COVID-19 & HVAC: How to Reduce Your Risks

Posted by Stuart Heisey on Thursday, August 13, 2020

Air Return Vent with Surgical Mask

Aerosol spread of COVID-19 has been confirmed by the CDC as a transmission source. There are things that can be done with HVAC systems to reduce risk in occupied spaces. Additionally, every one of these strategies also improves the indoor air quality of your occupied spaces, which has a long-term benefit on the health, productivity, and cognitive abilities of all of the occupants.

On-Demand COVID Webinars:

How Safe is it Inside? Calculating & Reducing COVID Aerosol Transmission Risk in Your Building Spaces (25 min + Q&A)

Every business and organization has interior spaces that are (or will be) needed for successful continued operations. With the CDC’s recent confirmation of aerosol transmission of COVID-19, you might be taking a fresh look at your in-person activities, including how you use the space, how your mechanical systems perform, and how you can reduce risk. COVID is not going away before winter weather sets in, which will force more activities indoors, increasing the risk of aerosol transmission... more info

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Successfully Operating In-Person During COVID: Roundtable (1 hr)

There is no direct substitute for some in-person activities. Learning, teaching, collaborating, and many other activities all benefit from physically being present. It’s true that alternatives exist, but none without tradeoffs. Our Roundtable panel will discuss COVID-19 mitigation strategies and changes made to continue in-person activities, while limiting risk. Everything from scheduling and occupancy to HVAC and foot-traffic patterns is on the table.... more info

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Entech has been working with institutions to help them assess their high-risk spaces and systems and then develop an implementation plan using the appropriate strategies. For each strategy indicated in this document, it is important to understanding its effectiveness when applied to a given space and system as well as the schedule and cost of implementation.

How to determine which COVID-19/Air Quality strategies apply:

  • Identify – Determine the most likely areas that pose the highest risk for in-person operations
    • Create a list of the Buildings, Systems, and Spaces with the highest risk factors
  • Analyze – Utilizing the Airborne Infection Risk Calculator (AIRC) made available by Alex Mikszewski and colleagues, determine the level of risk for the identified spaces.
    • Based on the current conditions and documented assumptions, determine the level of occupancy for each space to align with acceptable risk
  • Respond – Develop and Implement strategies to reduce the risks to acceptable thresholds
    • Where the occupancy is below the desired need or the risk is too high, assess the impact of various strategies to reduce risk and/or increase occupancy
    • Develop pricing/implementation documents to execute the recommendations

Entech can work with your team to clarify tasks and tools to accomplish the basic approach indicated above. Give us a call if you would like more information on how we can create more informed decisions for your institution’s return to in-person operations.

Strategies to Improve Indoor Air Quality & Reduce Airborne Contaminates

The following strategies, when applied correctly, are proven to improve indoor air quality and reduce contaminants, including viruses.

  1. Increase system occupied run-times: Usually a relatively simple and manageable change.
    1. Record all changes made and revisit run-times at change of seasons and at change of occupancy (breaks, downtimes, etc.).
    2. Interior moisture control could become a problem during high humidity weather.
    3. Cold weather will impact your systems differently. Revisit as winter approaches.
  2. Increase outdoor air ventilation: To the extent possible for each system, increase the amount of outside air (OA) during occupancy.
    1. Disable any demand controlled ventilation sequences.
    2. Record all changes made and revisit OA damper positions at change of seasons.
    3. Reset Economizer limits to enable 100% OA for a wider range of conditions.
    4. Interior moisture control could become a problem during high humidity weather.
  3. Pre and post occupancy purge: Run HVAC systems for an hour or two prior to and after occupancy each day, providing as much outside air (OA) as possible during that time.
    1. Systems have a heating and cooling capacity that was not designed for 100% outside air (OA), so if temperatures and humidities are extreme, exercise caution on how much OA your provide.
    2. Many systems are designed to run morning warm-up/cool-down modes without any OA, so a change in the control system might be required to accomplish this.
    3. If exhaust fans are associated with the HVAC system, these should be run during the purge to assist in removing contaminated air.
    4. Few systems have a post-occupancy runtime included in their programming, so again, some controls changes might be needed.
    5. Interior moisture control could become a problem during high humidity weather.
    6. Consider running at least the circulation fan during janitorial cleaning after hours.
  4. Filtration: Virtually all systems have some form of existing filtration, usually a minimally effective filter to catch significant debris and keep the coils relatively clean.  You can easily increase the filtration effectiveness by purchasing a more dense media, but this will also add pressure drop to your system, decreasing maximum available airflow.  If your system is oversized (like most are) this decrease in airflow will not be detrimental.  However, if your system struggles to maintain space conditions during peaks, you should probably keep the current filtration levels (and maybe change your filters more often…).
    1. Most commercial system filters will be MERV 8.  These can often be replaced with MERV 11 or MERV 13 in the same filter rack depth.
    2. Increasing filter effectiveness will increase fan energy on most variable air volume commercial systems.  So this may not be the first strategy you want to pursue.  (If you have a constant volume system, increasing filter pressure drop will decrease airflow and fan energy, so win/win if you have adequate capacity.).
    3. Portable, free-standing high-efficiency particulate air (HEPA) filters can also significantly improve the capture and containment of particles and aerosol droplets. This would be a filtration solution for higher occupancy spaces where the existing HVAC system cannot reach the level of fresh/filtered air changes desired.
  5. Humidification: While the mechanisms and causes are not completely understood, maintaining indoor air humidity levels above 40% RH has a significant impact on the spread of viral diseases.  This could be a blog topic in and of itself, so I will refer you to the excellent presentations and information available from ASHRAE Distinguished lecturer, Dr. Stephanie Taylor, MD.  Suffice to say, humidification should considered a key component of making our buildings safer for occupancy (or re-occupancy) during the current conditions.
    1. Humidification does not need to be an expensive endeavor. While high quality system level humidification might be a longer term investment (and require funding), an in-room, residential style humidification option can be applied. The closer to 40-50% RH that can be maintained in the winter, the better your occupants will resist infection.
  6. Airflow modifications: Changes to the airflow within the space can have a major impact on the spread, dilution, removal, and concentration of contaminants from in-room sources. These changes should be considered for “typical” spaces or repetitive space layouts (such as classrooms, labs, etc.) and can benefit significantly from computational fluid dynamics (CFD) and from case studies.
  7. For classrooms & training rooms: Provide audio amplification to reduce the emission rate of presenters(decrease quanta emissions rate factor of 6.4 for loud speaking vs. normal speaking).
  8. Energy recovery to enable greater increased outside air (OA): Energy Recovery is a required component of many HVAC systems where high amounts of outside air are necessary for the building’s expected occupancy.  However, even when not required, outside air quantities can be increased to improve indoor air quality, and energy recovery can be applied to offset the increase heating and cooling load, enabling you to retain your current HVAC unit/capacity while increasing the fresh air you introduce to the space. Ensure your selected Energy Recovery unit does not enable significant leakage from the exhaust airstream to the fresh airstream.
  9. Bi-Polar ionization: This technology has been around for a long time and it is worth another look if you have passed it by before.  In very simplistic terms, this technology reduces air-borne contaminants by encouraging small particles to group together through opposite charged particles making them drop out of the air for cleaning or more easily captured by filtration.  In addition, the charge ion particles break down odors and VOCs and harm the DNA of viruses and bacteria.
    1. Energy savings can be accomplished with this technology by following the Ventilation Compliance method of quantifying indoor air quality, permitting the reduction of minimum outside air (OA) required.  While Entech does not recommend aggressive reduction of OA, this strategy can create a cashflow to fund the first cost and operational cost of the technology.  Even if you maintain the current code required OA, and don’t have any energy savings, you will significantly improve your buildings indoor air quality.
    2. With a new driver to encourage organizations to pursue better indoor air quality, the operational and employee benefits of some of these technologies has a different financial analysis than just a year ago.  To learn more, contact Entech or check out this technology at or other bi-polar ionization providers.
    3. This technology can (should) be paired with indoor air quality monitoring.
  10. UV lights: There are many manufacturers of commercial UV lights (UV-C to be specific) made to be installed within the Air Handling portion of the HVAC system, typically immediately downstream of the cooling coil.  Their primary advantage is that they kill microbial growth on the cooling coil, improving heat transfer and reducing pressure drop.  If the UV lights are a high enough wattage, they can also disable the reproductive capability of mold spores and viruses.  This is beneficial for obvious reasons…
    1. UV-C lights are also sometimes used as a disinfectant for healthcare and/or laboratory spaces. While they should NOT be used when the spaces are occupied, they can be energized after hours to kill organisms within their line of sight.
    2. In HVAC systems, the UV lights first cost and power consumption can be offset/paid-back by decreased fan energy and increased heat transfer efficiency of the cooling coil.
    3. With no skin in the game, I highly recommend Sterile-Aire UV emitters (and have them installed in my home) due to their high output and long life relative to many options on the market.  If you are interested in learning more, contact Entech Engineering or visit
  11. Monitor:  You can’t control/manage/fix what you don’t measure/monitor.  There are excellent monitoring options out there that measure common indoor contaminants.  One is provided by AtmosAir Solutions (who also provides the BiPolar Ionization technology noted in Item 4 above).  Whatever monitoring option you choose, I recommend getting measurements prior to implementing any of the changes above (if possible).
  12. Controls: Adjusting or improving AHU controls can improve the indoor air quality (IAQ) across the board. With proper controls you can monitor the OA intake while also monitoring indoor and outdoor air humidity levels to provide maximum ventilation for the current conditions. With the increase of cleaning due to Covid 19 the air quality can be affected by total volatile organic compounds (TVOC) given off by cleaning products which can lead to headaches and discomfort. CO2 levels increase when spaces are occupied. TVOC and CO2 levels can be monitored and ventilation adjusted accordingly to provide the proper ventilation. Focus can be placed on energy efficiency when not occupied, or after the COVID 19 pandemic has passed.

Other system options include:

  • AHU or Upper-room UVGI (with possible in-room fans as a supplement to supply airflow) for high density/occupancy spaces.
  • Local exhaust ventilation for source control.
  • Personalized ventilation systems for certain high-risk tasks.
  • Portable, free-standing high-efficiency particulate air (HEPA) filters.
  • Temperature and humidity control.
  • Review cross-contamination of any energy recovery devices and bypass.

With all of the options available and the urgency of beginning implementation, it may be difficult to decide how to proceed! Entech Engineering can help! Contact me to discuss your organization’s unique situation!

Authored by:
Stuart Heisey, PE Photo

Stuart Heisey, PE


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Categories: Buildings & Campus

Tagged: Mechanical  |  Health & Safety  |  COVID-19

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