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Mechanical System Solutions to Help Mitigate Spread of COVID

By Michael Walsh, PE, LEED AP
July 9, 2020

Communities, corporations, and educational institutions are drafting plans to reopen during the COVID-19 pandemic, employing policies around personal protective equipment; keeping a 6-foot distance from co-workers, peers, and clients; and reconfiguring their physical spaces. Another potential mitigation strategy is to adjust a facility’s mechanical system to better protect the occupants, though that should not be considered the primary solution, cautions Michael Walsh, PE, LEED AP, senior mechanical engineer and principal at R.G. Vanderweil Engineers, LLP. “There’s no silver bullet,” he says.

Viruses are transmitted in three ways: direct person-to-person contact; indirect contact, for example from a doorknob to a person’s hands to their nose, mouth, and eyes; and in droplets spread in aerosols between people in close proximity. An NIH study has shown COVID-19 is detectable in aerosols for up to three hours, so, according to ASHRAE, it is sufficiently likely that it is transmitted through aerosol when people sneeze or cough.

General dilution ventilation will not help with aerosols, says Walsh. It doesn’t influence the larger droplets that fall onto surfaces that people touch, but there are smaller particles less that 10 micros that can be captured in the air and moved around a room. Ventilation and filtration, therefore, can reduce the airborne concentration of the virus.

First and foremost, ASHRAE recommends maintaining indoor temperature and humidity in comfort range. “They’re promoting maintaining good indoor air quality so occupants stay healthy, in order to help fight the virus,” explains Walsh. “It has less of an effect on reducing exposure to the virus itself.”

Walsh recommends that facilities operators start with the most common, simplest maintenance protocols:

  1. Make sure HVAC systems are running properly. One of the key sources of infection in the SARS epidemic was in a nursing home where the floor drain traps had dried out. In that case, flushing the toilet could spread the disease, so make sure all drain traps are primed. “You might want to run your water every day,” says Walsh. “Plumbing systems are critical. Conduct basic maintenance tests.”
  2. Maintain ventilation rates at least at the levels of codes and good engineering practice guidelines.
  3. Contain the virus within those high-occupancy spaces. The simplest recommendation is to replace the return registers with filter grilles that contain a 2-inch thick MERV 13 filter. “A number of studies say you get a very good capture of viruses and bacteria at level MERV 13 or higher,” says Walsh. At higher levels of filtration, however, the pressure drop starts to accelerate, and fan systems aren’t designed for that. MERV 13 allows a very low pressure drop option, contains the material within the high-occupancy spaces, and allows the filter to be spray-decontaminated before removal and disposal. It also protects the duct system from pulling air back and spreading it to other rooms within the building.
  4. Where it’s not feasible to replace the return grille, ASHRAE recommends replacing the return filtration at the air handler with MERV 13 or higher, if the fans can handle the added pressure drop. In that case, cautions Walsh, the return air ductwork becomes contaminated, because the system is pulling the aerosols through it, and it can settle there.
  5. Consider air pressurization in the building. The highest potential for generating aerosols is in a high-occupancy space. Maintain high-density spaces at a negative pressure relative to the area around it. “That doesn’t mean put a huge exhaust fan in the lobby, because you’d be pulling outdoor air in through the front doors, which creates a lot of other issues, like pulling in humid moist air in the summer, and cold dry air in the winter,” says Walsh. Instead, create high-pressure zones around the lobby or around the conference room, in order to contain the air; and then take it back to filtration in the air handler or exhaust it through an exhaust fan, if that’s what the system is designed to do.

Carrier has developed a portable negative air machine called the OptiClean™, which is designed to create negative pressure in a space that was not initially intended for infectious isolation, such as overflow rooms in hospitals treating COVID patients. It contains a flexible ducting system to exhaust the air after it passes through a HEPA filter.

“There are several concerns with this approach, the first being that code requires you to provide makeup air in the space outside the negative pressure room if you are going to exhaust it,” says Walsh. “If that makeup air is not increased, the main hospital can become negative to the outdoors, bringing in unconditioned and unfiltered outside air, which can cause unwanted moisture issues in humid weather and inadequate heating and humidification in cold weather.”

Operational Changes

It is also possible to add a layer of protection by running an existing HVAC system differently.

  1. Disable demand control ventilate sequences to maintain full ventilation rates round the clock.
  2. Shut down energy recovery systems that have the potential to cross-contaminate supply. Walsh says that only a very tiny bit of air gets cross-contaminated through an energy wheel, so with a MERV-13 filter installed before the energy recovery, it may be considered safe to keep it running.
  3. Run the system 24/7 or consider shutting it down for shorter periods of time at night, so you can flush the building with ventilation air.

“All of these will drive up your energy use, so you have to keep in mind the impact of that,” says Walsh.

What about simply clearing a room more often? In hospital settings, the jury is still out on the effectiveness of increasing air changes per hour combined with HEPA-filtered recirculation units. “ASHRAE notes that it’s unclear how quickly you can reduce the count before the infection rate goes down,” says Walsh. “It’s an unproven technology.”

ASHRAE’s Epidemic Task Force Healthcare guidance includes a CDC chart indicating that a patient room at the typical 4 ACH takes 69 minutes to remove 99 percent of the aerosol contaminants, and 104 minutes for 99.9 percent. For an office with 6 ACH, it takes 46-69 minutes. Higher air changes are needed to reduce this time frame. “That is why an OR runs at 20 ACH,” says Walsh. “That can clear the room in about 20 minutes. You really need high air changes for that particular recommendation to make sense. You need to be careful when applying a recirculating HEPA filter and claiming that it’s going to reduce the aerosol within the room.”

That note of caution is important for those who are considering simply replacing standard HVAC filters with HEPA filters as a way to reopen public spaces. For example, New York Gov. Andrew Cuomo has recommended making it mandatory that shopping malls install HEPA filters. Upgrading the filters without upgrading the entire system—particularly the fans—could have unintended negative consequences. If the fans are not sufficiently powerful to force enough air through the tight HEPA filters, the facility could become starved of conditioned air. A better solution would be to upgrade to a MERV 13 filter.

Florida State University, for example, is replacing all air filters with MERV 13 where there is recirculating air.

Generally speaking, the goal is to increase the quality of indoor air. To that end, increase the amount of outdoor air wherever possible. WELL™ Building standards recommend a minimum of 30 percent above code. “Our recommendation, if you are going to consider doing that, would be to run your system at 100 percent outdoor air until your capacity runs out and you can’t maintain temperature and humidity control,” says Walsh. “As you start to lose temp control, start to reduce outdoor air. That will occur only a few times during the day, or over a weeklong period, so you can run your system probably at 100 percent most of the time. But, again, energy use is going to go through the roof.”

The rule of thumb for years has been to maintain humidity at between 30-60 percent in order to minimize the concentration of bacteria and allergens and optimize human comfort. Newer studies from microbiologists, however, are showing that 40 percent relative humidity helps reduce the infection rate even though the aerosols are still there. Walsh cautions that the higher humidity can cause condensation on windows when the outside temperature is below 20 degrees, even with double glazing. “You also have to be careful that your building includes vapor barriers,” he says. “You really need to understand the building envelope before you go to those higher levels of humidity.”

A recent Columbia University study shows that UVC light can be used to kill airborne coronaviruses that are structurally similar to COVID 19. UVC lamps can be placed up high in a room, so that as the slow-moving return air goes up towards the return grills, the UVC kills the virus. Putting UV lights into a duct system is less effective, however, and requires a much higher wattage and an extended length of ductwork, says Walsh. Walsh recommends using UVGI (ultraviolet germicidal irradiation) in high-occupancy spaces, but cautions that they need to be carefully placed to avoid direct exposure of skin and eyes.

Containment First

The lesson learned from the design of biohazard labs is that the solution to pollution is not dilution but containment of infectious materials. Primary containment is the biosafety cabinet, and the secondary containment is the room itself while still requiring use of personal protective equipment (PPE). The Centers for Disease Control and the World Health Organization, for example, recommend that standard testing for the coronavirus agent be done in a BSL-2 laboratory, but under BSL-3 protocols for PPE, including masks, face shields, and gloves. For labs performing virus isolation in cell cultures, the CDC recommends performing this work in a BSL-3 laboratory.

“Wearing masks is really important to prevent you from spreading the disease,” says Walsh. “It’s important to deal with the aerosol side of it, but ASHRAE notes that good hygiene and proper distancing are the most effective methods of prevention, not the HVAC system.”