UV STUDIES | Modeling methods to address office building clean air targets

Airborne pathogens seem to be hitting in waves, whether it’s the evolving strains of COVID, RSV, or influenza. Recent outbreaks have raised awareness that our nation’s commercial buildings are often under-ventilated and lack adequate protection from airborne diseases for workers and students. In response, the push to protect people in public spaces has moved to the forefront through such means as improving indoor air quality (IAQ) in buildings.

To address the spread of COVID-19 and other pathogens within indoor spaces, ASHRAE and the Centers for Disease Control and Prevention (CDC) developed clean air standards and targets to limit indoor transmission. ASHRAE Standard 241, introduced in 2023, defines minimum equivalent clean airflow rates per person (ECAi) for infection risk mitigation in different space types. These rates are to be met during infection risk management mode (IRMM), which is to be applied when infection risk levels are higher or if policies require this operation. The ECAi is defined in terms of airflow per occupants (e.g., cfm/person), so the room occupancy is important in calculating the target equivalent clean airflow rate.

By contrast, CDC’s 2023 guidance, “Ventilation in Buildings,” recommends at least five equivalent clean air changes per hour (eACH) for occupied spaces in buildings and does not specify targets for different space types, other than supplemental ventilation requirements for healthcare facilities. Unlike ASHRAE Standard 241, the clean air target is not dependent on occupancy but rather the volume of the room. For example, multiplying five eACH by the volume of the room gives the (volumetric) clean airflow rate (e.g., cfm) needed for that specific room.

Both the ASHRAE and CDC targets can be achieved using in-room air-cleaning systems directly — e.g., in-room germicidal ultraviolet (GUV) or portable air cleaners (PACs) — or centralized HVAC system measures, including outdoor air ventilation, HVAC filtration, or in-duct GUV. Such measures can positively impact occupant health while also affecting comfort and energy use. But how well do these measures work?

Considerable progress has been made in research evaluating airborne pathogen mitigation measures individually, but knowledge gaps remain with how to best achieve the ASHRAE Standard 241 and CDC clean air targets. Information is needed about how measures such as GUV and PACs compare to filtration and outdoor air ventilation, and how multiple measures — including GUV technologies — compare in terms of impacts on occupant health, comfort, and energy consumption. Often, studies focus on either infection risks or energy impacts but don’t consider both along with other impacts, such as thermal comfort.

Pacific Northwest National Laboratory (PNNL) recently completed simulation-based analyses to assess the ability of various airborne pathogen mitigation scenarios to meet ASHRAE and CDC’s clean air targets. Results of the analyses have just been published. The analysis used models developed in Modelica language using the Modelica Buildings library. PNNL incorporated the models into an existing prototypical office building model and assessed the following seven scenarios on their ability to meet clean air targets:

• MERV 8 (Baseline) Scenario: Using minimum ASHRAE 62.1 ventilation combined with a Minimum Efficiency Reporting Value (MERV) 8 filter
• MERV 13 Filter Scenario: Using a MERV 13 filter and where HVAC zone airflow can be increased as necessary to attempt to meet the new targets
• Maximum Outdoor Air Scenario: A scenario where the HVAC outdoor air fraction and HVAC zone airflow can be increased as necessary to attempt to meet the new targets
• In-duct GUV Scenario: Using an in-duct GUV system where HVAC zone airflow can be increased as necessary to attempt to meet the new targets
• Upper-room GUV Scenario: Using an upper-room 254-nm GUV system along with ASHRAE 62.1 level ventilation to attempt to meet the new targets
• Whole-room GUV Scenario: Using a whole-room 222-nm GUV system along with ASHRAE 62.1 level ventilation to attempt to meet the new targets
• PAC Scenario: Using PACs placed in rooms along with ASHRAE 62.1 level ventilation to attempt to meet the new targets

Study results — CDC

Figure 1 shows the median eACH delivered per scenario to achieve the CDC target of 5 eACH and the corresponding energy use. The HVAC system-only scenarios cannot meet the CDC target despite supplying clean air (via increased filtration, disinfection, or outdoor air fractions) and increasing zone airflow setpoints to their maximum because the standard size of the prototypical office building HVAC system in this study is unable to provide enough airflow to meet the CDC target in all the zones simultaneously. The HVAC system would need to be upgraded with larger fans, coils, and/or ducts to achieve CDC targets. For other buildings, especially those with oversized HVAC systems, meeting the CDC clean air target using only the HVAC system could be more feasible.

Conversely, the scenarios with in-room measures — upper-room GUV, whole-room GUV, and PACs — can meet the CDC clean air target in this study because they are not limited by the capacity of the existing HVAC system to supply more airflow. In other words, upper-room and whole-room GUV and PACs can be feasibly designed to meet the CDC target with the normal operation of the HVAC system.

In terms of energy use, there are large increases in energy needed for the HVAC system measures to attempt to meet the CDC clean air target, due to high increases in fan energy to supply enough airflow. This increase in energy depends on system sizing and may be less severe depending on system design. In this study, the GUV and PAC scenarios consume 75–80% less energy per unit of equivalent clean air, highlighting a large potential energy-efficiency opportunity for these technologies.

Study results — ASHRAE Standard 241

Figure 2 shows the median ECAi delivered by each scenario to achieve the ASHRAE Standard 241 target of 30 cfm/person and the corresponding energy use. In contrast to the CDC analysis, nearly all scenarios achieve the ASHRAE Standard 241 clean air target for offices. The MERV 8 baseline case only meets the ASHRAE 241 target ECAi 74% of the time during occupied hours, while the other scenarios meet the ECAi target more than 99% of the time during occupied hours.

Energy consumption does not increase significantly for the ASHRAE 241 cases compared to the baseline, except for the Maximum Outdoor Air Scenario, because significant heating/cooling energy is needed to condition the higher amount of outdoor air. This study is applied for a cool and humid climate (Chicago, in ASHRAE climate zone 5A), so the increase in energy can be smaller for a milder climate or more significant for a climate with more extreme weather conditions.

It should be noted that the ASHRAE Standard 241 ECAi target of 30 cfm/person for the prototypical office building is lower than targets for many other space types — such as classrooms (40 cfm/person), restaurants (60 cfm/person), retail (40 cfm/person) — and has a lower assumed occupant density. Analysis of these other space types may show results more similar to the CDC case.

Discussion and next steps

The results of the study illustrate a significant potential energy-efficiency opportunity provided by GUV technology to realize healthier buildings. The ASHRAE Standard 241 and CDC targets are approximately 1.5–10X higher than conventional ASHRAE Standard 62.1 ventilation targets to which commercial buildings have historically been designed. Existing building HVAC systems may not have capacity to achieve the CDC and ASHRAE targets without major upgrades, as was seen with this CDC office analysis. Further, achieving the targets through HVAC-only approaches could substantially increase building HVAC energy use and carbon emissions. GUV technologies, which can be easily installed into existing buildings, might enable buildings to achieve the CDC and ASHRAE targets with much lower energy use.

PNNL is currently expanding this analysis to other building types, including a school, restaurant, and healthcare facility, and additional climate zones. A later study will conduct an economic analysis of the results. Findings will help building owners and operators understand what methods are most effective, energy efficient, and cost-effective to achieve CDC and ASHRAE targets and provide healthier buildings to students and workers.

REFERENCE

C.A. Faulkner et al., “Comparison of effectiveness and energy use of airborne pathogen mitigation measures to meet clean air targets in a prototypical office building,” Building and Environment, Vol. 257, 111466 (1 June 2024).

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