PNNL

Abstract

The prescriptive path is the most widely used approach for commercial code compliance in the United States. Though easy to implement, prescriptive approaches do not typically discriminate between minimally compliant, high-performing and poorly performing HVAC system configurations, thereby permitting the installation of systems with high energy consumption and carbon emissions. The problem is compounded in the US by federal preemption rules, prohibiting state and local energy codes from mandating more efficient equipment than is defined in federal standards. Hence, in order to meet aggressive energy and carbon reduction goals, it is clear that energy codes will need to transition from prescriptive to performance-based approaches. However, this transition is riddled with several challenges including the additional complexity and cost of performance based approaches, lack of confidence in energy modeling results, gamesmanship on the part of applicants, lack of qualified reviewers, challenges in promoting de-carbonization, and the inequity of trading long lived envelope efficiency for short lived measures such as building controls. This paper discusses a new HVAC system-based performance approach (HVAC System Performance) which provides a simpler solution compared to whole building performance, while keeping tradeoffs limited to specific building systems. The Total System Performance Ratio (TSPR) is a metric for evaluation of overall system efficiency instead of individual component efficiency, a solution to the existing component-based evaluation approach which could also eventually facilitate the transition to a performance-based code structure. TSPR is a ratio that compares the annual heating and cooling load of a building to the annual energy consumed by the building’s entire HVAC system. A calculation software tool has been developed as a module of the US Department of Energy’s Building Energy Asset Score tool, for determining a building’s TSPR. Already incorporated into the 2018 Washington State Energy Code, this approach is also being evaluated by ASHRAE Standard 90.l Project Committee and has the potential to provide a comprehensive performance-based approach for HVAC system evaluation and analysis.