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Prepping Your HVAC System to Lower COVID Transmission this Winter

by 7NOX Team | Jan 20, 2022 | COVID-19, HVAC, Maintenance

Colder weather often brings spikes in COVID-19 and influenza cases. With this in mind, we should continue promoting vaccinations, mask wearing, social distancing, surface cleaning and handwashing inside your buildings. However, we shouldn’t forget about our HVAC systems; they also play an important role in stopping the spread of COVID. In fact, if not properly managed, these systems significantly contribute to virus transmission. To properly protect your facility’s visitors and workers this winter, prep your HVAC system the right way by following these guidelines.

Use an Air Dilution Strategy

Viruses like SARS-CoV-2 travel within tiny liquid droplets expelled through our coughs and sneezes. These droplets can range in size from larger particles (5-10 μm) to smaller ones (less than 5 μm). Larger droplets fall to the ground quickly, while smaller aerosols linger in the air much longer. Their hang time presents both a problem and an opportunity. The problem is that these tiny airborne particles can easily cluster together, becoming concentrated within small areas like offices. Concentration makes them more potent and contagious.

However, these clusters are also easily dispersed or “diluted” by adequate air flow. So, an effective dilution strategy is to keep a good mixture of air within every part of your building. It’s a similar idea to running vs standing water. Which is safer to drink? Here are some tips for an effective dilution strategy.

Increase Outside Air Flow

Increasing outside air flow helps dilute recirculated interior air and break up any high concentration particle clusters. The CDC recommends the following tips when introducing outside air flow into your interior spaces:

Disable demand-controlled ventilation (DCV) systems
Open outdoor air dampers beyond minimum settings
When conditions allow, open windows and external doors
Use stand-alone fans inside windows
Set indoor AC unit fan speeds to “on” instead of “auto”
 Run your systems longer, 24/7 if possible

CAVEAT: Increasing outside air flow during very cold or very warm weather raises your energy costs and puts added stress on your system to maintain set points. So, some actions may only be practical during milder weather. Another concern is the introduction of pollutants and pollen into the building. For occupants with allergies, outside air could contain possible health risks from contaminants. Increasing outside air flow during very cold or very warm weather raises your energy costs and puts added stress on your system to maintain set points. So, some actions may only be practical during milder weather.

Another concern is the introduction of pollutants and pollen into the building. For occupants with allergies, outside air could contain possible health risks. Most higher-rated filters can catch pollen (which is between 5 -11 μm) so introduction of outside air through fans, open windows and doors pose the greater risk. 

Target 5 Air Changes Per Hour

Your air change rate (ACH) is a measure of how often you replace the air within a space. However, ACH is a bit misleading since one cycle doesn’t equate to 100% removal. In fact, it takes longer than you’d expect to vacate any contaminants from a room.

“When we change air in a room,” explains Lance Jimmieson, of Jackson Engineering, “we’re not magically taking out all of the air that’s there and replacing it with fresh air. It comes in, mixes and turns over, and typically mixes between perimeter and center zones. So, we’ve got to remove it.”

Jimmieson advises targeting a minimum of 5 air changes/hr (12 cycles) and bases his recommendation on CDC data (Table B.1). “Even with ten air changes an hour, i.e. every 6 minutes, it’s still going to take half an hour to get rid of any traces of an aerosol in the room, so air change rates need to be relatively high,” he explains.

Table showing air changes per hour — Source: CDC Airborne Contaminant Removal Table B.1.

Upgrade Air Filtration Systems

The choice of filter matters when trying to arrest droplets containing small contaminants like viruses. ASHRAE recommends a minimum MERV-13 grade or better for commercial buildings. MERV-13 through 16 can achieve a 95-99% average removal efficiency for particles from 0.3 to 1.0 μm.

High Efficiency Particulate Air (HEPA) filters have an even higher performance, capturing 99.97% of particles with a size of 0.3 μm. However, their superior efficiency creates more pressure drop in your system, which will reduce airflow rates and therefore system performance.

CAVEAT Pressure drops from upgrading filters can have a significant impact on your HVAC system. In fact, most managers and owners will find it too difficult or impossible to retrofit their HVAC systems with HEPA filters without a costly or significant redesign. This hurdle is why ASHRAE recommends using portable systems with HEPA filters. Also, higher grade filters are costly and single use, so expect an uptick in operating costs.

Consider UV Germicidal Irradiation

Ultraviolet germicidal irradiation (UVGI) systems use short wavelength UV-C light to kill viruses and bacteria before they’re distributed by your ventilation system. UVGI systems for HVAC are usually mercury-based lamps or LEDs. As viruses pass through the HVAC system, the lamps “inactivate” any viruses captured by high efficiency filters or that move through the AHU.

UVGI lamps contribute to air sterilization, especially where outside air flow is restricted and/or dilution efforts are insufficient. However, UV-C does have limits. Jimmieson notes that one critical restriction that’s often overlooked is particle size. “By and large, a good rule of thumb is that if you’ve got a particle size that is bigger than 5 μm then you’re going to struggle to nuke that particle with UV light.”

It’s a fact that has implications for your filtration system, since low efficiency filters allow particles greater than 5 μm to pass through. If those larger particles are hosting viruses, then they may not be neutralized by your UVGI. “You really want to position the UVGI system downstream of a good quality filter to take the lumps out of the air,” Jimmieson recommends.

Positive vs Negative Pressure Rooms

by 7NOX Team | Nov 4, 2021 | COVID-19, HVAC, Industry Trends

Many industries use pressurised rooms to stop cross-contamination between one area of a building and another. For example, semiconductor makers use positive pressure rooms (PPR) to ensure their integrated chips are free of contaminants in the air. Hospitals and clinics employ negative pressure rooms (NPR) to contain the spread of infectious diseases. The difference between positive vs negative pressure rooms is mostly one of pressure differential and air flow. Both approaches use air pressure differentials to control ventilation and contamination.

Pressure Differential

Anyone who’s ever let go of an un-knotted balloon has witnessed the propensity of air to move from a higher pressure area to a lower one. The bigger the pressure differential, the faster the balloon will fly around the room. Building managers use HVAC equipment, fans and ventilation systems to control this natural propensity of air to escape—to keep the “balloon” knotted as it were.

The natural movement of air without the aid of mechanical equipment like a fan is called “passive” air flow, and techs use passive air flow to keep debris and contaminants from entering or exiting a room. If done correctly, the result is a stable environment with lower or higher air pressure than the surrounding area.

What’s a Negative Pressure Room?

To create a NPR, HVAC professionals must move air out at a faster rate than it comes in. That is, a negative quantity of air maintained. The purpose is to control the direction of passive airflow. When someone opens the door of an NPR, negative pressure draws passive air inside, forming a barrier against the escape of pathogens or dust. Interior air then moves through a filtration system to remove contaminants before safely exiting the pressurised environment.

What’s a Positive Pressure Room?

Positive pressure rooms maintain a higher air pressure inside than the surrounding environment. Air escapes the room without letting in outside contaminated air. PPRs exist within surgical theatres and in vitro clinics where contamination is possible. PPR hospital rooms often house immunocompromised patients susceptible to infection or disease. Because PPRs form barriers to outside spaces, their HVAC systems must filter out any contaminants from the interior air while ensuring optimal pressure and safe air quality.

Air Tightness

Pressure room designers try to keep rooms as air tight as possible, but some leakage occurs through gaps in doors, windows and electrical outlets. Designers often outfit NPRs with ante rooms to minimise leakage. These entryways are also safe areas for removing PPE or as a failsafe against pressure loss. Airtightness is also a cost issue. The more leakage, the more energy required to maintain a room’s negative or positive pressure.

Air Comfort

Like any conditioned environment, pressurised rooms must also maintain humidity and air temperature to ensure comfort and safety. Air quality is particularly important for medical facilities, since suboptimal humidity levels can contribute to illness. To aid air quality, HVAC technicians design HVAC systems to include specific numbers of air changes per hour (ACH) based on the size of the room. ACH is a measure of how often air within a space is replaced every hour and is essential to combating contaminated, stale and unhealthy air.

Testing and Monitoring

Smoke tests are a common way to test the effectiveness of a pressurised room. They’re cheap and easy to administer, but aren’t continuous or highly accurate. During a smoke test, technicians create puffs of smoke next to known intakes like registers or under doorways. If the smoke flows inside or outside, then a pressure differential exists. The smoke just needs to move in the right direction. Electronic pressure monitors offer continuous, accurate monitoring, but they’re expensive to purchase and install. Still, accurate testing and consistent monitoring is the best way to maintain the effectiveness of a pressurised room. Inadequate or infrequent testing puts patients and others at risk.

Conclusion

The COVID-19 pandemic has extended the use of pressurised rooms to combat the disease. The idea has extended beyond the hospital room to include waiting rooms, triage, bathrooms and other areas that could contain contaminants or susceptible people.

While pressurised rooms are helpful for health care workers, patients and staff, they also present challenges to HVAC techs and facility managers. Expanding the number and size of pressurised areas in any building means paying more attention to resulting issues like high humidity levels, sticky entryways, mold growth, and increased energy costs. These are new challenges FMs and engineers will need to address as the built environment evolves to meet social change.

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