Written by Ruturaj Patil
Monday, August 21, 2023
Importance of ASHRAE 55 Compliance
By
Ruturaj Patil
“Heatwaves set more records across Europe, Asia and US” - ALJAZEERA
“Officials warn about high indoor temperature risks in Seattle heatwave” - CBS News
With the surge in global temperatures these kinds of news articles are the ones that we hear of nowadays.
Everyone of us has felt the heat of the blazing sun when you are sitting next to a window on a hot summer day ? Have you ever wondered why this might be the case?
What do we do when we face such a conundrum ? Go To Solution for most people is to crank down the thermostat leading to more energy consumption.
Lack of thermal comfort has become a contentious issue in the worldwide quest to become more energy efficient in all facets of life in order to ensure HVAC systems use less energy.
What is the Root Cause behind Thermal Dissatisfaction ?
A common misconception in the HVAC industry is that cooling load calculations are proxies for comfort calculations, and that A/C design is comfort design. Despite the fact that both are necessary for duct routing, equipment selection, and the layout of mechanical rooms, none of these activities determine the levels of discomfort that building occupants feel.
Can I say that ? Yes, Because these activities don't ask what people thermally feel based on what they're doing, what they're wearing, or how they're positioned relative to the architecture and geometry of the space. For that analysis, the designers must look through the lens of ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy.
ASHRAE 55
ASHRAE Standard 55 specifies conditions for acceptable thermal environments and is intended for use in design, operation, and commissioning of buildings and other occupied spaces.
- The environmental factors addressed in the standard are temperature, thermal radiation, humidity, and air speed; the personal factors are those of activity and clothing.
- It is intended that all of the criteria in this standard be applied together, as comfort in the indoor environment is complex and responds to the interaction of all of the factors that are addressed herein.
- This standard specifies thermal environmental conditions acceptable for healthy adults at atmospheric pressure equivalent to altitudes up to 3000 m (10,000 ft) in indoor spaces designed for human occupancy for periods not less than 15 minutes.
This standard does not address such nonthermal environmental factors as air quality, acoustics, illumination, or other physical, chemical, or biological space contaminants that may affect comfort or health.
Standard 55 Usage Flow Chart
Compliance with ASHRAE 55
ASHRAE 55 requires the practitioner to evaluate and comply to the limits specified in the standard for the following -
| Category | Factor |
|---|---|
| Personal | Metabolic Rate (Met) and Clothing Insulation (Clo) |
| General Discomfort | Mean Radiant Temperature, Dry Bulb Temperature, Relative Humidity and Air Speed |
| Local Discomfort | Radiant Temperature Asymmetry, Vertical Air Temperature Difference, Floor Surface Temperature and Draft |
Factors affecting Thermal Comfort
Thermal Comfort Assessment
So considering that selection of equipment, layout mechanical room components and duct routing is finished, how can HVAC designers assess Thermal Comfort and comply with ASHRAE 55.
Tools which can be used for Thermal Comfort Assessment are -
- 1D tools like CBE online thermal comfort tool
- CFD Softwares like Autonomous HVAC CFD
- Physical Surveys
Thermal Comfort Assessment Methods
Surveys can be used only after designing and commissioning of the system is completed, making the assessment method irrelevant during the design stage.
I am sure that now you might be wondering about how to assess thermal comfort at design stage ?
Well there 3 ways to do so -
- 1D Analysis -
Using tools like Centre for the Built Environment (CBE) online thermal comfort tool.
- 3D Analysis -
- Using Generic CFD softwares to predict Temperature, Air Velocity and Relative Humidity.
- Using Autonomous HVAC CFD to predict Thermal Comfort Indices like Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD).
1D Analysis
The 10 factors identified in Table 1, when known by calculation and specification, are inserted into the Centre for the Built Environment (CBE) online thermal comfort tool (Figure 2), which advises what a representative occupant might sense and perceive in a space and whether that experience would be in compliance with Standard 55. Equally important is that the assessment identifies sources of potential discomfort and can lead the designer to consider possible solutions.
Thermal Comfort Assessment on any such 1D tools requires the designers to perform the analysis for a particular type of Representative Occupant. A representative occupant is determined by Personal Factors and Location in space.
CBE Comfort Tool
Local Discomfort Assessment - CBE Comfort Tool
While CBE thermal comfort tool can be used to predict thermal comfort for a representative occupant but it has its own limitations. It is difficult for the designers to evaluate thermal comfort of all occupants which are located at different locations using the tool mentioned above. It might happen that the air conditioning system is designed for a representative occupant who is located near a large pane of exposed glass and has high MRT values but due to this occupants who sit far away from the windows might feel very cold.
To avoid such a phenomenon, 3D analysis can be performed on the project to evaluate thermal comfort and ensure that more than 80 % of occupants inside the space are comfortable.
3D Analysis
As mentioned before there are 2 tools to perform 3D Analysis :- Generic CFD Softwares and Autonomous HVAC CFD.
Let’s start by understanding how generic CFD softwares can be used for Thermal Comfort Assessment ?
Majority of the CFD softwares provide Temperature, Air Speed and Relative Humidity Results when any space is simulated. But as we have established above that for compliance with ASHRAE 55 requires spaces need to be simulated for thermal comfort indices like PMV, PPD, Mean Radiant Temperature, Radiant Temperature Asymmetry, Vertical Air Temperature Difference, Floor Surface Temperature and Draft.
To do so CFD engineers will have to develop complicated user-defined functions to produce the results for above mentioned parameters. Doing so will lead to much more project time utilisation.
How Autonomous HVAC CFD Calculates Thermal Comfort and Evaluates Local Discomfort ?
Autonomous HVAC CFD has the following features catered to help designers comply with ASHRAE 55 -
- Results for all Thermal Comfort indices
- Auto Generated Compliance Reports
- Results Viewer
Autonomous HVAC Features for Standard 55 Compliance
Results Viewer provides results for all Thermal Comfort and Local Discomfort Indices at a glance.
Designers can evaluate thermal comfort for all locations which are going to be occupied by occupants. Using this approach against the representative occupant method helps gauge thermal comfort more accurately.
Users can either check compliance for all parameters or just selected parameters in the results viewer using the variable dropdown. As visible in the below figure, each occupant has a probe assigned which displays either Red, Green or Grey colours depending on whether the occupant is Compliant,Non Compliant or Not Applicable respectively. Not Applicable legend is used for occupants for whom compliance can’t be evaluated according ASHRAE 55.
Result Viewer
Designers can hover over probes to get detailed information about calculated results of Thermal Comfort Indices.
Auto-Generated ASHRAE 55 Compliance Reports cover all the necessary details to show compliance against all parameters mentioned in the standard.
The report provides an executive summary of the entire design to start with where designers can evaluate system performance at glance across all the spaces in the project. Each space and design configuration are colour coded based upon the percentage of compliant occupants. For instance, 100 % of occupants present in Conference Room are compliant to ASHRAE 55 for both air distribution system design Swirl Face and Square Plaque as shown in the image below.
Besides executive summary, thermal comfort compliance results specific to each metric are also present in the report.
The auto-generated ASHRAE 55 Compliance Report can be used to obtain LEED Credits specific to Thermal Comfort.
Executive Summary in ASHRAE 55 Compliance Report
PMV Results in ASHRAE 55 Compliance Report
References
- ASHRAE Standard 55-2020
- ASHRAE Standard 55-2013 User Manual
- CBE Comfort Tool
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