Overview: Increasing the amount of outdoor air and cleaned filtered air introduced to educational spaces is one of the key methods for reducing the risk of disease transmission in schools, and for improving indoor air quality. From the start of the current COVID-19 pandemic, the CDC, WHO and local health departments have recommended increasing the minimum outdoor air settings on air handling equipment and disabling energy-saving functions such as demand-control ventilation that reduce outdoor air ventilation based on sensed occupancy levels or CO2 sensors.
Removes, Dilutes, or Inactivates Pollutants: Increasing the amount of outdoor air will not inactivate pathogens, but the increased ventilation will dilute pathogens to reduce the risk of transmission. Increased outdoor air will also dilute indoor air pollutants like dust, odors and VOCs reducing the health impacts of these pollutants as well. Health agencies are recommending that school buildings complete a flush cycle each morning and evening before and after occupancy. The general recommendation is 2 hours before and after occupancy, but a more specific recommendation is the amount of time to achieve 3 air changes between occupancy periods. Early in the pandemic, demand control ventilation (DCV) was recommended to be disabled to prevent the reduction in outdoor air based on CO2 readings, which may not properly account for pathogen emissions in low-occupancy spaces. However, a balance between increased ventilation and energy can be achieved in buildings where DCV can maintain a CO2 setpoint of 800 ppm within each occupied room, instead of the previous industry standard of 1100-1200ppm. This doubles the amount of outdoor air provided by systems controlled by DCV controls and reduces the energy and comfort impacts of conditioning large volumes of outdoor air. Additionally, the CDC, WHO, and health departments have recommended increasing the minimum outdoor air setpoints on air handlers as much as possible without impacting the ability of the systems to maintain interior comfort conditions, as thermally stressed occupants are more susceptible to infections. Clearly there are multiple ventilation approaches, and an engineer should be consulted to determine the available capacity of your HVAC system and provide guidance on the best approach to ventilation in your building.
Harmful By-products: Increasing outdoor air does not create harmful byproducts. However, building operators should be aware that, depending on the building location, outdoor air may have its own pollutants that can impact indoor air quality, such as pollen, smoke, smog, ozone, and vehicle fumes.
Using a central system to increase outdoor airflow will filter particulates but not gaseous pollutants. Humidity control is also important to consider as air that is too humid or too dry can have its own adverse impacts on health and transmission risk. Consult with HVAC engineers to ensure systems can maintain appropriate humidity levels when increasing outdoor air flow, particularly in the winter months. Energy recovery can provide good humidity control without requiring additional energy and maintenance for humidifiers.
Added Energy: Increasing outdoor air flow into a building will increase heating and cooling loads on mechanical equipment, as well as increasing the amount of fan energy to move the air. An EPA study found that for an educational facility with a central air conditioning system, increasing outdoor airflow from 5cfm per person to 15 cfm per person increased cooling energy costs by 20% and heating energy costs by over 500%. The advantage of using central systems to introduce outdoor air is that systems can be modified with energy recovery to limit the energy impacts of increased outdoor air. Proper addition of energy recovery devices can potentially negate any additional energy impacts from increased ventilation and provide energy savings if energy recovery did not already exist for a system.
Install Cost: For schools with central building automation systems, the costs to increase outdoor airflows may be limited to an engineering study and modification of existing ventilation system programming setpoints. Schools without central automation, or relying on older pneumatic control systems, may require time and labor to update ventilation system capabilities. Costs can range from the scale of moderate building repairs, up to major capital upgrades.
Looking Beyond the Pandemic
Beyond the pandemic, increased outside air ventilation has been shown to improve student and staff productivity. Designers and architects for schools may want to look at heat recovery systems to provide a greater volume of outdoor air than code minimum while avoiding the energy penalties. Heat recovery can also adapt existing systems to provide greater capacity while mitigating the additional energy costs of increased outdoor airflow. Unlike with opening windows, which provides no tempering of additional outdoor air, mechanical systems can be updated, modified, or designed to provide increased ventilation and maintain comfort without increased energy consumption.
For DCV systems, future recommendations are expected to be set to maintain CO2 at about 800 ppm instead of 1100-1200 ppm that is typical based on ASHRAE 62.1 designs. Again, this provides a means to supply additional outside air and reaping those benefits, while still providing some occupancy-based control to mitigate additional energy requirements (see IDPH Guidelines for Indoor Air Quality).