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NTT Blog

12/09/2020

COVID-19 and HVAC Systems

RD Holder Eng

Scientists still don’t fully understand how COVID-19 is transmitted. Though there are still questions about the effect of ventilation systems on infections, it’s well known that indoor air quality has a significant impact on people’s physical and mental health. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is one trusted source that continuously updates its technical information and resources page in addressing the impact of COVID-19 on HVAC systems.

In a recent ASHRAE position paper, when it comes to reducing the spread of the virus the “basic principles of social distancing, surface cleaning and disinfection, hand-washing and other strategies of good hygiene are far more important than anything related to the HVAC system.”

That said, it is a possibility that HVAC systems can help contain the spread. There are steps building owners and operators can take to retrofit existing buildings to help reduce bacteria, viruses and other indoor airborne pollutants:

1. Determine whether the existing HVAC system is operating as designed.

This requires a commissioning process that assesses the current performance of the building’s HVAC system. Air and Water Balance the HVAC system.

2. Consider increasing the amount of outdoor air to dilute airborne contaminants.

Consider increasing the OSA to at least 20 to 30 cfm per person.  Bathrooms have an exhaust air of at least 2 to 3 cfm per square foot.

The building HVAC system must be able to handle the surge in cooling and heating loads associated with a boost in outdoor air. Increasing the outdoor ventilation rate above what a HVAC system is designed to properly de-humidify in humid climates, for example, could cause an unpleasant and undesired increase of humidity in a building.

3. Improve central air filtration.

Filters with a Minimum Efficiency Reporting Value (MERV) rating of less than 13 do not remove particles in the size range of most viruses that are smaller than 0.3 microns. MERV-13 filters improve filtration efficiency of small particles and are approximately 50 percent efficient for particles from 1.0 micron down to 0.3 microns in size, while MERV-14 filters are 75 to 85 percent efficient in removing these particles and HEPA filters are more than 99 percent efficient. HEPA filters usually are not practical for office applications for reasons related to cost, energy use and operations. Though many newer office buildings already have MERV-13 or MERV-14 filters, older buildings may require more physical space to accommodate them as well as more powerful air-handling systems to overcome the added air resistance. ASHRAE also recommends sealing filter frames to reduce bypass around the racks and, in some cases, running HVAC systems longer though 24/7 operations would be costly and may not provide the commensurate benefit.

4. Maintain optimal humidity levels in the space.

Viruses thrive in low-humidity environments. While maintaining 40 to 60 percent relative humidity is ideal for health reasons, keeping it at a minimum of 40 percent is not practical for most office applications. The HVAC systems for most office spaces don’t incorporate humidification. The amount of moisture needed to offset dry outdoor air during the winter and in arid climates largely impacts the cost and decision not to include humidification. Condensation control also has to be considered when outdoor conditions are far different than indoor conditions, which is often the case when outdoor temperatures drop below freezing. The industry standard indoor high limit for most building types is 60 percent relative humidity, which is often achievable without much difficulty.

5. Negative pressure rooms prevent the spread of infectious contaminants in medical and research environments, but are not viable solutions for office environments.

Negative room pressure, which controls air flow into a segregated environment, is effective in applications where spaces like labs or hospital isolation rooms are intentionally designed for isolation or containment. But doing this, which requires rooms with physical separation like full-height walls and doors, would be impractical and cost prohibitive for most offices environments

6. Ultraviolet germicidal irradiation (UVGI) systems installed in HVAC systems can kill airborne particles that carry viruses.

Airstream UVGI is effective for mitigating the transmission of airborne viruses in office and indoor environments. Ultraviolet light devices inactivate a variety of viral, bacterial and fungal organisms. Airstream UVGI can be easily included in retrofit applications. An alternative to airstream UVGI is upper-room UV installations. These use less intense light than airstream UVGI systems designed to kill viruses, carry a higher cost and come with more risk if not properly designed, operated and maintained. Yet they do have the distinct advantage of being able to treat all surfaces in a room exposed to UV light.

7. Virus protection considerations alone should not determine whether office space uses an underfloor or overhead air distribution system.

Both systems have advantages and disadvantages. But neither has a definitive edge when it comes to limiting the spread of the virus. Organizations should opt for the reason they chose a particular air distribution system in the first place.

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