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Abstract: Design engineers commonly oversize HVAC systems with the justification of needing a reasonable safety factor to manage periods more extreme than the specific design conditions. Unfortunately, the safety factor easily becomes excessive. The design engineers minimize their professional risk, and by doing so they are actually asking the building owner to pay an immediate penalty due to increased first cost of equipment and an ongoing penalty due to maintenance and energy use implications. The penalties associated with excessive safety factors are often not communicated to the client. This paper presents the results of a study of “rightsizing” rooftop HVAC systems. The study included intensive interviews with HVAC designers investigating the design process and extensive field measurement of rooftop units (RTUs) during peak cooling conditions. This paper focuses on defining the signature of oversizing, i.e. how to use the physical measurements to quantify the degree of oversizing of an RTU and how to estimate the penalty of oversizing in terms of energy consumption and peak electricity demand. Utility companies incentive programs have not yet identified mechanisms for incentivizing rightsizing of HVAC system. The methodology described in this paper can be used as the basis for such programs. [ABSTRACT FROM AUTHOR]

• Thermal mass located near ventilation inlets lost more heat through NVM at night. • Automated NVM can improve energy efficiency, but also provides opportunity for error. • Periodic commissioning is recommended to ensure correct NVM function. Summer temperature extremes are expected to increase during the next century. Integrating night ventilation of mass (NVM) strategies in new 'smart' buildings could increase resilience to warming temperatures and reduce reliance on mechanical cooling. However, designing and operating buildings with NVM is challenging due to the complexity of interacting variables and difficulty of accurately measuring and predicting them, particularly when using automated controls and sensors. In this study we evaluated NVM performance by monitoring two night-ventilated classrooms in a mixed-mode smart academic building during October 2014 and June–July 2016. We investigated whether NVM systems were operating as intended, altered operation as needed to match design intent, and compared NVM efficacy during incorrect and corrected operation. We calculated the amount of heat NVM removed from each room overnight and visualized gradients of heat removal across the mass surface. We found that incorrect building automation system (BAS) operation during June 2016 resulted in overuse of mechanical cooling, while corrected operation later in July reduced the number of hours mechanical cooling was required, although average outdoor temperatures were higher. Our results also showed greatest overnight reductions in mass surface temperatures in locations nearest to the air inlets on the exterior south-facing walls. This critical evaluation quantifies the agreement between NVM performance as modeled during the design phase and during post-occupancy operation several years after construction. We conclude that the observed performance gap may be addressed through periodic or continuous re-commissioning. [ABSTRACT FROM AUTHOR]