technologies.

The HERS industry is not large enough to support multiple software providers developing and maintaining interfaces, compliance processes, and calculations engines.  Competing in all three areas either dilutes resources to the point of being ineffective, or results in a single provider dominating the market.  Neither benefits the industry.  Moving portions of this responsibility into a communal environment will reduce risk for the industry.  A public-sector, open-source building energy simulation engine, EnergyPlus, already exists and is supported by a large and broad group of stakeholders.  RESNET can leverage this resource, and build upon it a communally-developed and supported RESNET/ANSI/ICC Standard 301 calculation module.  RESNET can foster this development by providing seed funding, management and stewardship of the resource.

The California Energy Commission (CEC) has already completed such a uniform single-source software tool for code compliance.  The CEC organized and funded the development of the California Building Energy Code Compliance (CBECC) software tool for Title 24 Code Compliance.  CBECC is an open source compliance software tool utilizing Open Studio/EnergyPlus for non-residential buildings.  CBECC-Residential employs the California Simulation Engine (CSE).  CBECC does not collect building data but requires third party software tools to capture building data and send it to CBECC Compliance Manager via an SDD XML Standard.  Any third party code compliance software must utilize CBECC for residential or commercial Title 24 code compliance. The development of CBECC was a collaborative effort between CEC, software developers, industry players and utilities.  The idea was to provide a central platform housing software to provide data validation, ruleset regulation and compliance determination.  CBECC contains measure libraries and climate data that is utilized in the building models.  CBECC was developed to satisfy Section 25402 of the Warren-Alquist Act to “develop a public domain computer program which will enable contractors, builders, architects, engineers, and government officials to estimate the energy consumed by residential and nonresidential buildings.

The question remains, for Home Energy Ratings, would a similar style of public domain software improve HERS Index consistency?  Some may argue that an “interpretation layer” over the current rating software to translate building input data into the RESNET/ANSI/ICC 301 ruleset would provide better consistency.  Better training on the software could also provide better transparency without RESNET funding an ERI Software Module.  Maybe utilizing EnergyPlus to improve accuracy testing criteria will force existing software tools to be better aligned thus improving consistency.  There could be other arguments against a RESNET-funded public domain single source software for HERS Index consistency.  The industry needs open source collaboration to bring the software tools to the next level.  The NASA Open Government Initiative states the following about open source software development.

Open source development-which allows free access to software source code to allow anyone to make improvements-is revolutionizing the way software is created, improved, and used. The open source software movement is inherently transparent, participatory, and collaborative. Open source at NASA gives the public direct and ongoing access to NASA technology. Its adoption by NASA also helps lower the barrier to entry into space by enabling private industry to better make use of NASA investments. Although open source release has already provided numerous benefits to NASA and the public, the full benefits of open source can only be achieved when we establish the processes, policies, and corporate culture to favor open source development. This would shift our open source activities from its one-way direction of giving the public access to finalized software products, to allowing two-way collaboration as part of the development process. The benefits of allowing the public to assist in development of NASA software include increased software quality, accelerated software development, and a higher rate of technology transfer both to and from NASA.

RESNET has always relied on third-party HERS Rating software tools to calculate the HERS Index.   Each rating software tool utilizes their own development teams without sharing resources or technology.  RESNET is in the position to foster a new era of software collaboration in the HERS Industry by developing an open source ERI Rating module utilizing the EnergyPlus building simulation engine.  The ERI module will benefit from the collaboration of software developers inside and outside the HERS Industry.  It is believed that the ERI module will lowering the barrier for entry into the market as a HERS rating software tool.  At the time of this article, the RESNET Board has voted to approve the motion to develop the ERI Software Module. This will be a collaborative effort between RESNET, DOE/NREL and a task force of subject matter experts.  RESNET is taking a definitive step toward greater HERS consistency, transparency, and industry growth.  The process has begun and gaining momentum.  Future articles will offer status updates on the development process.


[1] For example, a HERS tool might capture a handful of characteristics about an air-conditioner (capacity, rated efficiency, SHR) where as a robust simulation engine requires hundreds of inputs to model the performance of the equipment over its full range of operation (performance curves as function of load and indoor/outdoor conditions, etc.).

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