Designing for Comfort: Implementing ASHRAE 55 with CFD and OpenFOAM
In the world of building design, “thermal comfort” is often treated as a subjective feeling. However, for engineers and architects, it is a precise technical requirement defined by ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy.
Meeting this standard is about more than just setting a thermostat; it’s about the complex interaction between air speed, temperature, and human activity. Today, we look at how CFD (Computational Fluid Dynamics) and OpenFOAM allow designers to move beyond simple HVAC calculations to create truly comfortable, energy-efficient spaces.
What is ASHRAE 55?
ASHRAE 55 specifies the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.
The standard focuses on the six primary factors of thermal comfort:
- Metabolic Rate: The level of physical activity.
- Clothing Insulation: The thermal resistance of what people are wearing.
- Air Temperature: The dry-bulb temperature of the air.
- Radiant Temperature: The heat emitted from windows, walls, and surfaces.
- Air Speed: The cooling effect of airflow (or the discomfort of a draft).
- Humidity: The moisture content in the air affecting evaporative cooling.
The core output of ASHRAE 55 is the PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied) indices, which quantify how a large group of people would perceive the thermal environment.
The CFD Advantage: Seeing Comfort in 3D
Traditionally, engineers used “lumped parameter” models—assuming the air in a room is perfectly mixed. But in modern architecture with large glass facades, high ceilings, or underfloor air distribution, this assumption fails.
CFD allows you to validate ASHRAE 55 compliance with surgical precision:
1. Draft Risk Assessment
One of the most common causes of thermal dissatisfaction is “draft”—unwanted local cooling of the body. CFD can pinpoint high-velocity air plumes from diffusers that might strike an occupant’s neck or ankles, allowing you to adjust diffuser placement before construction.
2. Vertical Temperature Stratification
ASHRAE 55 sets limits on the temperature difference between a seated person’s head and ankles (typically < $3^\circ C$). CFD simulates buoyancy and thermal plumes, showing you exactly how air layers within a space.
3. Mean Radiant Temperature (MRT)
Occupants near a cold window in winter will feel “cold” even if the air temperature is $22^\circ C$. CFD with radiation modeling calculates the exact radiant heat exchange between every surface and the occupant, providing a true MRT value for the PMV calculation.
Turbocharging OpenFOAM with the HVAC Extension
Standard CFD solvers provide velocity and temperature, but calculating ASHRAE 55 indices requires extra steps. The HVAC-for-OpenFOAM extension is a powerful tool for this specific task.
Why use this extension for ASHRAE 55?
- Direct PMV/PPD Output: It automates the complex iterations required to solve the Fanger comfort equations, giving you comfort maps directly in your post-processing.
- Specialized Boundary Conditions: It includes boundary conditions tailored for HVAC diffusers and human thermal manikins.
- Energy Efficiency: By visualizing exactly where cooling is needed, you can optimize your HVAC setpoints to meet ASHRAE 55 while minimizing energy consumption.
High-Performance Comfort on CloudHPC
Simulating an entire office floor or a complex theater for ASHRAE 55 compliance requires high mesh resolutions to capture thin boundary layers and air jets. This is where CloudHPC.cloud comes in.
By leveraging the cloud, you can:
- Run Multiple Scenarios: Simultaneously test “Summer” and “Winter” cases, or different occupancy layouts, in parallel.
- Handle Large Meshes: Solve million-cell models that would crash a standard workstation.
- Interactive Reporting: Generate 3D heat maps that make it easy to explain comfort risks to clients and architects.
Conclusion
ASHRAE 55 is about more than avoiding complaints—it’s about occupant health, productivity, and sustainable design. By combining the rigorous physics of OpenFOAM with the accessibility of CloudHPC, you can ensure that every occupant in your building stays in the “comfort zone.”
Ready to visualize thermal comfort? Sign up for CloudHPC and start your OpenFOAM HVAC simulations today.
CloudHPC is a HPC provider to run engineering simulations on the cloud. CloudHPC provides from 1 to 224 vCPUs for each process in several configuration of HPC infrastructure - both multi-thread and multi-core. Current software ranges includes several CAE, CFD, FEA, FEM software among which OpenFOAM, FDS, Blender and several others.
New users benefit of a FREE trial of 300 vCPU/Hours to be used on the platform in order to test the platform, all each features and verify if it is suitable for their needs
