Author - aaqadn

What is a HEPA filter?

HEPA stands for High Efficiency Particulate Air, and a true HEPA filter is widely regarded as the ultimate filter.

In World War II the Atomic Energy Commission needed a filter to protect researches from radioactive dust particles that might present a health hazard to them. The HEPA filter was born. It traps particles as tiny as .3 microns with an efficiency rating of 99.97%.

To give you an idea of the size of a micron, it takes 25,400 microns to equal 1 inch (2.54 cm). Conversely, 4/100,000ths of an inch equals one micron. In metric terms, a single micron is 1 millionth of a meter.

A particle of 10 microns is invisible to the naked eye. Pollen ranges between 5-100 microns. Human hair between 70-100 microns.

The rating for a HEPA filter is based on capturing nearly all microns .3 in size, verses .1 or even .001 because .3 microns are the hardest size to trap and the optimal size for passing into the human respiratory system. Therefore the .3 micron efficiency rating sets the highest standard.

A HEPA filter is so efficient that for every 10,000 particles that enter the filter within its filtering range, only 3 particles will get through. Surprisingly, HEPA filters become even more efficient the longer they are in use.

For this reason HEPA filters are used in environments that rely on high clean-air standards. Surgical facilities, tuberculosis wards, NASA clean rooms, and laboratories are a few examples of environments that utilize HEPA filters. HEPA filters are particularly useful for allergy sufferers.

Many imitations of HEPA filters pervade the marketplace, usually called HEPA -type filters. They are less expensive than true HEPA filters, rated lower, and do an inferior job of filtering the air. If your health requires the specific benefits and efficiency of a HEPA filter, check the packaging to make sure the filter is rated at an efficiency of 99.97% for microns .3 in size. If it’s a true HEPA filter, it will have this rating clearly marked.

Difference Between Air Cleaners And Air Purifiers

What is the difference between an air cleaner and an air purifier? . Air cleaners filter the air (HEPA air cleaners for instance) while air purifiers sanitize the air by emitting negative ions, ozone, utilizing heat (Airfree air purifiers) or with UV or UVC lamps.

Air cleaner strengths: Filters the air and collects dust, especially the larger dust (mostly dead skin) and pet dander.

Air cleaner cons: Most air cleaners feature a fan that on high can make a lot of noise pollution. Expensive replacement filters, expensive to purchase. Most do not address the negative ion issue.

Air purifier strengths: Kills airborne pathogens that cause allergies and sickness. Many air purifiers offer silent operation. Less expensive to operate than air cleaners.

Air purifier weaknesses: Little or no dust removal capabilities. Without good air circulation, can only purify the air in a portion of one room.

Air cleaners are often judged by a CADR (clean air delivery rate) number or by the number of air exchanges per hour. CADR however only tests for larger particles (pollen, dust, over .3 micron in size) and not for viruses, mold, mildew, bacteria, VOC’s, chemicals, or cigarette smoke….in other words, they test for things that make us sneeze, not things that make us sick or even kill us.

Air Purifiers on the other hand have a different mission. Their job is to purify or sanitize the air of things that cause odors or make us sick. Some use heat (Airfree), others UV or negative ions, yet others use ozone. Ozone generators are often sold as air purifiers which has the EPA’s panties in a wad, but for good reason. Most are sold by MLM direct marketing to little old ladies who buy the sales pitch …”it’s like a rain forest in your home” they say. However, ozone, while it’s not a toxic gas or smog, it is a very strong oxidizer and harsh on the lungs in large quantities. A little bit of ozone, that’s fine, monther nature makes ozone in clean environments. But in those clean environments there are not any nitrous oxides, a by product of ozone generators (in high concentrations) and there is not a lot of ozone. Ozone generators are best used for shock treatments in empty rooms, that’s it. Don’t let anyone tell you otherwise. What if you smoke? Well then I guess it’s better to breath ozone than second hand smoke. But that’s the exception to the rule. However, you’d be better served with UV technology. The Air Oasis Xtreme for instance can clean the air as well as an ozone generator without the high ozone levels. The Ionic Zone PCO Sanitizer can also remove VOC’s and odors from the air.

How do UV air purifiers purify the air? They utilizes hydrol radicals and super oxide ions or high intensity UV lamps (UVC induct units) to fry the DNA of microorganisms passing by. Often referred to as “mother natures broom”, hydrol radicals are very short lived, but very effective at purifying the air. UV and photocatalytic oxidation is gaining ground as the weapon of choice in the war against indoor air pollution. the old standard, negative ions (ionizers) on the other hand is losing ground mostly due to the negative press from the Ionic Breeze. However, the Ionic Breeze is not an ionizer as it produces a positive charge, not negative to attach particles to their negatively charged plates. Regardless, negative ions are stil an important component of indoor air and worthy of your consideration.

The best air purification devices in the world utilize air cleaning and air purifying technologies together to address a myriad of indoor air quality issues. One of our favorites is the NQ Clarifier. It utilizes 2 UV lamps along with 15 lbs of carbon and 80 sq ft feet of medical grade HEPA. It runs on under 100 watts and has a dial speed control which allows for ultra silent operation. We also love the Austin Air Allergy Machine and the Airpura I-600 for dust and general odor abatement. While those units are air cleaners, they can also purify the air with carbon and UV germicidal lamps.

We hope this article helped you understand the difference between air cleaners and air purifiers and the different technologies available today. If you still have any questions, give us a call and we’ll be happy to assist you with the perfect solution to your particular indoor air quality concern.

What Is An Air Ionizer?

An air ionizer is a relatively new device that is intended to purify the air. As its name implies, instead of using fans and filters, an air ionizer creates ions which remove microscopic particles from the air. Rooms are healthier for those suffering from asthma, allergies, impaired immunity, or respiratory ailments, without as many allergens circulating in the lungs.

Air ionizers rely on the chemical properties of particles. An ion is just a particle that is charged, either negatively or positively. This commercial device, a tabletop or stand alone unit, creates negative ions using electricity. The ions flood the room and seek out positively charged particles, such as dust, dander, bacteria, pollen, mold, smoke, chemical vapors, and many other allergens. Once bonded (remember, opposites attract), the particles are too heavy to float around where they can easily be inhaled. As a result, the harmful airborne particles become larger pieces of dirt on the ground where they can be cleaned by normal means.

Most particles near the ground, are positively charged. Natural phenomenon, like lightening or waterfalls, generate negative ions and ozone. This is the “fresh” smell you might encounter in an electrical storm or white rapids. Ozone is a naturally occurring gas related to oxygen. Our buildings’ insulation interferes with atmospheric air circulation, so an air ionizer seeks to compensate for this disequilibrium.

Often outdoor pollutants get a lot of attention. We do not want car exhaust or factory emissions to cause health hazards. But indoor air pollution remains a serious problem. Dust and mold collects inside heating and air conditioning ducts. Higher humidity inside allows bacteria to thrive. An air ionizer addresses these home and office sources of pollution and odor. The ozone that is created when negative ions are generated, battles pollution by breaking it down into smaller, harmless components. Ozone makes it more difficult for germs to grow, and deodorizes as well.

Ionizers conserve power and run silently, unlike fan-driven air purifiers. Another advantage over other purifiers is that no parts need regular replacement, because there are no physical filters. Even though most air ionizers don’t have a motorized fan, the ionization creates a faint breeze that helps to distribute the ions throughout a room. Some are also outfitted with screens or prongs that catch the particles of dust as they fall to the ground.

What Is Activated Carbon?

The primary raw material used for activated carbon is any organic material with a high carbon content (coal, wood, peat, coconut shells). Granular activated carbon is most commonly produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing to the correct size. The carbon-based material is converted to activated carbon by thermal decomposition in a furnace using a controlled atmosphere and heat.

The resultant product has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place. The walls of the pores provide the surface layer molecules essential for adsorption. Amazingly, one pound of carbon (a quart container) provides a surface area equivalent to six football fields.



Physical adsorption is the primary means by which activated carbon works to remove contaminants from water. Carbon’s highly porous nature provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to. The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores. The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are.

With this understanding of how the adsorption process works, we must then understand why it works, or why water contaminants become adsorbates.

Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution. Those compounds that are more adsorbable onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and a neutral or non-polar chemical nature. It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate.

Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions. This reaction is important to POU treatment because this conversion of chlorine to chloride is the basis for the removal of some common objectionable tastes and odors from drinking water.

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What is PCO?

Photo Catalytic Oxidation incorporates the usage of photons, packets of energy that make up light or light energy, combined with a photocatalyst, titanium dioxide, to eliminate airborne contaminates such as bacteria, viruses, mold, spores, fungi, allergens and gases.

Simultaneously, Volatile Organic Compounds, including solvents, alcohols, dyes and fuel oils are broken down into basic elements, water and carbon dioxide. The titanium dioxide photocatalyst becomes highly reactive when exposed to precisely controlled wave lengths of ultraviolet light (medical grade UVC). When organic pollutants become present, the photocatalyst attacks the chemical bonds of the bioaerosol pollutants, therefore converting the toxic compounds into benign constituent.

Ultraviolet (UV) Light

AAQ Malaysia offers commercial and residential UV light air purifiers. The Airsopure product line users a specific 253.7 nm wavelength UV-C lamp to ensure a maximal germicidal effect without generating any negative by-products like Ozone.


UV-A is the least harmful and most commonly found type of UV light, because it has the least energy. UV-A light is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light – thus appearing to glow in the dark. Most phototherapy and tanning booths use UV-A lamps.

UV-B is typically the most destructive from of UV light, because it has enough energy to damage biologic tissues, yet not quite enough to be completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since most of the extraterrestrial UV-B light is blocked by the atmosphere, a small change in the ozone layer could dramatically increase the danger of skin cancer.

Short wavelength UV-C is almost completely absorbed in air within a few hundred meters. When UV-C photons collide with oxygen atoms, the energy exchange causes the formation of ozone. UV-C is almost never observed in nature, since it is absorbed so quickly. Germicidal UV-C lamps are often used to purify air and water, because of their ability to kill bacteria.

Guidance for Building Operations During the COVID-19 Pandemic

By Lawrence J. Schoen, P.E., Fellow/Life Member ASHRAE

From ASHRAE Journal Newsletter, March 24, 2020

The HVAC systems in most non-medical buildings play only a small role in infectious disease transmission, including COVID-19.1 Knowledge is emerging about COVID-19, the virus that causes it (SARS-CoV-2), and how the disease spreads. Reasonable, but not certain, inferences about spread can be drawn from the SARS outbreak in 2003 (a virus genetically similar to SARS-CoV-2) and, to a lesser extent, from transmission of other viruses. Preliminary research has been recently released, due to the urgent need for information, but it is likely to take years to reach scientific consensus.

Even in the face of incomplete knowledge, it is critically important for all of us, especially those of us in positions of authority and influence, to exercise our collective responsibility to communicate and reinforce how personal choices about social distancing and hygiene affect the spread of this disease and its impact not just on ourselves, but on our societal systems and economy. The consequences of overwhelming the capacity of our health-care systems are enormous and potentially tragic. The sooner we “flatten the curve,”2 the sooner we can return to safer and normal economic and personal lives.

According to the WHO (World Health Organization), “The COVID-19 virus spreads primarily through droplets of saliva or discharge from the nose when an infected person coughs or sneezes….” Talking and breathing can also release droplets and particles.3 Droplets generally fall to the ground or other surfaces in about 1 m (3 ft), while particles (aka aerosols), behave more like a gas and can travel through the air for longer distances, where they can transmit to people and also settle on surfaces. The virus can be picked up by hands that touch contaminated surfaces (called fomite transmission) or be re-entrained into the air when disturbed on surfaces.

SARS infected people over long distances in 2003,4 SARS-CoV-2 has been detected as an aerosol in hospitals,5 and there is evidence that at least some strains of it remain suspended and infectious for 3 hours, suggesting the possibility of aerosol transmission. However, other mechanisms of virus dissemination are likely to be more significant, namely,

  • direct person to person contact
  • indirect contact through inanimate objects like doorknobs
  • through the hands to mucous membranes such as those in the nose, mouth and eyes
  • droplets and possibly particles spread between people in close proximity

For this reason, basic principles of social distancing (1 to 2 m or 3 to 6.5 ft), surface cleaning and disinfection, handwashing and other strategies of good hygiene are far more important than anything related to the HVAC system.7 In the middle-Atlantic region of the United States where I work, malls, museums, theaters, gyms and other places where groups of people gather are closed and there are “stay at home”8 orders. This is a “game” of chance, and the fewer individuals who come in close contact with each other, the lower the probability for spread of the disease. Since symptoms do not become apparent for days or weeks, each of us must behave as though we are infected.

Other public buildings, considered essential to varying degrees, remain open. These include food, hardware and drug stores, and of course, hospital and health-care facilities (which are beyond the scope of this article). Anecdotally, some universities are allowing some or all faculty, staff and graduate students to conduct essential research and online classes. Banks and other service organizations are open to staff and are receiving customers by appointment only, and private and government workplaces are open with work at home for some or all encouraged or mandated.

For those buildings that remain open, in addition to the policies described above, non-HVAC actions include:

  • Increase disinfection of frequently touched surfaces.9
  • Install more hand sanitation dispensers, assuming they can be procured.
  • Supervise or shut down food preparation and warming areas, including the office pantry and coffee station.
  • Close or post warning signs at water fountains in favor of bottle filling stations and sinks, or even better, encourage employees to bring their water from home.

Once the basics above are covered, a few actions related to HVAC systems are suggested, in case some spread of the virus can be affected:

  • Increase outdoor air ventilation (use caution in highly polluted areas); with a lower population in the building, this increases the effective dilution ventilation per person.
    • Disable demand-controlled ventilation (DCV).
    • Further open minimum outdoor air dampers, as high as 100%, thus eliminating recirculation (in the mild weather season, this need not affect thermal comfort or humidity, but clearly becomes more difficult in extreme weather).
  • Improve central10 air filtration to the MERV-1311 or the highest compatible with the filter rack, and seal edges of the filter12 to limit bypass.
  • Keep systems running longer hours, if possible 24/7, to enhance the two actions above.
  • Consider portable room air cleaners with HEPA filters.
  • Consider UVGI (ultraviolet germicidal irradiation), protecting occupants from radiation,13 particularly in high-risk spaces such as waiting rooms, prisons and shelters.

Construction sites present unique challenges. Much, but not all, construction work has the recommended social distancing; much, but not all, is outdoors or in partially enclosed and therefore well-ventilated buildings; and many, but not all, workers already use personal protective equipment such as masks14 and gloves. Governments in some locations have mandated closure of construction sites, while in others work proceeds.15 Engineers who perform field observations, commissioning or special inspections must consider what work can be postponed, performed remotely, or conducted using photographic documentation, and what personal precautions to take when site visitation is unavoidable.

If you, the reader, are called upon to advise building operators, please use the above general guidance, and be sure to combine it with knowledge of the specific HVAC system type in a building and the purpose and use of the facility. Like all hazards, risk can be reduced but not eliminated, so be sure to communicate the limitations of the HVAC system and our current state of knowledge about the virus and its spread.

We all have a role to play to control the spread of this disease. HVAC is part of it and even more significant are social distancing, hygiene and the influence we can have on personal behavior.

Thanks to William P. Bahnfleth, Ph.D., P.E., Presidential Member/Fellow ASHRAE, Lew Harriman, Fellow ASHRAE, Yuguo Li, Ph.D., Fellow ASHRAE, Andrew K. Persily, Ph.D., Fellow ASHRAE, and Pawel Wargocki, Ph.D., Member ASHRAE for their review of preliminary drafts of this article. Any errors that remain are the author’s alone.

Lawrence J. Schoen, P.E., is president and principal engineer at Schoen Engineering, Inc. in Columbia, Md. He was chair of the committee that wrote the most recent version of the “ASHRAE Position Document on Airborne Infectious Diseases.” The position document is undergoing revision.

ASHRAE Issue Statement On Relationship Between COVID-19 and HVAC in Buildings

ATLANTA (April 20, 2020) – ASHRAE has published two statements to define guidance on managing the spread of SARSCoV2, the virus that causes COVID-19 disease (Coronavirus) with respect to the operation and maintenance of heating, ventilating and air-conditioning systems in buildings.

“In light of the current global pandemic, it’s critically important that ASHRAE responds with guidance on mitigating the transmission of the virus, as well as ventilation and filtration recommendations,” said 2019-20 ASHRAE President Darryl K. Boyce, P.Eng. “ASHRAE has a significant role to play in ensuring safe and healthy building environments and these statements offer the expert strategies needed at this time.”

ASHRAE developed the following statements in response to widening false statements surrounding HVAC systems. ASHRAE officially opposes the advice not to run residential or commercial HVAC systems and asserts that keeping air conditioners on during this time can help control the spread of the virus. The official statements are below.

ASHRAE’s statement on airborne transmission of SARS-CoV-2/COVID-19

Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.

ASHRAE’s statement on operation of heating, ventilating, and air-conditioning systems to reduce SARS-CoV-2/COVID-19 transmission

Ventilation and filtration provided by heating, ventilating, and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air. Unconditioned spaces can cause thermal stress to people that may be directly life threatening and that may also lower resistance to infection. In general, disabling of heating, ventilating, and air- conditioning systems is not a recommended measure to reduce the transmission of the virus.

HVAC filters, along with other strategies, help to reduce virus transmission while removing other air contaminants that may have health effects.

ASHRAE’s Environmental Health Committee also developed an Emerging Issues Brief to support the two above statements:

There is great concern about the real possibility of transmission through the air of various pathogens, especially SARS-CoV-2, among staff and administration in healthcare facilities, office workers, retail workers and patrons, manufacturing workers, and residents in private and public facilities and the general public in outdoor settings and in public transportation.

ASHRAE has created the Epidemic Task Force, comprised of leading experts to address the relationship between the spread of disease and HVAC in buildings during of the current pandemic and future epidemics. The ASHRAE Environmental Health Committee’s Position Document Committee also updated a Position Document on Infectious Aerosols.

“ASHRAE, working with its industry partners, is uniquely qualified to provide guidance on the design, operation, and maintenance of heating, ventilation, and air-conditioning systems to the COVID-19 pandemic as well as to prepare for future epidemics,” said ASHRAE Epidemic Task Force chair, ASHRAE Environmental Health Committee voting member and 2013-14 ASHRAE Presidential Member Bill Bahnfleth.

Please  visit  the  newly  updated  ASHRAE’s  COVID-19 Resources  webpage at for additional details. The page includes frequently asked questions and the latest information on the ETF’s guidance for healthcare facilities, residential buildings and other issues related to the COVID-19 pandemic.


Founded in 1894, ASHRAE is a global professional society committed to serve humanity by advancing the arts and sciences of heating ventilation, air conditioning, refrigeration and their allied fields.

As an industry leader in research, standards writingpublishingcertification and continuing education, ASHRAE and its members are dedicated to promoting a healthy and sustainable built environment for all, through strategic partnerships with organizations in the HVAC&R community and across related industries.

ASHRAE is celebrating 125 years of shaping the built environment. Become a member of ASHRAE by visiting For more information and to stay up-to-date on ASHRAE, visit and connect on LinkedInFacebookTwitter and YouTube.


ASHRAE (2020).  ASHRAE Issues Statements on Relationship Between COVID-19 and HVAC in Buildings

ASHRAE Guidance for COVID-19 Building Reopening

ASHRAE Offers COVID-19 Building Readiness/Reopening Guidance


ATLANTA (May 7, 2020) – The ASHRAE Epidemic Task Force has developed guidance on mitigating potential health risks during the reopening of buildings closed during the COVID-19 pandemic.

“We have reached a time where planning for a safe return to normal activities has become a priority,” said 2019-20 ASHRAE President Darryl K. Boyce, P.Eng. “Safe operation of HVAC and building water management systems are critical components of building readiness and reopening, and ASHRAE’s resources provide a framework for developing plans in a variety of building types.”

ASHRAE’s recommendations for reopening buildings are outlined in the frequently asked questions section of its COVID-19 Resources webpage. Recommendations for building readiness and reopening include the following:

  • Create a strategic plan prior to opening a building. The plan should include measures to make occupants feel safer, ensuring supply chain for critical items such as filters and communication plans for building support and safety measures for occupants.
  • If the building opening takes place when Personal Protective Equipment (PPE) requirements are still in place, ASHRAE’s Occupancy Guides can be referenced to deal with functioning buildings during the epidemic.
  • Review HVAC programming to provide flushing two hours before and post occupancies. This includes operating the exhaust fans as well as opening the outside air dampers. For buildings without the capacity to treat large quantities of outside air and when outside air conditions are moderate, open all windows for a minimum of two hours before reoccupation.
  • Ensure that custodial scope includes proper cleaning procedures built from EPA and CDC guidance on approved products and methods:
    • Disinfect high-touch areas of HVAC and other building service systems (e.g. on/off switches, thermostats)
    • Disinfect the interior of refrigerated devices, e.g. refrigerators, where the virus can potentially survive for long periods of time.
  • Run the system on minimum outside air when unoccupied.
  • Garage exhaust, if any, should run two hours before occupancy.


“Key elements of a strategy to limit the spread of the COVID-19 virus are to perform needed heating, ventilating and air conditioning (HVAC) system maintenance, including filter changes, and to run HVAC equipment, prior to re-occupancy,” said ASHRAE Epidemic Task Force chair, ASHRAE Environmental Health Committee voting member and 2013-14 ASHRAE Presidential Member Bill Bahnfleth.

A decrease in water usage in buildings closed or with limited access during the pandemic can increase the risk of bacteria growth in building plumbing and associated equipment. Facility managers and building owners can help mitigate the risk of waterborne pathogens, such as Legionella bacteria, the cause of Legionnaire’s disease, by developing a water management plan. ANSI/ASHRAE Standard 188-2018, Legionellosis: Risk Management for Building Water Systems establishes minimum legionellosis risk management requirements for building water systems.

“ASHRAE’s building readiness guidance empowers building owners with resources and practical guidance for safer operation of HVAC systems as we cautiously transition into a post-COVID-19 world,” said ASHRAE Epidemic Task Force chair of Building Readiness guidance Wade Conlan.

The task force also recommends guidance released in the newly updated ASHRAE Position Document “Infectious Aerosols,” as well as the Emerging Issues Brief.

For extensive resources and strategies on safe building reopening, visit org/COVID19.


Founded in 1894, ASHRAE is a global professional society committed to serve humanity by advancing the arts and sciences of heating ventilation, air conditioning, refrigeration and their allied fields.

As an industry leader in researchstandards writingpublishingcertification and continuing education, ASHRAE and its members are dedicated to promoting a healthy and sustainable built environment for all, through strategic partnerships with organizations in the HVAC&R community and across related industries.

ASHRAE is celebrating 125 years of shaping the built environment. Become a member of ASHRAE by visiting

For more information and to stay up-to-date on ASHRAE, visit and connect on LinkedIn,FacebookTwitter and YouTube.

Adjusting HVAC Ops to Help Mitigate COVID-19 Spread

Adjusting HVAC Ops to Help Mitigate COVID-19 Spread

Increased ventilation, improved ltration e ciency, and electronic air cleaning are the keys in both commercial and educational settings, writes Greenheck’s Nick Pearce.

DEC 09, 2020

By NICK PEARCEApplication Engineer, Greenheck, Schofield WI

In response to the COVID-19 pandemic, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has published guidelines for HVAC system operation in commercial and educational buildings to help mitigate the spread of COVID-19 via airborne respiratory droplets. The guidelines fall into three general categories:

  1. Increased ventilation;
  2. Increasd filtration effiiency;
  3. Electronic air cleaners.

These HVAC system upgrades can pose challenges to building operation and energy usage. However, properly designed dedicated outdoor air systems (DOAS) and variable air volume (VAV) systems help minimize the challenges. DOAS and VAV systems provide high percentages of conditioned outdoor air into buildings. Many of the upgrades recommended by ASHRAE are easily implemented with these systems.


Increased ventilation dilutes the concentration of indoor contaminants, including infectious respiratory droplets, and mitigates the spread of COVID-19 via airborne transmission.1 DOAS and VAV systems are well suited for increased ventilation recommendations because each has features available to control and condition high percentages of outdoor air efficiently.

DOAS and VAV systems deliver conditioned outdoor air based on building occupancy levels. DOAS consists of two parallel systems, as shown in Figure 1.

Figure 1: DOAS includes a dedicated ventilation unit and multiple AHUs

A dedicated ventilation unit provides 100% outdoor air and controls the latent load, while air handling units (AHUs) control the space sensible load. VAV systems mix high percentages of outdoor air with return air to maintain indoor temperature and humidity. Figure 2 shows a single-zone VAV system serving a gymnasium.

Figure 2: A gymnasium served by a single-zone VAV system.


Maintaining indoor relative humidity is key to controlling the spread of viruses.

Viruses have the most difficulty surviving in 40-60% relative humidity.1 DOAS and VAV systems maintain this humidity range across a wide range of outdoor conditions by offering modulating compressors, hot gas reheat, and head pressure control. Modulating compressors (digital scroll or variable speed) adjust capacity to match the cooling or dehumidification load. Modulating hot gas reheat maintains the desired relative humidity levels. Modulating head pressure control maintains a constant refrigerant pressure by modulating the condenser fan speed. In mild temperatures, this reduces the speed of the condenser fans to improve hot gas reheat performance.


Operate HVAC systems longer. Increase the time that HVAC systems operate in normal occupied mode. ASHRAE recommends up to 24/7 operation to prevent the buildup of infectious particles.1 This is an easy strategy to implement, as it only requires scheduling changes.

Implement a pre- and post-occupancy flush sequence. If 24/7 operation is not feasible, use an outdoor air flush sequence. This sequence involves flushing the building with the peak outdoor air rate before and after occupancy. ASHRAE recommends providing three outdoor air changes, meaning that the air in the space should be replaced completely three times. As an alternative to calculating the air change rate, flushing periods of two hours for pre- and post-occupancy is another option.2

Disable Demand Control Ventilation (DCV). DCV saves energy by reducing ventilation during periods of light occupancy. Disabling DCV maximizes outdoor air regardless of occupancy.1 Blowers should operate at the maximum design speeds, and the outdoor damper should remain at its maximum position.

Open outdoor air dampers on VAV systems. Open the outdoor air damper as much as cooling and heating capacities will allow while maintaining desired space conditions.1 Consider this strategy carefully before applying in humid or cold climates. Use caution in polluted areas, as increasing ventilation may introduce unwanted pollutants into the building. DOAS and VAV offer optional high-efficiency filters to reduce the amount of pollutants that enter the building.

Use energy recovery. The preceding ventilation recommendations increase ventilation and mitigate the spread of COVID-19. However, these strategies also increase energy usage. To reduce energy costs associated with ventilation, energy recovery can be incorporated into DOAS and VAV systems.

Figure 3: Dedicated ventilation unit with an energy wheel that transfers energy between the outdoor and exhaust airstreams.

Figure 3 shows a total energy wheel installed in a dedicated ventilation unit. Total energy wheels transfer sensible and latent energy between the outdoor and exhaust airstreams. In summer, the outdoor air is cooled and dehumidified as it passes through the wheel. The opposite occurs in winter; the outdoor air is heated and humidified. This preconditioning of the outdoor air reduces the cooling and heating loads, resulting in lower energy costs. ASHRAE recommends that energy recovery devices remain operating and are essential for maintaining ventilation rates during this pandemic.3

Existing DOAS and VAV systems can benefit from the addition of energy recovery. Adding an energy recovery device reduces the outdoor cooling and heating loads, allowing for increased ventilation while maintaining space conditions. Figure 4 shows an energy wheel installed in the outdoor air intake of an outdoor ventilation unit, with exhaust air being pulled through the wheel from a restroom.


ASHRAE recommends filters with a rating of MERV 13 or higher, which capture at least 85 percent of airborne respiratory droplets.4 Use MERV 13 and MERV 14 filters in VAV systems to capture infectious respiratory droplets from the return air. Highefficiency filters do not affect the spread of COVID-19 when used in DOAS ventilation

Figure 4: An energy recovery wheel installed in the outdoor airstream of a VAV system reduces the mechanical cooling and heating loads.

units. These ventilation units provide 100% outdoor air, and thus no infectious respiratory droplets are recirculated from the space. The only time DOAS ventilation units could require high efficiency filters is in polluted areas.



In addition to the ventilation and filtration strategies outlined above, electronic air cleaning devices can supplement HVAC systems to clean indoor air and further mitigate the spread of COVID-19. Although several types of electronic air cleaners exist, the industry has focused on two in particular: ultraviolet lights and bipolar ionization. These technologies can be applied to VAV systems that handle a mixture of outdoor and return air. Electronic air cleaners do not affect the spread of COVID19 when used in DOAS ventilation units, which do not recirculate air from the space.

Ultraviolet (UV-C) Light

UV-C light can inactivate microorganisms. Although it has not yet been proven effective against COVID-19, it is effective against other coronaviruses.4 Therefore, ASHRAE recommends incorporating UV-C lights into HVAC systems.

The effectiveness of UV-C light against microorganisms is a function of exposure time multiplied by light intensity. It is commonly used for surface disinfection of cooling coils and drain pans. These components are kept free of mold and bacteria growth when irradiated with UV-C light. Low light intensities can be used due to the essentially infinite exposure times of the surfaces.5

Use of UV-C lights is possible for airstream disinfection, but this requires higher light intensities due to shorter exposure times of moving air. ASHRAE suggests a minimum 0.25 seconds of exposure time to avoid excessive light intensities and the associated high energy costs.5 The ability to meet this suggested exposure time in an AHU is dependent on physical space constraints and airstream velocity. If this is not possible, mounting the UV-C lights in the ductwork is another option. Avoid mounting UV-C lights near filters because the lights can burn holes in the filter media. Caution should be used while servicing these devices, as exposure to UV-C light may cause skin redness and eye irritation.6

Bipolar Ionization (BPI)

Building operators may consider using BPI, especially if other mitigation strategies are not feasible due to blower or tempering capacity limitations. BPI generates positively and negatively charged ions, which can inactivate airborne microorganisms by stripping them of hydrogen. The ions also cause airborne particulate to cluster together into larger masses that can be captured by filters.7 BPI devices come in varying sizes and are easily installed in VAV units.

Some BPI devices generate ozone as a by-product of their operation, which is harmful to human health and can cause respiratory problems. When selecting BPI, check if the device meets UL 2998, which validates that electronic air cleaners produce no ozone.8


As commercial and educational buildings reopen, simple HVAC system adjustments can help minimize the risk of airborne COVID-19 transmission. DOAS and VAV systems can efficiently meet ASHRAE’s recommendations for increased ventilation. High- efficiency filters and electronic air cleaners can further supplement the effect that DOAS and VAV systems have on mitigating the spread of COVID-19. Building owners can breathe a sigh of relief, knowing that their HVAC system operations are helping minimize the impact of COVID-19.

Upgrade Your HVAC System

Upgrade your HVAC system to incorporate high- percentage outdoor air units suitable for use in DOAS and VAV systems. Table 1 illustrates how specific features of these systems meet ASHRAE’s recommendations for mitigating the spread of COVID19.


  1. ASHRAE, “ASHRAE Position Document on Infectious Aerosols,” 14 April 2020.
[Online]. Available: usaerosols_2020.pdf.
  1. ASHRAE, “Building Readiness,” 2020. [Online]. Available:
  2. ASHRAE TC 5.5, “Practical Guidance for Epidemic Operation of Energy Recovery Ventilation Systems,” 9 June 2020. [Online]. Available:
  1. ASHRAE, “Filtration and Disinfection FAQ,” 2020. [Online]
  1. ASHRAE, “Ultraviolet Air and Surface Treatment,” [Online]. Available: covid19/si_a19_ch62uvairandsurfacetreatment.pdf. [Accessed 18 September 2020].
  2. UV Resources, “UVC | Ultraviolet C Light | Frequently Asked Questions,” 2020.
[Online]. Available:
  1. Waddell, “An Overview of Needlepoint Bipolar Ionization,” 28 February 2019. [Online]. Available: uploads/customerresources/Service-Logic/GPS-WhitePaper-NPBI-Explained.pdf. [Accessed 16 September 2020].
  2. ASHRAE, “ASHRAE Position Document on Filtration and Air Cleaning,” 29 January 2015. [Online]. Available:

Source: HPAC Engineering


Distributor of Electronic Air Cleaners and Ultraviolet Light in Malaysia:

56, Jalan 4, Kawasan Perindustrian Ringan,

Pandan Indah, 55100, Kuala Lumpur.

Hotline: 03 – 4295 3295





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