Effective COVID-19 mitigation for HVAC systems
There are many technologies available for HVAC systems to combat the spread of COVID-19, but not all systems apply to all building types
- Become aware of the different HVAC technologies available to mitigate the spread of COVID-19.
- Understand constraints for application of each system to mitigate spread of airborne pathogens.
- Learn about the factors to consider for a specific project including cost, duration, space usage, etc.
With a flood of information about heating, ventilation and air conditioning technologies to combat the spread of airborne viruses such as COVID-19, the consulting engineer might ask, “How do I select systems and equipment to best meet the owner’s requirements and specific project goals?” The following list of variables should be considered:
- Permanent versus temporary.
- Acceptable level of maintenance.
- New construction or renovation project constraints.
- Equipment requirements to achieve published “effectiveness” or “kill rate.”
The consulting engineer should look at a few of the specific questions/criteria that make the difference between simply doing something and being effective when designing HVAC systems to mitigate the spread of COVID-19.
Combating coronavirus spread
The coronavirus that causes COVID-19 is spread both by airborne and surface transmission. Airborne transmission of respiratory droplets containing the virus is the main form of transmission, according to the Centers for Disease Control and Prevention. A study by the National Institutes of Health determined that if the droplets land on surfaces, the virus may remain stable and active for several hours to several days, depending on the surface material.
In selecting the most effective technology for a specific project, the engineer needs to consider the space use and the susceptibility of the people that will occupy the space. Are the rooms constantly occupied or are there significant times when unoccupied? If constantly occupied, systems that treat the air supply may be the best or only option. But if the rooms are sometimes unoccupied, systems that can decontaminate surfaces by direct exposure may be more effective, especially if the existing HVAC system cannot be modified to accommodate a system that treats the air supply.
Project goals and constraints
Different projects have different goals and constraints. The solution for a church or assembly facility may be different from a school or hospital application. Some determining factors to consider:
- If the project is a retrofit to an existing system, existing HVAC systems may need to be modified for the new equipment. The technology may require more space or increased capacity (fan horsepower or cooling capacity) than what is currently available.
- For a new project, it is easier to provide the required accommodations to optimize the technology’s effectiveness.
- Introducing the new system may require changes in facility operations and occupancy patterns. Some systems cannot be used when the room is occupied.
- Is a permanent solution needed or does a portable/temporary solution meet the need? Or does it need to be permanently installed with the ability to switch on and off?
- The facility maintenance staff should agree that they have the resources to maintain the equipment.
- The owner’s budget may dictate which systems and equipment are viable options.
The best time to incorporate the system is during design of a new project. Depending on the equipment application, retrofit work may involve shutdowns of existing equipment, which might result in needing temporary air supply from other sources (adjacent air handling unit systems or temporary AHU and ductwork). The project engineer should discuss with the owner how much, if any, downtime is acceptable.
Some systems have a maximum air velocity and/or minimum exposure time to be effective. If the maximum air velocity is less than the intended design velocity for the duct or AHU, resizing may be necessary. A minimum exposure time in the airstream would also impact AHU layout because of increased air path length. Both of these constraints could result in a larger AHU footprint, which has the subsequent consequences of needing more mechanical room space or more space for ductwork above the ceiling. Referring again to retrofit projects, the existing HVAC system should be observed to verify that it can be modified to accept the new equipment that there is space available for the necessary modifications.
For some projects, combating airborne viruses is a constant need, such as in a hospital. For others, it could be a temporary concern, such as a church, which can take other steps — including changes in facility operation and occupancy patterns — to address the concern.
Table 1: This shows the breakdown of permanent and temporary heating, ventilation and air conditioning options that could reduce the spread of COVID-19. Courtesy: Smith Seckman Reid Inc.[/caption]
Mitigating COVID-19: technologies and methods
Systems and equipment to mitigate spread of airborne pathogens such as coronavirus can be grouped by method and technology:
- Flush: fresh air supply, exhaust contaminated air.
- Capture: filtration.
- Deactivate/kill: bipolar ionization, UV light (either direct exposure or UV-produced oxidation).
Flush: Fresh, outside air is relatively germ-free. Exhausting room air and replacing with all fresh air keeps the room air “washed.” However, quality of room air is dependent not only on the fact that the air is fresh but on how often the room is refreshed. Most bacteria and germs in a space come from the occupants so, to be effective, a relatively high rate of air change is needed to alleviate concerns of germs building up in the room air.
As a frame of reference, isolation rooms in hospitals require 12 air changes per hour, which indicates the range of air flow needed to effectively “wash the room” with air. This comes with a high energy price tag to heat and cool/dehumidify 100% fresh air. One option, as noted above, is to automate an HVAC system where certain rooms can, upon activation, be totally exhausted and supplied with all fresh air. When not needed, the system is switched back to the return air system.
One temporary measure that has been implemented at hospitals during this period is the installation of small, plastic sheeting arranged as a capture hood at the head of a hospital patient bed. Canopies at the head of a bed provide some capture of germ laden air (presumably at the source) and exhaust it out of the building. This reduces the impact on the HVAC system, both in equipment and in the energy penalty of using fresh air.
Capture: HEPA filters remove 99.97% of particles at a 0.3 micron size but can be effective at removing particle sizes smaller than 0.3 microns. Most viruses are smaller than 0.1 microns, therefore, HEPA filters are very capable of removing these from the airstream. Due to the fact that HEPA filters add between 1 and 2 inches external static pressure to a duct system, existing or new fan capacity should be sized for the additional static pressure.
HEPA filters can be installed several ways: in the AHU, at a return grille or in a portable fan/filter unit installed in a room. Adding HEPA filters in an AHU will add length to the unit. Because HEPA filters are usually rated at 500 feet per minute (or less) and most duct systems are sized for higher velocities, adding HEPA filters in a duct requires modification to enlarge a section of duct to accommodate the filter. HEPA filters can be installed at return grilles and when this is done, the return grilles and the filter can be oversized to reduce velocity, which reduces the added static pressure on the fan system. HEPA filters can also be used in portable units that recirculate room air.
Deactivate/kill: Bipolar ionization technology generates millions of positively and negatively charged ions in the airstream which act on organisms (viruses, bacteria, mold, volatile organic compounds, etc.) in the air. The ions destroy the organism surface structure. Particles of opposite polarity are attracted to each other, creating larger, benign particles that are more efficiently filtered out of the airstream.
There are two types of bipolar ionization systems: tubular and needlepoint, with the name indicating the shape of the equipment used. They can be located in AHUs or in ductwork and have maximum airstream velocity requirements that may require modifications to an AHU layout. All bipolar ionization methods require airflow to work and are typically powered when the air handler is running.
One side benefit of bipolar ionization is that it also aids in particulate and odor control. Outside air ventilation requirements can be reduced when implementing bipolar ionization. However, this strategy is not recommended when fighting COVID-19.
An engineer should evaluate the level of ozone, if any, generated by the bipolar ionization process and determine with the owner to determine what is acceptable for the area being treated. The 2019 version of ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality requires air cleaning devices to be ozone-free (UL 2998 listed and labeled).
UV light, more specifically in the shorter wavelengths of UV-C, has been proven to kill bacteria, viruses, algae and other organisms. (UV light is classified according to wavelength bands UV-A, UV-B and UV-C. Most sunlight that reaches the earth is UV-A with a little bit of UV-B. UV-A and UV-B are not as effective at killing organisms as UV-C.) It has been used for many years to inhibit organic growth on wet cooling coils. The recent crisis has elevated UV as an effective method to combat the coronavirus.
UV can be provided either by direct exposure to the air or photocatalytic oxidation. Direct exposure consists of locating the lamps in the airstream and providing enough exposure time and intensity to kill the pathogens in the airstream. PCO produces changes in the organism when the UV light reacts with a catalytic material to produce oxidation products (hydroxyls and anions). These oxidation products react with organic compounds including viruses to break up the compounds into more stable substances such as water vapor and carbon dioxide. The oxidation products work in an airstream and may also disperse into a room to interact with organisms in the room. PCO may also help to reduce odors and particulates.
Several caveats should be examined when looking at UV to kill organisms:
- The intensity needed to kill pathogens in a moving airstream is higher than what would be installed to keep a cooling coil clean.
- UV exposure to the air increases by extending the air path length, reducing velocity or a combination of both. Either will affect AHU and duct size.
- Direct exposure to UV lamp can result in damage to skin and eyes so any equipment using UV must be shielded from occupants.
- UV radiation, depending on the wavelength, may generate ozone, which can cause irritation to the respiratory system.
- Some PCO systems use titanium dioxide, which is considered possibly carcinogenic if inhaled.
Both direct exposure and PCO systems can be located to cleanse the air in a room instead of locating the lamps in an AHU or duct. When locating direct exposure lamps in a room, the room must be unoccupied to avoid harmful exposure if the lamps are focused on surfaces in the occupied area of the space. Direct exposure units that focus away from the occupied area — upper room UV germicidal radiation, and PCO units, which contain the UV light within the equipment — can be located in an occupied room.
COVID-19 cleaning effectiveness
Some manufacturers make numerical or qualitative claims about effectiveness, with some even claiming a 99% kill rate.
Read the fine print. Understand how the test was conducted that resulted in the numerical claim. Identify the testing agency that performed the test. Some manufacturers may claim a 99% kill rate, only to find out it was for a test of air in a closed room for 60 minutes. If that fits the situation for a specific project (for example, a church building that sits unoccupied much of the time) it might be relevant. Not all claims apply to all applications. Reach out to the manufacturer or vendor for information on how the tests were conducted. Relevant testing will always be performed by an independent third party.
Test conditions (intensity of the unit output, density of units in the test area, modifications to the AHU or ductwork) all contribute to the test results. Those conditions should be compared to the conditions for the specific project.
As noted above, some systems may generate ozone. Manufacturers may acknowledge this and usually will, upon request, provide some answer. The answer may be quantitative or qualitative. Two good benchmark standards are UL 867, which addresses the safety of permanent and portable electrostatic air cleaning equipment and ozone generation, and UL 2998, which is a validation claim procedure for “zero ozone emissions.”
Bottom line: Acknowledge the advertised effectiveness but be wary of expecting similar results for each specific project.
Applying COVID-19 solutions
Understanding the goal of the project helps determine how to select and apply the technology. Permanent solutions intending to provide best available solutions will typically involve installation in the AHU and/or duct systems. Temporary or portable solutions for interim applications will usually be room located.
When considering applications to an AHU or duct system, it is important to realize the possible changes in equipment and/or ductwork to obtain required exposure time, which is dependent on air velocity and air path length. The installed cost can be a significant factor in these applications, not only to the AHU but to other building aspects such mechanical room space. Applying these technologies also impacts the facility’s energy costs due to increased fan horsepower or other additional equipment loads.
Determine the maintenance effort and costs. HEPA filters may go three months, UV lamps a year and bipolar ionization tubes two years before needing replacement. Identify facility staff maintenance efforts and costs of replacement parts.
Impact on operating cost will vary. Filters carry a static pressure penalty, which translates to increased fan energy. Lamps and tubes have relatively insignificant static pressure loss and small increases in electrical power load.
For applications located in the occupied room, changes could be needed in room layout, for access to the equipment. There also may need to be limitations on occupied times. Interlocked operation with a building automation system, building security, nurse call, access control and other building occupancy systems may be necessary or recommended if avoidance to exposure is required. Consideration should be given to where to store the equipment.
The HVAC system can be a significant weapon in combating the spread of airborne transmitted contaminates but it needs to work in conjunction with the building layout and architectural impact. The key is separation — isolating areas where occupants may have the disease and limiting access to those areas. This may involve designating suites or pods of rooms to be containment areas and providing limited access or dedicated entries to those areas. Once an area of isolation and containment has been designated, it is more effective to implement some type of mitigation within that area.
HVAC decision steps for combating COVID-19
Follow these five steps when modifying HVAC systems to address COVID-19 issues.
- Determine the goal of the project. Decide if the best available solution is required or if some lesser level of commitment can provide the desired effectiveness.
- Discuss with the owner how much the facility can change its operation in terms of occupancy times, access and containment.
- Is a permanent solution needed or can temporary/portable systems and equipment meet the project goal?
- Acknowledge the maintenance effort to keep the system operating at desired effectiveness and confirm that the owner can provide that effort and expense.
- Finally, once a technology is selected, get specific information and guidance from the manufacturer for the specific project application.
- Ask if a product complies with standards such as UL 867 or UL 2998.
- Have the manufacturer review the proposed installation and respond that it includes the necessary constraints (air velocity, air path length, equipment location and arrangement, etc.) for their product to be effective on that specific project. A manufacturer will likely not respond with a guaranteed number for effectiveness for a particular project. However, the engineer should obtain some degree of assurance that the installation of an air cleaning system will be beneficial and not just an attempt to do something.
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