The ASHRAE Fundamentals Handbook is the go-to reference for HVAC professionals when it comes to load calculations. The handbook offers unique calculations methodologies for residential versus commercial load calculations. Two key chapters — Chapter 17 (Residential Cooling and Heating Load Calculations) and Chapter 18 (Nonresidential Cooling and Heating Load Calculations)—outline these distinct approaches tailored to different building types. While both chapters rely on fundamental heat transfer principles, their methodologies diverge significantly due to the unique characteristics of residential and nonresidential buildings.
Chapter 17 focuses on residential buildings, which typically have:
Chapter 18, on the other hand, addresses nonresidential buildings, which:
In addition to these differences, residential applications have the benefit of occupants having more control over their body and surroundings. If occupants are uncomfortable, they can easily add or remove clothing, open windows, grab a blanket, or turn on a fan. In commercial buildings the occupants do not have this luxury, meaning that thermal comfort is much more difficult to adjust from an individual perspective.
Cooling load calculations are essential for designing HVAC systems that maintain indoor comfort. While both chapters rely on fundamental heat transfer principles, their methodologies differ in complexity and application.
Chapter 17 provides simplified cooling load procedures suitable for manual calculations. The two primary methods used are:
The Residential Heat Balance (RHB) Method is based on fundamental heat transfer principles and accounts for:
This method considers the moderating effects of thermal mass, meaning that heat absorbed by building materials during the day is gradually released, reducing peak cooling loads.
The Residential Load Factor (RLF) Method is a simplified approach that estimates cooling loads using predefined load factors. These factors are derived from empirical data and account for:
The RLF method is particularly useful for quick calculations but lacks the precision of the RHB method.
Chapter 18 employs more advanced techniques to account for the complex thermal dynamics of commercial buildings. The primary method used is the Radiant Time Series (RTS) Method.
The RTS Method is a rigorous approach that replaces older simplified methods such as:
The RTS Method incorporates:
A key feature of the RTS Method is its ability to convert radiant heat gains into cooling loads using time-series coefficients. This approach ensures accurate peak load predictions, making it ideal for commercial applications. This is done by splitting all sensible cooling loads in each space into radiant and convective portions. The radiant portions are then spread out over long periods of time to account for the absorption and release of heat by the building materials. A similar method is used in the CTS adjustment of conduction loads. For a more detailed description of the RTS calculation method, see our blog post here. You can also see a comparison of the RTS method with other commercial load calculation methods here.
| Aspect | Residential (Chapter 17) | Nonresidential (Chapter 18) |
|---|---|---|
| Calculation Complexity | Simplified methods (RHB, RLF) | Advanced methods (RTS) |
| Thermal Mass Effects | Considered to moderate peak loads | Accounted for in detailed calculations |
| Solar Radiation | Basic estimation | Detailed calculations of solar angle and sol-air temperature |
| Internal Heat Gains | Lower due to fewer occupants and equipment | Higher due to lighting, equipment, and occupancy |
| Zoning Requirements | Minimal zoning, often single thermostat | Precise zoning for thermal comfort |
Heating load calculations are much more similar between the two chapters.
Chapter 17 includes provision for distribution losses based on a few different common system types in residential buildings. The distribution losses are assumed to exit the system without achieving their useful purpose by being lost through the attic or basement. It also provides a simple method for estimating infiltration, which can be one of the largest contributors to heat loss.
Chapter 18 looks strictly at space load calculations, and therefore does not provide estimates for system losses. System losses must be dealt with at the system level of the calculations. Due to the geometry and architecture of commercial buildings, there is typically much less heat lost through poor system performance, and therefore it does not play as critical of a role as in residential buildings.
While both chapters share foundational principles, their methodologies reflect the distinct needs of residential and nonresidential buildings. Chapter 17 prioritizes simplicity and adaptability for residential applications, whereas Chapter 18 employs rigorous, data-driven approaches for complex commercial environments. Understanding these differences is crucial for HVAC professionals seeking to optimize system design and performance.
