Reducing Climate-Change-Induced Heat Strain and HVAC Performance Loss With Circulating Fans

By DAVID ROSE, AMCA Standard 230 Technical Committee, and WILL CONNER

In 1990, the Intergovernmental Panel on Climate Change (IPCC), the United Nations (U.N.) body for assessing the science related to climate change, released its First Assessment Report,1 detailing the impacts human activities were having on atmospheric greenhouse-gas concentrations and tropospheric air temperature. The report predicted global mean air temperature would rise by an average of 0.54°F (0.3°C) per decade in the 21st century, or 5.4°F (3.0°C) by the year 2100, an outcome that would be devastating for the environment and many of the world’s most vulnerable populations, which would suffer the effects of more frequent severe-weather events, sea-level rise of up to 3.3 ft (1 m), and food insecurity.

Flash forward to 2021 and the release of IPCC Working Group I’s contribution to the Sixth Assessment Report, which the U.N. secretary-general called a “code red for humanity.”2 While international treaties such as the Kyoto Protocol and the Paris Agreement were designed to cap mean-temperature increase at 2.7°F (1.5°C), the most likely greenhouse-gas-concentration trajectory adopted by the IPCC, the intermediate representative concentration pathway (RCP) of 4.5, sees a mean-temperature increase of up to 6.3°F (3.5°C) by the end of the century.3 In short, it is becoming increasingly clear that the 2.7°F (1.5°C) target will be exceeded, and it could happen as soon as 2050.3

This article will examine the predicted performance of buildings at significant risk from climate change and illustrate how the use of circulating fans can reduce cooling demand in conditioned buildings and heat strain in unconditioned ones under increasingly demanding climatic conditions.

Contextualizing Global Impacts and Existing Works

One of the greatest communication challenges in climate science is “consensus gap,” or the dissonance between analytical, scientific, and numerical effects of climate change (e.g., rise in mean temperature) and more human, contextual, and observational impacts of climate change (e.g., seasonal temperature, drought). Researchers at ETH Zurich leveraged connections between what populations subjectively know about and associate with their current climates to underscore the dramatic changes that will occur under the RCP-4.5 scenario by 2050, selecting 520 cities around the world and applying statistical models to 19 independent bioclimatic variables to determine the cities’ best future climatic analog.4

Distilling the results of the ETH Zurich study, Table 1 shows the future climatic analog of cities in 15 climate zones (CZ) used in U.S. Department of Energy (DOE) reference-model simulations and in ANSI/ASHRAE Standard 169-2021, Climatic Data for Building Design Standards.5 For context, 2017 annual heating degree-days based on 65°F (HDD65) and 2017 annual cooling degree-days based on 50°F (CDD50) from the ASHRAE Climatic Design Conditions database are given.

Credit: Quality Stock Arts/Bigstock

TABLE 1. Representative cities in continental-U.S. climate zones and their future climatic analogs.

TABLE 2. Monthly 2-percent cooling design conditions for Portland, Ore.; San Antonio, Texas; Jacksonville, Fla.; and Orlando, Fla. (Note: DB = dry bulb, MCWB = mean coincident wet bulb).

Indoor-air conditions of heated-and-ventilated-only warehouses of typical construction are driven primarily by the influence of envelope loads (i.e., internal loads typically are limited). This makes warehouses particularly vulnerable to changes in outdoor-air conditions, such as those caused by climate change. A representative warehouse occupant who is physically active and dressed to common standards (1.7 MET, 0.57 Clo) is at greater risk of heat strain than a worker in another environment with more adaptive clothing or who works at a lower metabolic rate. Heat-strain occurrence among warehouse workers is a growing concern and expected to become significantly worse across nearly every climate zone, if mitigating measures are not put in place.

Table 4 shows the results of building-energy-model simulations of the representative warehouse in the current climates of the 15 cities in Table 1 and under the future weather conditions of the cities’ climatic analogs. Note the shifts to the “very hot” extremes and away from neutral conditions. On average, the cities are expected to experience a 627-hour increase in dangerous heat-strain conditions, with those in climate zones 4B, 5A, 6A, and 6B expected to experience an average increase of more than 1,000 hours.

TABLE 4. Heat-strain comparison, no fans. (Note: NL = no limit; percent change is incalculable, as current conditions result in zero heat-stress hours.)

Unlike warehouses, the vast majority of smaller commercial spaces, such as offices, fitness facilities, restaurants, and stores, are conditioned. While the loading profiles of these spaces are inherently different than those of a warehouse (much smaller, with more internal and latent loads), the spaces are not immune to significant performance impacts from changing weather conditions, as it is possible their mechanical systems will be unable to keep up with envelope and ventilation loads they simply were not designed for and will require more energy to maintain desired performance, even in partial-load situations.

To examine the question of whether cooling effects provided by air-circulating fans can improve the resilience of existing mechanical systems while maintaining occupant thermal comfort amid changing climatic conditions, the medium-office DOE reference model was analyzed across the same 15 climate zones for a scenario with no fans and a standard cooling setpoint (72°F/22.2°C) and for a scenario with fans and an elevated setpoint (80°F/26.7°C). Occupant comfort was identical across the two scenarios for both current and future weather conditions. Two results—peak cooling load in tons of cooling required and annual HVAC cooling energy use in kilowatt-hours of electricity—were recorded for each simulation.

Figures 2 and 3 show that, without mitigation, peak cooling load and HVAC energy consumption increased in all but one climate zone (1A). On average, peak cooling load increased by 9.7 percent and HVAC energy consumption increased by more than 23,000 kWh (42.6 percent) relative to current climatic conditions. When air-circulating fans were added and the setpoint was increased, performance under future weather conditions was much more similar to that under current weather conditions, with multiple climate zones seeing decreases in peak load and energy consumption and others seeing significantly smaller gains than in scenarios without fans and an elevated setpoint. On average, with fans, cooling capacity increased by only 0.1 percent, while energy use relative to current conditions increased by only about 1,700 kWh (9.6 percent).

Conclusion

Returning to the two questions that formed the basis of the research presented in this article:

Can cooling effects provided by air-circulating fans mitigate climate-change-induced increases in occupant heat strain in heated-and-ventilated-only warehouses?

Yes. With a properly designed fan solution delivering airflow to occupants in a heated-and-ventilated-only warehouse, the effects of indoor conditions that could lead to greater occupant heat strain under future climatic conditions can be significantly reduced. In some cases, the addition of fans even improves future conditions relative to current ones.

Can cooling effects provided by air-circulating fans improve the resilience of existing mechanical systems while maintaining occupant thermal comfort amid changing climatic conditions?

TABLE 5. Heat-strain comparison, with fans. (Note: NL = no limit; percent change is incalculable, as current conditions result in zero heat-stress hours.)

FIGURE 2. Comparison of HVAC-system performance with no fans, future conditions.

FIGURE 3. Comparison of HVAC-system performance with fans, future conditions.