Designing HVAC systems to combat pandemics

It has been almost two years since the onset of the COVID-19 and, in that time, an incredible amount of knowledge has been developed on the coronavirus. From vaccine production, to personal protection, to the spread of the disease, society has learned how to deal with this virus.

The development of heating, ventilation and air conditioning solutions could be compared to the development of vaccines. Just as pharmaceutical companies used existing and relatively new technologies to develop vaccines such as think vector vaccines and messenger RNA, the HVAC industry has used a similar strategy to battle COVID-19 like high-efficiency particulate air filters and ultraviolet germicidal irradiation.

In the fight against COVID-19, the HVAC industry has employed many different strategies and technologies, some being safer or more effective or more practical than others.

Increasing ventilation to combat COVID-19

Based on the concept of dilution, the introduction of ventilation or outside air into the HVAC systems serving buildings is not a new concept. The International Mechanical Code, ASHRAE Standard 62.1 and other codes set requirements for the proper ventilation of buildings based on occupancy and function. But COVID-19 has taught us that even those code minimums or standards may not be adequate for the protection of building occupants depending on the type of HVAC system used or the virality of a specific pathogen like a novel coronavirus.

Almost all HVAC systems require forced air to accomplish the objective of properly cooling and ventilating the spaces they serve, and outdoor air is a critical component of that objective. Ensuring proper delivery of that ventilation air or even potentially increasing the amount of ventilation has become more important than ever with COVID-19.

For current and future HVAC system designs, providing the opportunity to increase ventilation could be the means to win the battle against the next pandemic with a philosophy of diluting the virus with ample amounts of outdoor air. There are obvious drawbacks to this, which include increased energy usage to heat or cool the increased outdoor air. Existing technologies for heat recovery can minimize and even reduce the amount of energy used to condition the ventilation air component.

Distribution and flow of air in HVAC systems

The distribution of conditioned air in any building plays an important role in maintaining good indoor air quality and in the case of controlling pathogens, this distribution is critical. Taking lessons from some of the more critical spaces designed by engineers, air distribution philosophies taken from hospitals can provide simple but important lessons.

In hospitals, the flow of air from clean to less clean areas is critical to controlling infections. In the case of operating rooms, filtered and conditioned air is typically introduced at the ceiling level through laminar airflow devices. As air flows down and over the surgeon, nurses and patient, it is drawn to opposite corners of the OR and collected at low wall return air devices and returned back to the air handling unit. At the AHU, a certain portion of the air is exhausted from the building, fresh air is introduced, the mixed air is conditioned and then filtered through MERV 14 (minimum) filters to be reintroduced to the OR. For other, less critical spaces, air is still distributed at the ceiling level but then returned from the ceiling level (see Figure 1).

Figure 1: Operating room layout. Example of airflow from clean to less clean areas. Courtesy: WSP

From a design perspective, understanding the distribution and flow of air in any space is an important aspect of design. Design engineers should constantly question how to distribute air to specific spaces and determine the best way to introduce and return air from those spaces. Taking a lesson from the design of HVAC systems for critical spaces, viewing airflow from clean to less clean areas can be an effective way to limit the potential transmission of pathogens.

The operation of the AHU system as referenced above is almost identical in applications serving many different occupancies in the built environment. For health care applications, the differences can be found in the types and levels of filtration and the ability of the system to condition and move large quantities or air.

From the perspective of filtration, spaces dedicated to inpatient care, treatment and diagnosis must be served by two filter banks, the first with a minimum MERV 7 filter rating, located upstream of any heating or cooling coil such that all mixed air is treated. The second filter bank, downstream of all wet-air cooling coils and the supply fan, must be MERV 14 or better. This second filter bank ensures the cleanest air is distributed to the inpatient care spaces.

The filter ratings are key to providing clean, conditioned air. MERV 7 filters are successful in arresting more than 90% of particles that are larger than 3 microns in diameter. These filters typically capture mold spores, dust and pollen.

MERV 14 filters arrest greater than 98% of all particles that are larger than 0.3 microns in diameter and capture bacteria, droplet nuclei and most tobacco smoke. Many AHUs serving critical spaces in hospitals are fitted with filter frame assemblies that will accept MERV 15 or higher filers, also known as HEPA filters. MERV 15 filters arrest greater than 99% of all particles 0.3 microns in size or larger.

As part of its response to the COVID-19 pandemic, ASHRAE provided technical guidance for filtration and disinfection standards associated with respiratory droplets and droplet nuclei. In its COVID-19 response, ASHRAE stated, “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 1.0 µm in size and larger.”

Respiratory droplets and droplet nuclei 1.0 µm in size and larger typically do not travel large distances and by weight fall out the air in short distances and are more easily trapped by filter media.

By understanding the nature of transmission of a specific pathogen, in this case Sars-CoV-2, the ability to install and/or use MERV 14 filters or better, can be an effective way for designers of HVAC systems to capture and potentially control that and future pathogens.

However, increasing filter efficiency leads to an increase pressure drop, which can lead to a decrease in airflow and more energy usage by the fan assembly to overcome the increased filter resistance. Careful consideration must be taken in the design of future pandemic-ready systems but at a minimum, the use of the highest MERV rating filter can provide results in the control of pathogens in HVAC systems depending on air patterns and pressurization within specific buildings.

Adding UVGI lighting to HVAC systems

UVGI devices have been applied in the past within AHUs to control the growth of mold and bacteria. Typically, an array of UVGI lights shine UV-C light continuously on cooling coils to irradiate and disinfect the coil surface over time, and has been used to control mold growth but not to disinfect the air. This technology also has been used for irradiating ductwork and filter media. An important design consideration with UVGI lighting is the intensity of the UVGI light source and the duration of exposure of a pathogen to that source as the kill rate can be greatly impacted by those factors.

ASHRAE’s position document on filtration and air cleaning indicates, “Experience suggests that control of a moving airstream does not provide favorable killing rates because of the short dwell time (of the air in the range of the UV light).”

ASHRAE further recommends that UVGI devices should be paired with mechanical filtration of the highest MERV rating as practical . Use of a UVGI system must be carefully considered and designed to provide maximum effectiveness. The velocity of the airstream across filer media or the cooling coil must be fully understood as a much higher intensity of UVGI needs to be provided to be effective against a pathogen like COVID-19. When paired with a MERV filter assembly, the filter media also must be safe for UV-C light exposure.

Along the same thinking of UVGI in the AHU, upper room UVGI devices create an irradiation zone with UV-C light (200 to 280 nanometers) in areas with high ceilings. An application like this can disinfect the room air as it is cycled through the room by the HVAC system. This application can be a supplement to deactivate and destroy COVID-19 if increasing room ventilation is not an option.

UV-C light is harmful to human tissues and any UVGI system must be installed at the proper height and in strict accordance with manufacturers recommendations.

Pandemic operations

Another important factor in HVAC system design to battle future pandemics is the idea that the HVAC system should be flexible in its operation. The use of variable air volume strategies allows the HVAC system to react to the required heating or cooling loads, ventilation rates or building occupancy. A VAV system also provides flexibility for pandemic mitigation as it can adjust quickly and accurately to changing conditions. This can be extremely beneficial when outside influences demand the HVAC system react differently.

In addition to the use of VAV systems, there are several design approaches that can be implemented for future use or activation to battle a pandemic like COVID-19. Approaches such as one that integrates a dedicated exhaust system or one where automatic temperature control sequences allow the HVAC systems to operate temporarily in a 100% outdoor air mode could be essential to controlling the next pandemic. Design issues such as how to integrate a dedicated exhaust fan into the HVAC system or how to provide adequate cooling and heating capacity for a 100% outdoor air mode need to be considered but should not be roadblocks to implementing similar strategies.

From an operations standpoint, developing strategies that isolate portions of a building or create areas where increased ventilation or exhaust rates can be maintained are beneficial in battling a future pandemic (see Figure 2).

Figure 2: Operation strategy. Floor plan with “negative pressure” zones shaded to show containment strategy. Courtesy: HKS

Retro-commissioning, testing and balancing, HVAC recalibration

Typically, in commercial buildings the primary purpose of HVAC balancing is to provide proper air distribution to avoid occupant comfort complaints due to temperature, noise or drafts. Verification of ventilation rates and space pressurization in these buildings, although required by building codes, were generally not considered critical to building occupants and facility staff.

The documentation and routine verification of ventilation rates and pressurization was generally provided for health care, laboratory and other special building types or programs with specific industry or regulatory requirements for such documentation. More recently, periodic critical space testing services for health care occupancies have become required for accreditation of those health care facilities. When considering the importance of HVAC system operation in a pandemic mitigation strategy, HVAC system testing, balancing and commissioning takes on greater importance for all building types.

Designing and operating HVAC systems for pandemic mitigation often will require changes in ventilation, space pressurization relationships and/or filtration methods on a temporary or “surge” basis or modifications to existing systems that will operate on a permanent basis. In either case, once the HVAC system is installed or modifications to existing systems completed, system balancing and commissioning are required to verify that the increased ventilation rates and space pressurizations have been achieved and that control strategies to maintain systems in operation operate correctly.

The installation of additional filtration media or equipment also will impact the operation of existing HVAC equipment and post installation test and balance is required to confirm that system airflow capacities have not been affected by the additional filtration system. A complete and detailed testing and balancing report that provides a summary table and indicates actual measured room air exchange rates (measured in air changes per hour) and pressurization relationships is essential for building owners and operators to demonstrate the measures in place to protect occupants during a pandemic.

HVAC systems and equipment are dynamic and are constantly changing based upon occupant requests and based on indoor and outdoor environmental conditions, so after systems are initially constructed and tested, routine follow up testing is required to verify the systems are continuing to operate as intended for pandemic mitigation.

Many critical facilities such as hospital and laboratories are required to perform annual testing of systems to verify operation, ventilation rates and space pressurizations. This same strategy should also apply to HVAC systems designed to operate for pandemic mitigation as confirmation that the HVAC systems mitigation strategies remain in operation.

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