CH-18-C018 -- Commercial HVAC Load Sizing Calculations Gone Wrong PDF

Commercial software vendors of building performance analysis tools receive tens of thousands of technical support queries per year. This paper attempts to provide some lessons learned for the all-too common mistakes that go unnoticed by the consulting engineering community, as it pertains to HVAC sizing calculations. It is not the intent of this paper to compare under-sized or over-sized HVAC systems against the calculated system sizes. It is the intent to focus only on the load calculations, from the perspective of the design engineer. In addition, we will attempt to provide summaries of common HVAC sizing workflow issues, technical lessons learned, misunderstood methodology queries, and typical mistakes associated with the process of HVAC system sizing. While this type of analytical data is useful to the development direction of software vendors, the data often remains hidden from the design community. To calculate HVAC sizes of non-residential buildings accurately, three core competencies are required by the mechanical design engineer:

1. Understanding of Building Physics and Energy Flows
2. Understanding of Local or ASHRAE-based Code Minimums
3. Understanding of Building Performance Analysis Software

The first competency of 'Understanding of Building Physics and Energy Flows’ is rarely communicated back to the general design community, either because of the time-constraints associated with the typical schedule of the consulting engineer, or because of the stigma associated with the engineer who must demonstrate a misunderstanding to their peers in order to educate their peers. In order to anonymously publicize common misunderstandings of the HVAC-sizing process; this paper will highlight ten major topics of analysis, which include:

• Loads Sizing Methodology
• Gains, Schedules & Diversity
• Design Day Weather Data
• "Rules of Thumb"
• Zoning for HVAC
• Solar Shading
• Trusting your Engineering Principles
• HVAC Sizing by Simulation
• Envelope Thermal Characteristics
• Misunderstanding Unmet Load Hours