FAQs

Critical questions to address to reduce spread and infection. Click here to view videos.

Yes, the COVID-19 illness is caused by the Coronavirus SARS-CoV-2 which according to the Centers for Disease Control (CDC) is thought be transmitted via respiratory droplets from infected persons via the air. Although the majority of these droplets are heavy enough to drop out of the air within 6 feet of the infected person, there is likely a percentage of droplets small enough to remain airborne and circulate through the HVAC system. A properly selected HVAC filter becomes the determining factor on if these pathogens are recirculated back into the air flow of your system.

Yes, several studies have shown that an HVAC system with a properly selected filter can reduce the spread of bacteria and viruses. One of these studies performed in 2013 modeled the spread of the influenza virus in an office environment and showed that HVAC systems with the appropriate filtration can offer a 7X reduction in spread of the virus.1, Although there has not been any significant research completed specifically on SARS-CoV-2 yet, there are studies on similar pathogens which have shown that HVAC systems can impact the spread of virus particles.

1“HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs”

Yes, with the proper type of filter selected. Virus particles, or virons, can be as small as 0.02 to 0.4 micrometers. This is 150 times smaller than the diameter of a typical human hair. Most typical HVAC air filters are not designed to remove a significant number of these extremely small particles, however there are filters such as HEPA and ULPA filters which remove close to 100% (99.97%+) of these particles when properly installed.

However, most virus particles including the ones that cause COVID-19 rarely, if ever, travel on their own. They are almost always attached to much larger moisture droplets which are expelled from the infected individual. The capture rate of these larger particles is much greater and therefore it can be stated that air filters with higher MERV ratings (13+)2 are effective tools to capture the airborne particulate that carries airborne virus.

2 ASHRAE Position Document on Infectious Aerosols – April 14, 2020

There are several new technologies to consider! Since HVAC systems can impact the spread of virus particulate in several ways, understanding those factors is an important part of infection control protocols. First off, and most importantly, you will want to consider which type and performance rating of air filters you have installed (it is recommended to have at least MERV 13 filters installed)3. This choice will impact both the number and size of particles removed, along with the volume of airflow produced by the unit. Premium filtration formats like V-Bank Mini-Pleats, utilize modern filtration components which allow for higher removal performance without sacrificing airflow.

There are also technologies related to the cleanliness of an HVAC system that should be considered. These include both cleaning chemicals which can disinfect the HVAC system components, along with other devices such as UV-C germicidal lighting which can inhibit the growth of microbes within the unit.

In addition to the particulate air filters, there are technologies such as Filtration Group’s Purafil gas phase filtration modules which utilize chemical and molecular filter media to remove gas and chemical contaminants which are not captured by traditional particulate filtration. This works in conjunction with your standard HVAC system as a second line of defense.

3 ASHRAE Position Document on Infectious Aerosols – April 14, 2020

High MERV rated filters and those that utilize oxidizing media. To reduce the spread of the virus the viron particles need to either be captured or killed. Most HVAC filters will focus on capturing the virus. In this case, the higher the MERV rating, the greater the capture rate of potentially infectious particles. Although most HVAC systems were not engineered for optimum infection control, increasing the efficiency level above MERV 13 is recommended by the US National body who sets guidelines for the HVAC industry.4,5

Filters which utilize chemical filtration to inactivate the virus such as those with Purafil’s Puraward media, may further reduce the spread of the virus by killing the pathogens in addition to just capturing them.

4 ASHRAE Position Document on Infectious Aerosols – April 14, 2020

5 “HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs

Before deciding on any changeout cycle for an air filter, it is important to understand why you are changing the air filter. Changing an air filter accomplishes several goals. First off it removes the contaminants that have been captured from the HVAC system. When changed properly, this removal should ensure that those particulates will not re-enter the climate-controlled space. In addition, by replacing the used filter with a clean air filter, the resistance to airflow will be reduced which will allow the HVAC system to continue to operate properly. If air filters are not changed on an appropriate schedule, the particles captured will “clog” the filter to the point in which the appropriate amount of airflow will not longer be able to move through the HVAC system. Changing a filter more often does not necessarily mean that it will provide better protection, it will however accomplish the goals listed above.

Air hygiene can be impacted through Filtration, Dilution, and Source Control measures.

Filtration: Clean air is the key to superior indoor air hygiene. The contaminants left after source control and dilution efforts have been implemented (see below) can often be mitigated through proper filtration. This includes both particulates which are removed through various filtration media formats, and gaseous contaminants which can be removed through adsorption and chemisorption using materials like activated carbon and potassium permanganate.

Standalone filtration equipment such as air scrubbers can be deployed in areas where the HVAC system is not set up to provide enough clean air to meet the needs of the occupants. Occupants often keep these stand-alone units close to places where they will be over extended periods of the day (i.e. an office desk or a near a bed).

Source Control: Are you doing everything that you can to remove the source of potential contaminates from your building. This can include limiting or screening visitors that may be contagious. Using materials which are hypoallergenic in furnishings and building materials. Restricting activities such as smoking, cooking, and others which produce contaminants.6

Dilution: Are you utilizing outside air to ventilate your space? When appropriate amounts of fresh air are not introduced into a building, unwanted levels of CO2 and other contaminants tend to build up within the occupied space. Adding fresh air through either natural ventilation (open windows or doors), additional fresh air brought in through the central HVAC system,, or a Dedicated Outdoor Air System will help improve the indoor air hygiene in most applications

6 https://ehs.colorado.edu/resources/ventilation-and-air-quality/

There are certainly actions you can, and should, take. The first thing to ensure is that you are following your local government health authority’s recommendation (or the US CDC as an example). These include regular hygiene habits like washing your hands regularly for 20 seconds with soap and warm water, utilizing an alcohol based hand sanitizer with at least 60% alcohol after touching surfaces, avoid touching your face, and ensure social distancing measure are in place (at least 6 feet from others).

Beyond these personal hygiene matters, the indoor air hygiene as well as regular cleaning of hard surfaces is highly recommended. The COVID-19 virus is believed to stay airborne for hours after someone with the virus coughs or sneezes. The steps addressed in these FAQ’s outline several recommendations for improving HVAC filtration and augmenting with localized air scrubbers.

Yes, The American Society of Heating Refrigeration and Air Conditioning Engineers have recommended increasing outdoor air exchange rates, increasing filtration levels to MERV 13 or higher where possible, and reducing or eliminating recirculated air as much as possible.7 These recommendations are based on the physics of how an HVAC system operates paired with research which has determined the methods of how virus spreads among humans.

In addition, studies have been performed which show the impact of an HVAC system on the spread of various virus. These have shown that the air movement created by the HVAC system can impact the spread of the virus in different ways, and that the filtration levels used in an HVAC system can reduce the risk of infection in a conditioned space.8

7 ASHRAE Position Document on Infectious Aerosols – April 14, 2020

8“HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs”

There are several metrics which can indicate the level of indoor air quality. Some of those are commonly measured such as temperature and humidity. Keeping these basic metrics in a comfortable range are commonly the main goals of an HVAC system. In addition, measurements such as CO2, CO, and VOC levels can be measured using sensor technology which is readily available and may already be implemented in your building controls system. Particulate levels such as PM 2.5 can be monitored both through periodic sampling or in real time using laser particle counting equipment. These particulate levels should be lower than what is found in the outside environment. If the levels of these particulates are higher in the occupied space, then an investigation into the source of the particulate and the performance of the filtration system is warranted.

Filtration: Clean air is the key to superior indoor air hygiene. The contaminants left after source control and dilution efforts have been implemented (see below) can often be mitigated through proper filtration. This includes both particulates which are removed through various filtration media formats, and gaseous contaminants which can be removed through adsorption and chemisorption using materials like activated carbon and potassium permanganate.

Standalone filtration equipment such as air scrubbers can be deployed in areas where the HVAC system is not set up to provide enough clean air to meet the needs of the occupants. Occupants often keep these stand-alone units close to places where they will be over extended periods of the day (i.e. an office desk or a near a bed).

Source Control: Are you doing everything that you can to remove the source of potential contaminates from your building. This can include limiting or screening visitors that may be contagious. Using materials which are hypoallergenic in furnishings and building materials. Restricting activities such as smoking, cooking, and others which produce contaminants.6

Dilution: Are you utilizing outside air to ventilate your space? When appropriate amounts of fresh air are not introduced into a building, unwanted levels of CO2 and other contaminants tend to build up within the occupied space. Adding fresh air through either natural ventilation (open windows or doors), additional fresh air brought in through the central HVAC system, or a Dedicated Outdoor Air System will help improve the indoor air hygiene in most applications

6 https://ehs.colorado.edu/resources/ventilation-and-air-quality/

NO, there is no set formula as the expense per occupant will vary based on the density of the occupied space, the environmental conditions both inside and outside of the building, and the design of the mechanical systems. Typically, the HVAC system is one of the most expensive systems to operate in a commercial building and therefore the costs of providing the appropriate indoor air hygiene may be significant, but likely nowhere near the costs of not doing so.

Relative humidity, or RH, is the ratio of the amount of water in the air at a certain temperature compared to the amount of water in saturated air at that same temperature. When saturated air contacts a cool surface, condensation occurs. Examples of condensation are dew, sweat (water appearing on cold surfaces), or fog.

The higher the relative humidity, the closer the water vapor in the air is to condensing on surfaces.

The lower the relative humidity, the drier the air.

As human beings, we can sense higher humidity as mugginess or dampness. Higher RH levels limit our body’s ability to cool itself by sweating and having the sweat evaporate – which absorbs heat from our body.

Well, if it is too wet, then your house becomes very attractive to critters like roaches, termites, and other pests.

Mold also requires high RH levels to grow.

If it is too dry, then family members complain of sore throats and dry skin. Also, low relative humidity potentially improves the survival rate of pathogens, like viruses and spores.

The Department of Energy, ASHRAE, and building scientists all recommend aiming for the Goldilocks zone –

not too dry and not too wet. This means staying between 40% and 60% relative humidity all year round.

To stay at 40% or above in climates with cold winters, supplemental humidification may be required based upon the construction of your home.

For homes in green grass climates, staying below 60% throughout the year will likely require supplemental dehumidification.

Too bad we can’t all live in San Diego, where the weather (including the relative humidity) is in the Goldilocks zone year-round.

Here is my rule of thumb: If there is enough rain and moisture to support grass growing outside, then there is enough moisture to support mold growing inside your home. They key to prevent mold grow – and to keep you home from looking attractive to pests and insects, is to always keep the relative humidity of your home below 60%.

That sounds easy enough. Here is how you do it: Your air conditioner, when it is set up properly, removes a tremendous amount of moisture from your indoor air. When there is a heat load, meaning the sun is shining, your AC system should be able to maintain an indoor RH of less than 60%.

The tough part is how to maintain 60% or less when there is no heat load to keep your air conditioning running. This occurs most nights as well as most overcast or rainy days. If you live in a green grass climate, supplemental dehumidification has proven to be the only solution that can always maintain the desired setting of 60% or less in your home.

Depending upon where you live and the construction of your home, you may suffer from exceedingly low RH in your home during the winter. Older, leakier homes built in cold climates often use supplemental humidification to increase the RH level inside the home during winter.

Supplemental humidification addresses the symptom – low RH – but not the cause. The real cause is that too much outside air is coming into the home due to leaky windows, fireplaces operating, and/or inadequate insulation. It is better to address the cause to solve the low RH problem than the symptom. Supplemental humidification can often lead to condensation problems in the coldest spots in the home.

In general, for a 2,600 square foot house in a green grass climate, you would need approximately 100 pints of capacity at AHAM-rated conditions (which is 80°F and 60%RH). Some dehumidification is better than none, and too much dehumidification doesn’t hurt – it just remains unused.

The classic answer is “It Depends.” It depends on where you live, how you house is constructed, how much fresh air ventilation is brought inside. It depends on how much you cook and shower. It depends upon how many people live in your house. All of the variables lead us to use rules of thumb.

Well, if you just have a fiberglass filter, you shouldn’t just replace it. You should upgrade it to a pleated filter.

Fiberglass filters should be replaced when dirty, which can be as often as every month. Better quality pleated filters can last longer, usually 3 to 6 months. High-end 4” to 6” pleated filters can last 6 to 12 months.

In my house, I write the date on each filter when installed using a Sharpie pen. My furnace has an upgrade Lennox 4” MERV-14 filter which I change annually on my birthday. My whole-home ventilating dehumidifier (Ultra-Aire 120V) has a 2” MERV 13 filter and a 4” MERV 14 filter.

HEPA stands for High Efficiency Particle Arrestment. HEPA-rated filters are required to remove 99.97% of particles with a diameter of 0.3 microns. The particle size of 0.3 microns was chosen because it is right around that size of particle that is hardest for filters to capture. Filter efficiency usually increases with particles smaller and larger than 0.3 microns.

Studies have shown that localized HEPA filter machines are the best option for purifying the air we breathe. A small HEPA machine by your bed, desk, and favorite location in your den will provide you with the cleanest air possible throughout most of the day. The machines are quiet and unobtrusive.

Outside air, once filtered properly, can be considered “fresh air”. It has the oxygen, carbon dioxide, and volatile organic compound levels (VOCs) found in the atmosphere. Inside air of inhabited spaces often has less oxygen, more carbon dioxide, and higher VOCs than outside air.

Outside air is used to flush the “stale” inside air to renew and freshen the indoor air. The goal is to get the inside air to approximate the same makeup as outside air but at a comfortable temperature and relative humidity.

Just flushing inside air to the outside – say by opening the window – does renew the air. However, proper ventilation air is filtered to remove particulate pollution and conditioned to remove excess humidity and possibly heat.

Almost all homes have bathroom fans and range hoods that exhaust moisture and VOCs (including odors) directly at the source. Most clothes dryers also exhaust inside air out of the home. Exhaust ventilation is usually the least expensive way to ventilate a home. You can have some control on your exhaust ventilation rate by running the bathroom fans on a timer.

Many homes have an outside air duct that connects directly to their HVAC furnace. The outside air duct usually has a damper that can be adjusted to increase/decrease the amount of outside air brought into the home. The HVAC blower, when on, draws outside into the furnace system positively adding outside air into your home.

Some homes used energy-efficient heat and energy recovery ventilators to control the amount of outside air brought into the home balanced with the amount of air exhausted from the home.

It is also important to know that in cold climates, home ventilate naturally due to the stack effect – air leaks out the roof and sneaks in at the ground level. In really hot climates, homes ventilate in the other directly due to the reverse stack effect.

This issue vexes the experts. There are ongoing arguments about how much ventilation is enough. A standard created by ASHRAE, the American Society of Heating, Refrigeration, and Air Conditioning Engineers, states that there should be a certain amount of outside air ventilation per square foot of your home plus a certain amount of outside air ventilation per person.

The exact amounts have changed (almost doubled) in the past 10 years; however, the debates continue on, and there is no strong consensus yet.

For an average 2,200 square foot home with 4 occupants and a dog, the outside air ventilation rate is about 100 cubic feet per minute (CFM). It used to be about 60 CFM. Please realize that there are lots of caveats and exceptions.

My own personal rule of thumb is to monitor the carbon dioxide (CO2) levels in my home. If I notice the CO2 concentration in my living room is often above 1200 parts per million (PPM), then I increase my ventilation. If it stays below 600 PPM, I may decrease my ventilation. Outside air is about 400 PPM.

Note that more outside air brought into your home, properly filtered and conditioned (temperature and RH are comfortable), the healthier your indoor air. However, you do reach diminishing returns and the cost to condition the air can become exorbitant.

Watch FAQ Videos

What is Relative Humidity?
Optimum Relative Humidity in Home
How Much HD Capacity Do I Need?
How To Stay Below 60%
Low Relative Humidity in the Winter

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