Our experts are on top of HVAC SYSTEMS vs COVID-19

As we work through the current pandemic caused by the novel coronavirus, SARS-CoV-2 – which causes the disease COVID-19, our clients now regularly ask us if we will be changing the way we design HVAC systems to mitigate the spread of COVID-19 and future diseases, and whether they can make changes to existing equipment to optimize their systems and limit viral spread.

The implications are significant for mechanical equipment: current theory holds that aerosols, airborne infectious particles, are likely to be spread through various air-handling and ventilating equipment. If those aerosols can indeed carry live SARS-CoV-2 cells over extended distances, unfiltered re-circulation of indoor air then becomes a potent way to spread the virus. Per Harvard public health scientist Joseph Allen in a recent Washington Post column:

“Airborne transmission – caused by small particles that can linger in the air for extended periods of time, unlike droplets from coughs, which settle quickly – is key to understanding why this disease spreads so rapidly in certain circumstances. It’s also key to figuring out how best to reopen our country.”

In April of this year, ASHRAE – The American Society of Heating, Refrigerating and Air-Conditioning Engineers, the organization that publishes standards for heating, cooling and ventilation – acknowledged that “transmission of SARS-CoV-2 through the air is sufficiently likely” and that “ventilation and filtration provided by heating, ventilating and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air.”  ASHRAE appointed a COVID-19 Epidemic Task Force that offers recommendations related to building systems.  Two important questions posed by ASHRAE were:

  1. “What are the engineering interventions that may be applied to minimize the spread of the disease through the air?”
  1. “How effective are those engineering interventions at minimizing the spread of disease?”

As scientists’ understanding of the virus and its mode(s) of transmission continues to evolve, there are many ideas about the ways in which an HVAC system might combat the spread of COVID-19.  Below, we analyze and discuss a few of the issues, as well as recent recommendations and suggestions.

L.R. Kimball is constantly monitoring current thinking and best practices to prepare clients for post-COVID-19 recovery and to return to business.

Dilute and Manage the Airflow Within a Building

Humans inhale oxygen and exhale carbon dioxide. One of the many purposes of ventilation (outside air) in a HVAC system is to replenish that oxygen inside a building. In addition, the constant supply of ventilation helps to dilute odors and prevents stagnation of the air. In their most basic form, ducted commercial HVAC systems mix return air from the conditioned space with outside air for ventilation. This mixed air is then filtered, conditioned (heated and/or cooled depending upon climatic conditions) and supplied to the space. The amount of ventilation provided is based on either the IMC (International Mechanical Code) or ASHRAE Standard 62.1. The referenced code or standard dictates the minimum required ventilation rate of the system in CFM (cubic feet per minute).  The average system is typically designed to provide only the minimum amount of outside air because conditioning additional outside air is more energy intensive then conditioning recirculated air. Various methods to further dilute the air within a building are as follows:

  • Increase the number of air changes per hour (ACH) within a space, i.e. increase the number of times per hour that the air in a space is replaced.
  • Operate the HVAC system without interruption – including overnight and on weekends.
  • Allow fresh air into the building through natural ventilation.
  • Leverage the fact that most buildings are currently operating at a significantly reduced occupant load and there may be spare capacity within existing systems to make temporary changes.

Changing indoor airflow patterns could also play a key role in reducing transmission.  Designs that incorporate laminar airflow are already commonly used in clean room and hospital operating rooms. A few potential methods to adjust airflow patterns are:

  • Replace fixed speed motors with variable speed motors to allow for reduced air speed. This may help decrease the spread of a virus across a room when high-speed air flows past an infected person.
  • Change the type of air devices used or increase the quantity of air devices to reduce air speed within a space.
  • Supply air at the floor level and return air at the ceiling level in lieu of the more common design which supplies and returns air at the ceiling level.

Better Filtration

An HVAC system “cleans” the air by filtering out particles that are captured in a filter for disposal. There are many different types of filters available and each is designed to serve a specific purpose. Filters are rated from 1 – 20 (higher is better) on the MERV (Minimum Efficiency Report Value) scale. The industry standard referenced above, ASHRAE 62.1, recommends using a filter with a MERV rating of not less than 8. Most new commercial buildings are designed for filters with a minimum MERV rating of 11. Filters in an airstream create a pressure drop which impedes airflow. Once an HVAC system is selected, it is designed to accommodate the pressure drop of the specific filters to be used, measured when they are “dirty”. The term “dirty” refers to a filter loaded up with particles over time, which increases the filter’s pressure drop. An increase in MERV rating will increase the pressure drop of the filter.

One common suggestion to combat the recirculation of airborne particles is to use a HEPA (High-Efficiency Particulate Arrestance) filter, which has a MERV rating of 17-20, and is designed to trap >99.97% of particles with an average size of 0.3 microns and is commonly used in hospital and clean room applications where the highest levels of filtration are required. While a HEPA filter will certainly filter the air more effectively, it cannot be simply installed in place of a standard filter. A typical HEPA filter is much thicker than standard filters found in HVAC equipment. Additionally, the pressure drop is approximately 3 to 7 times greater than a MERV 8 filter. Finally, due to their high cost, HEPA filters are typically provided with less costly pre-filters, which help to extend the life of the HEPA filter and require extra space and create an additional pressure drop.

Increasing the Humidity

Absolute humidity is the amount of water vapor in the air regardless of temperature, while relative humidity (RH) is the percentage of water vapor in the air at a given temperature. Thus, the same amount of humidity can result in different levels of relative humidity at different temperatures; relative humidity affects how we feel temperature. Currently, ASHRAE recommends maintaining relative humidity at 40% – 60% within a building. Scientific evidence shows that this humidity range is best for minimizing the spread of many airborne infectious organisms, including influenza, and most likely SARS-CoV-2.

The maximum relative humidity level in a building is typically limited as a byproduct of the cooling process during the cooling season. As the supply air is cooled, moisture is removed, and the humidity levels are kept under control. Different system types are better than others at controlling the humidity levels and there are certain conditions that can make humidity control difficult, but in general, maintaining ASHRAE’s current recommended maximum humidity level is not an issue.

During the heating season, supplemental equipment will likely be required to maintain the recommended minimum relative humidity setpoint. This is because the cold and dry outside air that is brought in for ventilation reduces the RH and most systems were not designed with the capability to increase the RH. The basic requirement to maintain minimum relative humidity levels during the heating season is a source of steam from either a campus loop or a steam heating system or a gas/electric humidifier to provide steam that can be introduced back into the supply airstream.

Ultraviolet Intervention

The sun emits ultraviolet (UV) light which can be both beneficial and harmful to humans.  UV radiation can be classified into three types based on wavelength: UVA, UVB and UVC. Ultraviolet light in the C spectrum (100-280 nm) is harmful to microorganisms such as bacteria, viruses, mold spores, mildew, and other pathogens, and can be used to combat the spread of disease through Ultra-Violet Germicidal Irradiation (UVGI). First developed in the 1930’s, UVC lights have primarily been used in healthcare settings to sterilize equipment and in residential applications to sterilize well water. UVC lights have been shown in the past to kill other corona-type viruses, and current research looks promising that they will be as effective at rendering SARS-CoV-2/COVID-19 harmless.  Researchers at Columbia University Irving Medical Center were able to kill 99.9% of two common corona viruses by exposing them to UVC light.

 L.R. Kimball can help walk you through the options …We can EVALUATE your systems and RECOMMEND mitigation measures customized to meet the specific needs of your organization

Get an Engineer Involved!

HVAC systems maintain healthy air quality, but cannot prevent anyone from becoming infected by airborne illnesses.  Before you make any changes to your system, you should engage a Mechanical or HVAC Engineer to evaluate your current system to understand its components and its present performance. Before you involve that engineer, you should collect all materials that may be pertinent to the investigation, including original design drawings or as-builts, approved equipment submittals, sequence of operations, controls diagrams, set points and trend data. The older a system is, the more we can assume that it will be operating differently than its original design intent or suffering from a lack of routine maintenance.  Your engineer can review the options with you, addressing system complexities, long-term fixes vs short-term fixes, and cost to implement.  In the long run, upgrading the HVAC system to safeguard the occupants may also result in a more efficiently operating system.

For further reading, ASHRAE has created a technical resource dedicated to their response to the COVID-19 pandemic: www.ashrae.org/covid19



This document summarizes current views regarding best practices and approaches to ventilation systems that may limit virus spread in buildings. Research into the airborne transmission of COVID-19 and the best methods for preventing viral spread is ongoing and being continuously updated. References to specific products or organizations are solely for illustrative purposes and do not constitute any endorsement or recommendation.

 L.R. Kimball’s COVID-19 Re-opening Guidelines are derived from guidelines issued by United States government agencies including the Centers for Disease Control and Prevention (CDC), the Occupational Health and Safety Administration (OSHA), the Environmental Protection Agency (EPA), the Food and Drug Administration (FDA), as well as those developed by the World Health Organization (WHO), the American Institute of Architects (AIA), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the Institute of Real Estate Management (IREM), and the Building Owners and Managers Association (BOMA). Each state or territory has additional minimum requirements to allow businesses to re-open. The recommendations by the aforementioned agencies are being continually updated and refined and are not consistent with each other in all respects. While not applicable to every business, property or circumstance, the guidance provided is intended to highlight relevant factors for consideration and informed decision-making to re-open a given building as quarantine restrictions are eased. While we seek to provide you with guidelines which are prudent and consistent with the guidelines of the agencies mentioned above based on current knowledge, the effects of the COVID-19 pandemic are unprecedented and extraordinary, and we cannot be held liable for actions or inactions taken based on this article or for the effects of any transmission of COVID-19 which may occur on your premises.  We recommend consulting with a qualified professional prior to implementing any measure discussed in this article to ensure safe and effective implementation and compliance with local rules and regulations.



Joseph Allen, Washington Post: “We cannot keep ignoring the possibility of airborne transmission. Here’s how to address it,” May 26,2020:


Pandemic COVID-19 and Airborne Transmission,” ASHRAE Environmental Health Committee, April 17,2020:


McKinsey & Company, “Can HVAC Systems Help Prevent Transmission of COVID-19?” July 9, 2020: https://www.mckinsey.com/industries/advanced-electronics/our-insights/can-hvac-systems-help-prevent-transmission-of-covid-19

ASHRAE Epidemic Task Force, Building Readiness, May 21, 2020: https://www.ashrae.org/file%20library/technical%20resources/covid-19/ashrae-building-readiness.pdf

2020 ASHRAE Handbook – HVAC Systems and Equipment, Chapter 22: Humidifiers, June 11, 2020:

https://www.ashrae.org/file library/technical resources/covid-19/i-p_s20_ch22.pdf

Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses, June 24,2020: https://www.nature.com/articles/s41598-020-67211-2

By: Ryan Meitzler, PE, LEED AP ID+C
Sr. Mechanical Engineer
L.R. Kimball