Courtesy of Greenheck
greenheck hvac

Airing it out

Feb. 1, 2021
Adjusting an HVAC system in educational spaces can help minimize the spread of Covid-19.

Keeping people safe and protected from contagious viruses has reached unprecedented levels in recent months as the Covid-19 pandemic continues to grip the world.

Leading organizations such as the Centers for Disease Control and Prevention (CDC) and medical professionals stress the need for common sense practices to slow the spread of Covid-19 and other contagious viruses. Adjusting HVAC system in schools, universities, and other educational facilities can reduce the likelihood of occupants becoming infected. Here are five tactics to help keep students and staff safer and more healthful. 

Increase Outdoor Ventilation

HVAC systems in nonresidential buildings frequently rely on outdoor air to freshen the conditioned air within the building. The amounts vary depending on a variety of circumstances relating to energy usage, environmental conditions and climate.

Outdoor air helps disperse and dilute indoor contaminants, including virus particles. Increasing the amount of outdoor air coming in through the HVAC system can decrease infection rates. It is possible to cut infection rates in half, simply by doubling the amount of outdoor ventilation. Therefore, maximizing the amount of outdoor air in educational facilities will minimize the spread of contagious diseases such as the flu and Covid-19.

How does one add more outside air? Here are some options.

Disable the demand control ventilation. Cafeterias, gymnasiums, auditoriums, lecture halls, and other large spaces often use a demand control ventilation sequence to vary the amount of ventilation based on occupancy. Fewer people usually translates into less ventilation. However, to achieve the goal of minimizing virus spread, a system should provide more outdoor air regardless of occupancy.

•Consider running the HVAC system longer, up to seven days a week and 24 hours a day. Doing so disperses virus particles.

Carry out a pre- and post-occupancy flush sequence. If 24/7 operation is not feasible, flushing the space with the maximum outdoor air rate before and after occupancy can disperse virus particles while minimizing energy usage. ASHRAE recommends flush periods of two hours for both pre- and post-occupancy for systems that meet minimum ventilation requirements.

Open the dampers on partial recirculation units more—up to the maximum 100%. However, make sure the cooling and heating systems are sized to handle the additional load.

•Increasing ventilation reduces the concentration of harmful particles in a building. Still, doing so could harm indoor air quality and diminish staff and student performance. Warmer air, cooler air, or humid air requires more conditioning, and outdoor air may contain other pollutants that also affect indoor air quality. 

Increase Filtration Efficiency

Increasing ventilation rates can dilute contaminants; introducing more efficient filtration can reduce the presence of contaminants. Few people would object to increasing ventilation in rural areas. Doing so in metropolitan areas where outdoor air contains significantly higher amounts of pollutants may not be as popular. Although increasing ventilation is beneficial, it is equally critical to ensure that a space maintains the desired conditions.

Drawing more outdoor air from areas that have higher pollution concentration can add to poor indoor air quality, and result in problems such as sick building syndrome. Occupants of buildings with this condition often experience symptoms such as coughs, headaches, and respiratory ailments. Sick building syndrome may exacerbate the spread of contagions such as the Covid-19 virus.

Improved filtering of the air can minimize the effect of polluted outdoor air by reducing contaminants in the space. Filter effectiveness is indicated by the minimum efficiency reporting value, or MERV, which ranks filters on a scale of 1 to 16.  Higher MERV ratings indicate that a filter is more effective at capturing airborne contaminants, including infectious respiratory droplets. Therefore, a higher MERV rating captures more pollutants and lowers the risk of infection.  A filter with a MERV 13 or higher rating works well in polluted areas and on systems that mix outdoor air with recirculated space air. MERV-13 filters capture most harmful particles.

The one significant drawback to increasing filtration is the ability of the equipment to handle the increased static pressure. Therefore, facility managers should make sure the blowers on HVAC equipment can handle the elevated static pressure.

Maintain Space Conditions

Research has found that certain space conditions in a building can create unfavorable survival rates for microorganisms while increasing immune response to respiratory infection. Maintain normal space temperatures and unit operating temperatures when increasing outdoor ventilation. Keep the space humidity levels between 40% and 60% relative humidity; many microorganisms, including most viruses, cannot survive in this humidity range.

Creating those conditions, however, may be a challenge in some climates. Excessive cold, heat and humidity all can cause problems for maintaining desired space conditions.

Use Energy Recovery

Energy recovery products such as the energy wheel and energy core are essential for maintaining desired space conditions while increasing outdoor ventilation and should remain operational. Energy recovery saves energy by using conditioned exhaust air to precondition the ventilation air, without increasing the potential spread of infectious particles.  Disabling or bypassing the energy recovery unit can result in uncontrollable indoor humidity and increase the potential spread of viruses like Covid-19. When compared with an air-handling unit running 100% recirculated air, energy recovery products have a significantly lower risk of reintroducing virus particles back into the building.

You can also disinfect some energy recovery products. For example, a 3% hydrogen peroxide mixture used on an energy recovery wheel will disinfect and kill virus particles.

Supplemental Electronic Air Cleaners

Electronic air cleaning devices can supplement HVAC systems to clean indoor air further and help minimize the spread of Covid-19. Several types of electronic air cleaners exist in the market, including UV-C lights, bipolar ionization, photocatalytic oxidation, electronic air filters, and gas-phase air cleaners.  These cleaning technologies have advantages as well as design considerations and can be applied in a variety of ways.  Consult with manufacturer-specific manuals and data for more information on the usage and effectiveness of these devices.

It is clear that Covid-19 will be with us at least for the near future. Seasonal flu and other viruses also break out during the school year. Therefore, it is important for schools and universities to do everything they can to minimize the effect on students and staff.

Adjusting building systems to improve indoor air quality and minimize virus transmission between people will help. The five different actions are simple ways to optimize your HVAC equipment.

Pearce ([email protected]) is an application engineer with Greenheck’s dedicated outdoor air systems group and an active member of the American Society of Heating, Refrigerating and Air- Conditioning Engineers.

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