If chemical disinfectants are used, they should only be applied with the HVAC system turned off. In addition, disinfectants should not be applied to ventilation filters before continuing to use the filters within ventilation systems. The effects of disinfectants on filter performance are unknown. Filters should only be treated with disinfectants if they are to be removed from service and disposed of.
Ultraviolet (UV) systems are a great way to maintain the cleanliness of HVAC coils, drain pans, and other damp surfaces. These systems generally require more lamps, so they can provide significant UV doses in a short period of time. A typical one-pass inactivation efficiency is 85%, just like a good particulate filter, but systems can also be designed for inactivation greater than 99.9%. In addition, a well-designed UV air disinfection system within an HVAC system, and located adjacent to the cooling coils, can also provide surface disinfection benefits.
Another way to install UV is in a “top air” configuration. Specially designed wall-mounted fixtures create an irradiated area above the occupant and disinfect the air in the space, as the air circulates naturally, mechanically or through the HVAC system. CDC has approved this type of system for use in tuberculosis control for nearly 20 years, and there is guidance from NIOSH on how to design them. Mobile UV systems are also frequently used for terminal cleaning and surface disinfection in healthcare facilities and other spaces.
Systems such as these are commonly used in unoccupied spaces due to occupant exposure concerns. The design and sizing of effective ultraviolet disinfection systems can be a complex process due to the need to determine the dose delivered to a moving air stream or to an irradiated region of a room. In-duct systems are further complicated by the configuration of the air handling unit and ducts and surface reflections that can help achieve higher irradiation levels. Overhead air systems require proper air mixing to function properly while paying close attention to reflective surfaces that could cause room occupants to be overexposed to UV energy.
Accredited manufacturers and system designers can help by making the necessary calculations and designing specific systems for individual spaces. With the recommendation of a MERV 13 or higher, does a MERV 13 filter meet your needs? A MERV 13 filter is a step in the right direction and captures more particles than a typical MERV 8 filter. However, it's not as good at capturing small virus-sized particles as a HEPA can. A MERV 13 will trap less than 75% of air particles that are 0.3-1.0 microns in size (coronavirus is 0.1 microns). It is also difficult for many existing HVAC (HVAC) systems to adopt a MERV 13 because of the greater fan load of finer filter media, which can actually cause more harm than good and reduce airflow if your system is not designed to handle that type of filter. In general, finding a filter with a higher MERV rating will correlate with greater efficiency in capturing particles and contaminants in the air.
In cases where the ventilation system cannot handle a high-efficiency filter, you can use a portable air purifier while upgrading to the highest possible MERV rating with existing capacity. To help protect against the COVID-19 virus, ASHRAE changed its recommendation from MERV 8 filters to MERV 13+. The test procedure for the MERV classification uses 6 measurements and 12 particle sizes, resulting in a total of 72 data points. Good MERV sorting efficiency will improve indoor air quality and reduce the spread of virus particles.
MERV 14 and higher can capture particles that are even smaller than previous MERV ratings, ranging from 0.3 to 1.0 microns in size. The table below provides approximate relationships between ratings under ASHRAE, MERV and ISO 16890 test methods. Therefore, a filter with a MERV rating of at least 13 would efficiently capture COVID-19 virus particles and could help reduce the spread of COVID-19 indoors. When using a 4″ MERV 13 filter instead of a 1″ filter, the resistance is reduced and the flow rate increases from 320 fpm to 460 fpm.
One of their recommendations is to use air cleaners with at least a MERV 13+ rating, or a higher HEPA rating when possible. In addition, Camfil is leading the initiative to educate customers about the different MERV ratings and how to know if you are getting what you paid for. RP Fedder sells a full range of MERV 13+ filters and can help you determine what types of filters and efficiency ratings you need. Because HEPA filters are so efficient, they cause a greater pressure drop than MERV-rated filters.
MERV ratings 1 to 4 are generally only used as pre-filters, but can be found in many typical applications, such as commercial buildings, ovens, and window air conditioning units.