Health authorities agree that COVID-19 spreads mostly when infected individuals sneeze or cough, and others are exposed to droplets with the virus. These droplets can be inhaled directly at short distances, or they can fall on surfaces that are touched frequently. Large droplets normally settle within six feet, but smaller ones can stay airborne for longer. Small droplets can potentially be carried to other areas by HVAC systems, unless you deploy prevention measures for COVID-19 in your building.
Viral droplets with a size below 10 microns are of special concern for ASHRAE, since they can stay suspended in the air for many hours. Airborne transmission does not seem to be the main infection route, but it cannot be ruled out either. Many HVAC systems will create an infection risk unless they are reconfigured. This post will discuss three ways in which COVID-19 can spread through HVAC, and solutions for each case.
Get your HVAC system inspected, and fix weak spots against COVID-19.
Air Movement from Higher to Lower Pressure Areas
Pressure differences create air movement, and any particles suspended in the air will be carried along. If the 2019 coronavirus (SARS-CoV-2) is released in a room at higher pressure than its surroundings, it will tend to spread. Buildings with unbalanced ventilation systems are susceptible to COVID-19, since there is limited control over air movement. Unbalanced HVAC systems also waste energy, which is another reason to fix them as soon as possible.
However, HVAC engineers can also use air pressure differences as a prevention measure. Rooms with potential sources of coronavirus are kept at negative pressurization, since this holds the air inside. At the same time, other areas are kept at positive pressurization to keep the virus out. In fact, hospitals use this strategy in COVID-19 patient rooms to contain the virus.
Air Recirculation Through HVAC Ducts
Ventilation systems normally supply a mixture of outdoor air and recirculated air, which is called make-up air. However, during the coronavirus emergency, this creates a risk of recirculating air with viral particles into occupied spaces. Multiple areas served by the same air-handling unit are especially vulnerable.
To reduce the risk of COVID-19 transmission with recirculated air, ASHRAE recommends increasing the outdoor air supply as much as possible, while reducing recirculation. If possible, the building should be ventilated with 100% outdoor air and no recirculated air. However, this may be limited by weather conditions, and the capacity of heating and cooling systems.
An air filter upgrade also helps prevent COVID-19, and ASHRAE recommends at least MERV 13.
- These are hospital grade filters that can capture many airborne germ. MERV 16 is the highest rating, and HEPA filters are even more effective.
- However, the ventilation system capacity can limit the filter rating. The most efficient filters also cause the highest pressure drop, and they may restrict ventilation.
An experienced HVAC engineering firm can identify the highest filter rating that works with your ventilation equipment. You can also consider a fan upgrade to use more efficient filters.
Ultraviolet germicidal irradiation can kill not only coronaviruses, but also other harmful germs like Legionella bacteria and mold. HVAC engineers recommend UV disinfection systems for air handlers, where they can kill germs before they spread to other building areas.
Air Leaks in Energy Recovery Ventilators
Energy recovery ventilators increase the efficiency of HVAC, by exchanging heat and air humidity between the outdoor air supply and the exhaust air. However, leakage can occur between the two airflows, and viral particles in the exhaust could reach the air supply. This risk is reduced when the ERV system is well designed and maintained, since air leakage is typically 1-3%. This metric is called the Exhaust Air Transfer Ratio (EATR) in AHRI Standards.
ERV units should be inspected, and bypassed or repaired if the air leakage is too high. The risk is reduced even further when high-efficiency HVAC filters are used, since the air supply from an ERV must flow through the filter before it reaches indoor areas.