Assessing Local Thermal Discomfort with AHC for ASHRAE Standard 55 Compliance

The comfort of occupants in buildings is a crucial factor in determining the performance of a building. In conjunction with an occupant’s overall thermal satisfaction, local thermal comfort plays an important role in assessing the predicted percentage of dissatisfaction (PPD) in space. The risk of local thermal discomfort varies greatly with clothing levels, metabolic rate, and activity levels. Local Thermal Discomfort is primarily caused by the following factors:

The acceptable predicted percentage of dissatisfaction (PPD) due to above local discomfort parameters are is stated in ASHRAE Standards 55 as listed below:-

Table 1: Acceptable predicted percentage dissatisfied due to local thermal discomfort

In this blog, we explore how autonomous HVAC CFD (AHC) can help engineers and designers evaluate local thermal discomfort in buildings and recommend effective solutions to mitigate it. These simulations can identify problem areas and evaluate the effectiveness of various design and system modifications to ensure optimal thermal comfort for building occupants. Designing an HVAC system for large offices is a challenging task that requires careful consideration of various factors, including the size of the building, zoning, air quality, thermal comfort, etc. It should be designed and installed carefully to ensure optimal performance and thermal comfort.

Vertical Air Temperature Difference

The Vertical Air Temperature difference is expressed as the difference between the air temperature at ankle level and head level. Generally, it is unpleasant to be warm around the head whilst at the same time being cold around the feet, regardless of this being caused by thermal stratifications. ASHRAE Standard-55 defines limits for seated occupants and standing occupants as shown below table. Autonomous HVAC CFD (AHC) evaluates the Vertical Air Temperature difference and mark off occupants are in compliance with the criteria or not.

Table 2: Vertical Air Temperature limits for seated occupants and standing occupants

Figure 01: Vertical Air Temperature Difference

Red probe indicates occupants are not compliance with criteria and green probe indicates occupants are compliance with criteria as specified in ASHRAE Standard 55

Table 3: The number of occupants satisfies the vertical air temperature difference criteria

Draft Rate

The draft rate is the most common source of thermal discomfort and often affects exposed regions of the body such as the head/ neck region and leg/ ankle region. The draft rating may be expressed as the percentage of people dissatisfied with a draft. ASHRAE Standard 55 limits the draft rate to 20 %. Autonomous HVAC CFD (AHC) evaluates the draft rate over occupants in compliance with the criteria

Figure 02: Draft Rate

Table 04: The number of occupants satisfies the draft rate criteria

Radiant Temperature Asymmetry

The radiant temperature asymmetry is the difference between the plane radiant temperatures in opposite directions in an indoor space as perceived by occupants. In general, people feel more comfortable when the temperature of the surfaces around them is similar. When there is a large difference in the surface temperatures in a room, it creates a sensation of discomfort and may cause local discomfort. Generally, it is caused by cold windows, uninsulated walls, cold products, cool or warm machinery, heated or cold ceilings, etc. ASHRAE Standard 55 sets limits on the allowable temperature differences in vertical and horizontal directions as shown in the below table. Autonomous HVAC CFD (AHC) helps to evaluate the radiant temperature asymmetry over occupants in compliance with the criteria.

Table 05: Allowable temperature differences in vertical and horizontal directions

Figure 03: Radiant Temperature Asymmetry

Table 06: The number of occupants satisfies the radiant temperature asymmetry criteria

Floor Surface Temperature

Thermal Comfort can also be experienced by the occupants when the floor surface is too hot or too cold, ASHRAE 55 recommends that floor temperatures stay in the range of 19-29 °C, this criterion is based on occupants wearing lightweight indoor shoes and does not address situations where occupants sit directly on the floor.

Figure 04: Floor Surface Temperature

Table 07: The number of occupants satisfies the floor surface temperature criteria

In conclusion, local thermal comfort plays a vital role in ensuring the comfort and satisfaction of building occupants. Autonomous HVAC CFD (AHC) provides an effective solution to evaluate and mitigate local thermal discomfort caused by factors such as vertical air temperature difference, draft rate, radiant temperature asymmetry, and floor surface temperature. ASHRAE Standard 55 provides the criteria for an acceptable predicted percentage of dissatisfaction (PPD) due to local discomfort parameters. AHC can simulate and evaluate the effectiveness of various design and system modifications to ensure compliance with ASHRAE Standard 55.