Q1. What filters are recommended for HVAC systems? Q2: How

Q1. What filters are recommended for HVAC systems?

A: Our current recommendation is to use a filter with a Minimum Efficiency Reporting Value (MERV) of

13, but a MERV 14 (or better) filter Is preferred. Of course, the ultimate choice needs to take the

capabilities of the HVAC systems into consideration. Generally, increasing filter efficiency leads to

increased pressure drop which can lead to reduced air flow through the HVAC system, more energy use

for the fan to compensate for the increased resistance, or both. If a MERV 13 filter cannot be

accommodated in the system, then use the highest MERV rating you can.

Appropriate filters tested under the ISO 16890 Standard can also be used. The table below provides

approximate relationships between ratings under the ASHRAE MERV and ISO 16890 test methods. The

current recommendations would suggest using a filter with an ePM1 rating under ISO 16890.

*MERV-A will give closer results between the two standards. Charged media filters usually show a drop-off in efficiency with

use. ISO 16890 captures this with an IPA condition step. ASHRAE 52.2 can capture this drop if the test is done with the optional

Appendix J which gives the MERV-A. Thus the MERV and the ePM ratings do not reflect the same testing. For charged media,

the MERV will likely make the filter appear more efficient than the ePM rating.

Q2: How often should HVAC filters be changed?

A: Please refer to current ASHRAE guidance on HVAC System Maintenance and Filter Replacement

during the COVID-19 Pandemic.

Filters in HVAC systems should be changed according to the typical schedule or even left in place longer

than normal. Extending the time between filter changes is a strategy being employed during the current

COVID-19 pandemic. If lower rated filters were upgraded to meet ASHRAE recommendations of at least

a MERV 13 filter, there was likely an increase in airflow resistance (increased pressure drop) when the

new filters were installed. However, the upgraded filters may be capable of holding more dust than the

lower rated filters without a substantial additional increase in air flow resistance. Thus, extending the

time between filter changes may be beneficial. Either way, the risks from not changing filters right on

time are small. The filters may load more than normal, which could lead to slight reductions in air flow.

The secondary impact of that depends on the type of building and building occupancy (among others).

Q3: What is the size of the SARS-CoV-2 virus, and can it be

captured by ventilation filters?

A: Research has shown that the particle size of the SARS-CoV-2 virus is around 0.1 µm (micrometer).

However, the virus does not travel through the air by itself. Since it is human generated, the virus is

trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are predominantly 1

µm in size and larger.

ASHRAE currently recommends using a minimum MERV 13 filter, which is at least 85% efficient at

capturing particles in 1 µm to 3 µm size range. A MERV 14 filter is at least 90% efficient at capturing

those same particles. Thus, the recommended filters are significantly more efficient at capturing the

particles of concern that a typical MERV 8 filter which is only around 20% efficient in the 1 µm to 3 µm

size range. Filters with MERV ratings higher than 14 would capture an even higher percentage of the

particles of concern. High-efficiency particulate air (HEPA) filters are even more efficient at filtering

human-generated infectious aerosols. By definition, a HEPA filter must be at least 99.97% efficient at

capturing particles 0.3 µm in size. This 0.3 µm particle approximates the most penetrating particle size

(MPPS) through the filter. HEPA filters are even more efficient at capturing particles larger AND smaller

than the MPPS. Thus, HEPA filters are more that 99.97% efficient at capturing airborne viral particles

associated with SARS-CoV-2.

Q4: Should HVAC system maintenance, including filter changes,

be postponed during the pandemic?

A: Please refer to current ASHRAE guidance on HVAC System Maintenance and Filter Replacement

during the COVID-19 Pandemic.

It is not necessary to suspend HVAC system maintenance and/or filter changes during the COVID-19

pandemic, but some additional safety precautions are warranted.

At this time little is known about the penetration of SARS-CoV-2 into HVAC systems. The task force is

developing a research work statement to study just that. Neither ASHRAE nor CDC has posted guidance

on the decontamination of HVAC systems (to include air filtration systems) potentially exposed to SARS-

CoV2. The survival time of SARS-CoV-2 in air has been estimated at several hours and survival on

surfaces is, at most, several days. There is no evidence to date that SARS-CoV-2 penetrates to a

significant extent through central air-handling units to cause risk in spaces where infectors are not

present. Should HVAC systems actually become contaminated with viable virus, the most likely scenario

for disinfection is that the virus would lose its viability naturally within hours-to-days, and thus, there is

no guidance advocating proactive system shutdown for decontamination and/or filter replacement.

There are no published laboratory studies showing whether the SARS-CoV-2 virus can be re-aerosolized

from ventilation air filters or other HVAC surfaces. Laboratory studies indicate that re-aerosolization of

viable mycobacteria, which are of similar size to viral particles, from filter material is not probable under

normal conditions. However, the risks associated with handling filters or performing other maintenance

activities inside HVAC systems potentially contaminated with coronaviruses under field-use conditions

have not been evaluated. Therefore, it is recommended that workers performing maintenance and

replacing filters on any ventilation system with the potential for viral contamination wear a properly

fitted respirator (N95 or higher), eye protection, and gloves. The HVAC systems should be turned off

prior to entry for any maintenance activity. When feasible, filters can be disinfected by spraying them

with a 10% bleach solution or another appropriate disinfectant, approved for use against human

coronaviruses (see EPA List N), before removal and disposal. Disinfectants should not be used on

ventilation filters prior to continued use of the filters inside ventilation systems. The effects of the

disinfectants on filter performance are unknown. Filters should only be sprayed with disinfectants if

they are to be removed from service and discarded. Whether disinfected or not, filters removed from

HVAC systems with suspected coronavirus contamination can be placed into a regular trash bag (do not

bend, tear or crush the filters) and disposed of as normal trash. There are no special bagging/tagging

requirements or waste processing steps necessary, outside of the normal waste processing procedures.

When maintenance tasks are completed, maintenance personnel should immediately wash their hands

with soap and water or use an alcohol-based hand sanitizer.

Q5: Are there recommended procedures for disinfecting HVAC

equipment?

A: Neither ASHRAE nor CDC has posted guidance on the decontamination of HVAC systems (to include

air filtration systems) potentially exposed to SARS-CoV2. To date, there has not been compelling

evidence to demonstrate that viable virus is contaminating these systems. Should such systems actually

become contaminated with viable virus, the most likely scenario is believed to be that the virus would

lose its viability naturally within hours-to-days, and thus, there is no guidance advocating proactive

system shutdown for decontamination and/or filter exchange.

If you choose to use chemical disinfectants, it is important to use them properly. There are no

disinfectants approved specifically for use inside ventilation systems. However, there are hundreds of

EPA-registered disinfectants that are effective at killing human coronaviruses (see EPA List N). A good

fact sheet on the use of disinfectants to control the COVID-19 virus from the National Pesticide

Information Center can also be found here. It provides some tips for proper disinfectant usage, including

surface types, understanding the label, and proper dwell or contact times. It is important to remember

that most of these disinfectants, while effective at killing coronaviruses, do not continue killing long

after they are applied. So, while they are effective at killing viruses on HVAC surfaces, they will not

continue killing into the future. They would need to be reapplied.

If chemical disinfectants are used, they should only be applied with the HVAC system powered off. Also,

disinfectants should not be applied to ventilation filters prior to continued use of the filters inside

ventilation systems. The effects of the disinfectants on filter performance are unknown. Filters should

only be treated with disinfectants if they are to be removed from service and discarded.

Q6: Is ultraviolet energy (UV-C, ultraviolet germicidal irradiation,

germicidal ultraviolet) effective against the SARS-CoV-2 virus?

A: Ultraviolet energy (ultraviolet germicidal irradiation or germicidal ultraviolet) could be a powerful tool

in the fight against COVID-19. ASHRAE’s position on UVC is expressed in the Position Document on

Airborne Infectious Diseases: UVC air and surface disinfection is used in many different settings –

residential, commercial, schools, as well as healthcare. Germicidal light (particularly 254 nm UVC

produced by low pressure mercury vapor lamps, which operate near the most effective wavelength of

~265 nm) has not, to our knowledge, been tested on SARS-CoV-2, but it has been tested on an airborne

coronavirus (Walker 2007). The sensitivity of that coronavirus to 254 nm was high enough that it seems

like a good candidate for UV disinfection.

While UV systems are quite effective at maintaining cleanliness of HVAC coils, drain pans and other

wetted surfaces, properly designed systems can be quite effective at on-the-fly inactivation of

microorganisms in moving airstreams. These systems generally require more lamps so they can provide

significant UV doses in a short period of time. A typical single pass inactivation efficiency is 85%, much

like a good particulate filter, but systems can be designed for over 99.9% inactivation as well. Plus, a

well-designed UV air disinfection system inside an HVAC system, and located by the cooling coils, can

also provide the surface disinfection benefits mentioned above.

Another way to install UV is in an “upper-air” configuration. Specially designed fixtures mounted on the

wall create an irradiated zone above the occupant and disinfect air in the space as air circulates

naturally, mechanically or by means of the HVAC system. This sort of system has been approved for use

in control of tuberculosis by CDC for nearly 20 years and there is a NIOSH guideline on how to design

them.

Finally, mobile UV systems are frequently used for terminal cleaning and surface disinfection in

healthcare and other spaces. Systems such as these are typically used in unoccupied spaces due to

concerns of occupant exposure. All three system types may be relevant, depending on the building type

and individual spaces within the building.

The design and sizing of effective ultraviolet disinfection systems can be a complex process because of

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 air handling unit and ductwork configuration and

reflections from surfaces that can help achieve higher irradiance levels. Upper-air systems require

adequate air mixing to work properly while paying close attention to reflective surfaces that could result

in room occupants being overexposed to the UV energy. Reputable manufacturers and system

designers can assist by doing the necessary calculations and designing systems specific to individual

spaces.

For basic information on ultraviolet disinfection, we suggest that you consult some of the references

listed on the ASHRAE COVID-19 resources page (ashrae.org/covid19).

Q7: Do I need to seal my filter in place after I put it in the filter

slot?

A: A filter needs to be installed so that all the air goes through the filter. If this doesn’t happen, the air

that goes around the filter will not get cleaned. This can be the same as using a lower MERV filter.

Sealing filters to prevent bypass can be accomplished in many ways. For a residential filter in a slot that

is a bit loose, a simple piece of tape can be used hold it in place and allow the airflow to push the filter

forward to engage the flat surface in front of it. Commercial building filters with rigid frames may be

clamped in place. Gasketing can be added to most filters, either to the installation location or to the

frame of the filter. HEPA filters need much better sealing as even a tiny 0.3% leak will give 10x as much

penetration.