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COVID-19 Solutions: HVAC Systems

COVID-19 Solutions: HVAC Systems

Michael Morelli, PE

June 3, 2020

By now, we all know the basic measures for preventing the spread of COVID-19: wash your hands, wear a mask, and practice social distancing. Most of us have been working from home during this time. But what happens when we return to our offices?


Early data suggests that COVID-19 can be transmitted through commercial air conditioning systems. Changes to the operation of existing heating, ventilating and air conditioning (HVAC) systems could help reduce airborne exposures.


While we all need to keep practicing the basics, HVAC systems can help in the containment and reducing the spread of the COVID-19 virus through buildings. How?


  • Increasing outside air
  • Better air filtration
  • Adding ultraviolet light


While not every workplace can implement every measure, continue reading for details on making your building healthier.


Increasing outside fresh air to the occupied spaces


The name of the game here is keeping the indoor environment refreshed as often as possible. High volumes of fresh outside air will produce more ‘air changes.’ Air changes is the term used when the volume of outside air reaches the total air volume of the indoor space, thus changing 100% of the indoor air quantity.


Most packaged commercial air conditioning systems (furnaces, rooftop units, split system air handlers) are designed and sized to operate within certain ranges of fresh air intake to meet the code-required minimum fresh air requirements of the occupants. Some systems have controls that will allow large amounts (up to 100%) of fresh air to be admitted into the building under certain outdoor conditions of temperature and humidity to reduce the cooling load of the air conditioner. This happens when it’s temperate and comfortable outside; usually 55-70 degrees Fahrenheit and 50% relative humidity outdoors.


There are some issues that appear when trying to increase outside air to the indoors. First, as the capacity of HVAC units is constant, and it is rarely the perfect temperature outside, increasing outside fresh air will also have the negative effect of increasing humidity indoors. Packaged commercial units are normally not designed to remove this higher amount of humidity at a full rate of fresh air intake. Increased humidity in the building is undesirable, as it leads to biological growth of mold and mildew, and an uncomfortable environment. Without proper design, the equipment can be overwhelmed by high (or low) outdoor air temperatures and humidity.


Also, some systems have no capability to provide outside air to the areas they serve, such as wall and ceiling-hung air conditioners. These areas would require introduction of outdoor fresh air ductwork, heaters and fans, at considerable expense and disruption.


Though the percentage of outside air used by HVAC equipment cannot be increased beyond which it was designed, many energy-saving techniques have been implemented in the past 20 to 30 years to reduce the outside air quantities below what equipment is capable of handling. These energy-saving techniques can be reversed, particularly during this pandemic. Energy saving techniques can be eliminated or modified to accomplish more continuous fresh air flow into the building.


  • Night setback is generally the process of reducing the heating or increasing the cooling temperature set point while a building is unoccupied to save on energy costs. Some HVAC systems, including most newer rooftop units, have the ability to close the outside air damper at night to greatly reduce the heating or cooling load when no outside air is necessary. This is typically shown as “Recirculation Mode.”
  • Demand controlled ventilation is similar to night setback, where the system automatically reduces the amount of outside air drawn into a system based on occupancy as measured by carbon dioxide (CO2) concentration (a byproduct of occupant respiration). Any use of this in the short-term should be eliminated, and then all units should be set to run at the maximum outside air percentage that they are capable.  
  • An airside economizer in an HVAC system is a process of increasing the amount of outside air being brought into the building when conditions are such that outside air can be used to directly cool a space. Economizer controls can be modified to increase outside air quantity while still running heating or cooling systems. This could allow you to increase outside air during milder temperatures rather than reduce heating and cooling costs during milder temperatures. 


Recommendations to increase outside air include: 


  • Increase outdoor air ventilation in air conditioners equipped with outdoor air intakes by disabling demand-controlled ventilation and open outdoor air dampers to 100% as indoor and outdoor conditions permit. A balance of comfort and safety through careful calculation and design is necessary at this point.  The goal is to introduce as much fresh air into the building as possible, but occupant thermal comfort is also necessary for reasonable use of the building by the occupants. A room high and low temperature cutoff for the outside air damper to return to minimum condition should be introduced into the programming to prevent the room from becoming too hot or too cold. Each system will require field adjustments to the hardware and controls, as available, to maintain this balance.
  • Eliminate “Recirculation Mode” during unoccupied times.  Any use of “Recirculation Mode” during unoccupied times should be eliminated and even rooftop units with the simplest of thermostat controls should be set to run constantly.
  • Rooftop units with simple thermostat controls should be set to run constantly. If a fan setting has on/auto/off controls, all units should be set to “On”, so that the fan runs constantly even if heating or cooling is not required.  This will help continuous ‘air changes’ throughout the occupied space.
  • Eliminate any demand control ventilation. All units should also be set to run at the maximum outside air percentage allowable for that unit. 
  • Evaluate costs for systems not fitted with equipment for increased outside air or that cannot be automatically modulated. If the cost to add dampers and controls is prohibitive for a particular unit, adding higher efficiency filters and a UVGI unit only is likely a better solution.

Increase air filtration

Increasing air filtration


Adding high-efficiency filters will treat the recycled return air within facilities and have the capability to remove particulates as small as bacteria and virus containing particles. MERV (Minimum Efficiency Reporting Value) filters are generally used for less critical facilities, such as office buildings. HEPA (High-Efficiency Particulate Arrestance) filters are of a higher level filtration efficiency and used in critical facilities such as hospitals, where a maintenance staff have required preventative maintenance schedules for continuous replacement.


Increasing the higher efficiency filtration systems will cause a higher resistance to air flow. Fans in air conditioning and heating units are sized to overcome the resistance of all the internal devices (cooling coils, heating coils), ductwork, air diffusers, and grilles that it pushes air through. These fans are designed to overcome that back pressure and leave enough pressure to distribute the tempered air into the room.


When a new filter is added to the system, you run the risk of restricting the air flow to a point that it will not function properly. The higher efficiency MERV and HEPA filters may remove smaller particles such as viruses and bacteria, but will increase the back pressure on the system, which may have consequences in total air volume delivered to each space, causing thermal comfort issues for occupants.


Another option that people consider are portable air cleaners. There is no direct clinical evidence of the benefit of portable air cleaners for reducing infectious disease risk, but some benefit can be reasonably inferred for appropriately sized (e.g., their removal rate is appropriate for the room), maintained, and operated portable HEPA filters. As with building filtration, the details are important (e.g., efficiency and airflow rate of the air cleaner, sizing and placement within the space, maintenance and filter change, nature of space that is being cleaned) and appropriate portable filtration is only likely to be effective in concert with other measures.


Recommendations to increase air filtration include:


  • Install enhanced filtration (higher MERV filters) over code minimums in occupant-dense and high-risk spaces. Increase central air and other HVAC filtration to MERV-13 where filter racks and equipment will allow, or the highest level achievable, without compromising the unit heating and cooling capacities. Higher than MERV-13 rated filters will not substantially increase the protection from bacteria and viruses in a general use air handling unit setting, and are not recommended for general packaged air conditioning units.
  • In units with belt drives, replace the pulleys and belts with appropriately sized ones for the higher pressure conditions to overcome the increase in fan static pressure. For units with variable frequency drives (VFDs), adjust the motor speed upward, with a corresponding increase in pressure, and rebalance duct distribution system after the changes are made.
  • Keep systems running longer hours, 24/7 if the existing control systems will allow. If needed, upgrade existing controls to allow for 24 hour operation of the system.
  • If the central HVAC system will not allow additional filtration, add portable room air cleaners. Make sure the units have HEPA or high-MERV filters.

Adding ultraviolet light

Adding ultraviolet germicidal irradiation (UVGI) in the air stream


UV light contains several unique properties and characteristics, which can be used to disinfect viruses, bacteria, mold, spores, and other harmful microbes. How can light disinfect? Science! This sanitation method relies on UV-C wavelengths, i.e. UV bands within a range of 200 nm to 280 nm, to deactivate the cellular ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), as well as the reproductive capabilities of microorganisms. According to data from the International Ultraviolet Association (IUVA), UV treatment has been around for more than 40 years. As a tried-and-tested solution, UV disinfection is prevalent in hospitals, medical or healthcare centers, laboratories, food processing, wastewater facilities, HVAC system management and more.


Through published studies and documentation, researchers have verified UV-C sanitation to deactivate previous strains of coronavirus, such as Severe Acute Respiratory Syndrome (SARS-CoV) with the first human case appearing as far back as 2002, and Middle East Respiratory Syndrome (MERS-CoV) with the first human case appearing as far back as 2012. During the spread of these infections, UV light disinfection played a vital role in preventing the spread of disease.


Retrofitting ultraviolet lights into existing commercial air conditioners can prove difficult to achieve in a practical sense. These units could be retrofitted into supply air ducts when possible, but changes to existing sheet metal and new electrical wiring would most likely be required. The cost of purchase is relatively low, but installation may present a challenge, as these systems require supplemental power supply and often the best location for the installation is often in inaccessible areas.


Recommendations to adding UVGI in the air stream include:


  • Consider a UV system. A properly designed and maintained UV system has been shown to reduce infections from other viruses. UVGI removes 99.9% of harmful airborne viruses and bacteria moving through the HVAC system, and spore growth (mold) growing on coils, drain pans and UV compatible filters. The details of the system are very important (e.g., design of fixtures, lamp type, lamp placement airflow amount and mixing, etc.). Simply adding UV to an existing system without consideration of these factors has not been demonstrated to have a benefit.
  • Add duct or air-handling-unit-mounted UVGI where the central HVAC system will allow this modification. Prioritize these systems for systems serving high occupancy areas.
  • Use portable UVGI devices in connection with in-room fans in high-density spaces such as waiting rooms, lounges, and meeting rooms. Building cleaning contracts may have an option to include a UV-C cleaning device service, such as are used in hospital room cleaning procedures.  Considering its proven effectiveness, this may be a good option for densely occupied spaces, if cost effective.

And the most effective measure

The most effective thing you can do: reduce occupant density, practice social distancing, wear face masks, and wash your hands.


These measures are much more effective than HVAC adjustments and should be incorporated in a new safety program along with the HVAC changes described below.


Consider operational adjustments first, for example stop utilizing conference rooms for large groups.  If these rooms are less often used, updates to these HVAC zones can be omitted, saving upgrade and alteration costs.

Other suggestions

There are other options to consider as well, depending on your space.


Bypass or repair energy recovery ventilation systems that could be leaking potentially contaminated exhaust air back into the outdoor air supply. Properly installed and maintained Energy Recovery Systems have almost zero transfer of particles into the supply systems, and any transfer is limited to gaseous pollutants like tobacco smoke and other smells. Additionally, since the rate of transfer is not dependent on the speed of rotation of an energy recovery wheel, and energy recovery can greatly aid in increasing outside air delivery, disabling Energy Recovery Systems is not recommended.


Review HVAC programming to provide fresh-air flush out two hours before and post occupancies. The control program should provide a ramp up of the supply fan if VFD equipped, and opening the outdoor air damper to 100% two hours prior to building occupancy and two hours after everyone leaves the building for the day. This includes operating exhaust fans as well as opening 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, and run the central HVAC system on minimum outside air when unoccupied. This will help pre-condition spaces to better-than-occupied set point prior to the start of the workday.


For unconditioned spaces that may only have an exhaust system, such as garages and warehouses, exhaust should run to complete one full air change of the space or two hours minimum before and after occupancy if outside conditions do not allow for continuous operation while occupied.


Duct cleaning has been mentioned in many statements as a method to avoid the transmission of viruses. The Federation of European Heating, Ventilation, and Air Conditioning Association (FEHVA) recommends against duct cleaning, stating. “Duct cleaning is not effective against room-to-room infection,” and that, “viruses attached to small particles will not deposit easily in ventilation ducts and normally will be carried out by the air flow.”  So, skip the duct cleaning and invest in more effective measures.

Getting Started

Getting your building or office adapted for a safe return may be a daunting task. We’re here to help. Our experts will help you find the right fit for your space, your occupants, and your long-term plants.


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