Trial / Demo
As energy codes tighten and performance expectations rise, the consequences of getting HVAC loads and sizing wrong have become more significant. Mechanical engineers are increasingly expected to deliver systems that are not only code-compliant, but resilient, efficient, and capable of performing reliably across a wide range of real-world operating conditions.
In this context, traditional peak-load approaches, while still useful, are no longer sufficient on their own. To reduce risk and design with confidence, engineers need methods that better reflect how buildings and systems actually behave over time.
Why peak loads alone are no longer enough
Most HVAC systems operate at part load for the majority of the year. Yet many design decisions continue to be driven by a narrow set of peak conditions. As systems and controls become more complex, this disconnect makes it harder to predict performance, customize control strategies, and defend design decisions once a building is occupied.
Several converging trends are amplifying this challenge:
• Increased use of DOAS, heat recovery, variable flow, hybrid, and electrified systems
• Control strategies that strongly influence part-load efficiency and comfort
• More stringent requirements under standards such as ASHRAE 90.1 and local stretch codes
• Higher expectations around resilience, comfort, and low-carbon performance
• Greater legal and financial exposure when systems underperform
Static peak-load calculations provide limited insight into how these factors interact over time.
The role of dynamic thermal simulation
Dynamic thermal simulation helps bridge this gap by modelling buildings and HVAC systems across a full range of operating conditions. Rather than focusing on a single design hour in the winter and the summer, engineers can evaluate performance across seasons, loads, and control states.
This approach makes it possible to assess:
• Thermal mass and heat storage effects
• Weather variability and climate sensitivity
• Occupancy patterns and internal gains
• Interactions between adjacent spaces
• Part-load behavior, where many comfort and efficiency issues emerge
The result is a more realistic, whole-building understanding of performance.
Integrating loads, systems, and controls
Another important shift in practice is the move toward more integrated workflows. Engineers need to move quickly in early design, while still producing analysis that can be refined and defended as projects progress.
Platforms such as IES Virtual Environment (IESVE) support this by allowing a single model to evolve from schematic design through detailed system and control analysis. Rapid load assessments can be extended into more detailed simulations without rebuilding geometry or switching tools, preserving continuity and traceability.
This flexibility allows engineers to choose the appropriate level of detail at each stage of design, while maintaining a consistent analytical foundation.
Reducing sizing risk and supporting energy resilience
Improper system sizing remains one of the most common and costly risks in HVAC design. Oversized equipment increases capital cost and reduces operational efficiency, while undersized systems can lead to comfort complaints and reputational damage.
Dynamic simulation helps mitigate these risks by enabling engineers to:
• Generate more defensible load and sizing inputs
• Compare multiple system configurations and control strategies
• Understand system behavior under realistic operating conditions
• Right-size equipment while meeting code and resilience requirements
For projects subject to increased scrutiny, this level of analytical depth is increasingly seen as essential rather than optional.
Supporting practical adoption
To help engineers apply these principles in day-to-day practice, IES has published a Workflow Guide that walks through modern HVAC load and system design approaches, from early-stage assessment through detailed analysis. The guide focuses on practical workflows, decision points, and common pitfalls associated with sizing risk and system performance.
Download the Workflow Guide here
A shifting baseline for best practice
As clients, owners, and regulators place greater emphasis on demonstrable performance, the ability to explain how and why a system will work is becoming a differentiator. Dynamic thermal simulation is no longer just a specialist capability; it is increasingly part of mainstream HVAC design practice.
The tools and workflows engineers adopt today will shape how effectively they manage risk, compliance, and resilience in the future. The direction is clear: HVAC design is becoming more dynamic, more integrated, and more closely tied to real-world performance.
About the Author
Matthew Duffy is a Vice President of IES, a global climate tech company delivering innovative software solutions and consultancy services to decarbonize the built environment. An environmentalist at heart, Matthew works at the intersection of engineering, architecture, and decarbonization, advancing energy efficiency and renewable energy through effective communication and innovative simulation. Matthew has been a Board member of IBPSA-USA from 2020 to 2026 and is Chair of IBPSA’s Wisconsin Chapter. A resident of Madison, WI, Matthew holds a bachelor’s degree in mechanical engineering from the Milwaukee School of Engineering.
