In parallel with the recommended immediate measures above, long-range planning efforts should critically assess indoor air quality and consider investments in a range of HVAC system upgrades to increase air quality long-term. Taking into consideration the expected remaining useful life of existing systems and the availability of funding, we recommend considering the following long-term HVAC improvements:
Increase Filtration Efficiency
To determine how well your filter captures contaminants, check the MERV rating (Minimum Efficiency Reporting Value), which is the industry standard used to measure the overall effectiveness of air filters. In general, the higher the MERV rating, the more airborne contaminants are filtered.
MERV-13 is currently the minimum recommended filter rating to have any possible impact on capturing virus particles according to ASHRAE. Many HVAC systems found in schools and commercial buildings use lower MERV ratings and are not sized for higher filtration levels. In fact, depending on the design of the system, swapping in a high-efficiency filter may result in an undesirable reduction in airflow due to the additional pressure drop created by higher efficiency filtration. This can be addressed through modifications such as increasing the filter face area, increasing the frequency of filter changes to minimize loading, employing booster fans, or upgrading system fans.
At the high end of available air filtration, High-efficiency particulate air (HEPA) or Ultra Low Particulate Air (ULPA) filters will contribute most significantly to reducing airborne contaminants by filtration and have long been used in cleanrooms utilized for advanced manufacturing, research, and healthcare facilities. These systems are most effective when coupled with unidirectional downflow air distribution creating a piston effect whereby contaminants are pushed past the operational or breathing zone. This will typically require a system of HEPA/ULPA filters, either ducted or fan-powered, to be installed in the ceiling coupled with return air chases extending from the ceiling down to the floor. This type of installation will require careful planning to accommodate required seating and circulation space as well as available floor-to-floor heights. In cases where overhead filter installation is not feasible due to space constraints, floor-mounted filtration systems utilizing flexible ducting can offer a possible solution as well.
Improve Air Distribution
Bringing in more fresh outdoor air by increasing outside air ventilation rates is one of the most effective ways to minimize risk of infectious disease transmission through the air system. Commonly found in research laboratories, 100% Outside Air Systems are ideal, as they do not allow for any potential recirculation of contaminated air from within the building. When sized to meet a building’s full heating and cooling load, 100% Outside Air Systems, however, will result in high energy usage due to the additional heating, cooling, and filtration required to treat the outside air. A full conversion to 100% outside air may therefore not be feasible in many buildings due to high capital and operating costs.
Dedicated Outside Air Systems (DOAS) are a viable option to consider as they deliver only the required ventilation rates to specific building zones. Coupled with local recirculating air systems, these systems allow for limiting the recirculation of air to smaller zones, which can minimize the risk of pathogen spread when paired with proper filtration and air treatment. Consideration to personalized air terminals should be given where possible. Coupled with dedicated fan filter systems, these could be implemented as part of furniture modules or through careful placement of ducting and terminals.