Controlling airflow velocities, direction, and patterns—key to stopping the spread of the virus
As I write this, COVID-19 cases are soaring around the world. The best way to avoid infection is to follow the guidelines provided by the public health community as they evolve. The current advice is 1) wear face masks, 2) practice physical distancing and avoid gathering in large groups, 3) wash hands and sanitize surfaces and, 4) avoid unnecessary travel.
These strategies will help reduce the virus spread, not prevent it. Educating yourself as much as possible by following credible sources can help you against this particular beast that has been stalking us for almost a year now.
With accurate information, you will be able to identify hazardous situations and conditions, helping you to take steps to protect yourself.
I have been studying ways the HVAC industry can engage this enemy since day one. The American Society of Heating and Refrigeration Engineers (ASHRAE) is one good resource to use.
Experts are unsure of the viral “shedding rate” or the “infectious dose” and how either of these factors vary. They do know the health risks related to viral concentration in the air vs. how long it lingers, and in the length of time people are exposed. What we industrial combustion people call “dwell time” and “soak time.”
We know enough to take meaningful action to help reduce infectious spread. Early on in the pandemic, ASHRAE published the following statements based on evidence from peer-reviewed studies of a number of real-life spreader events, and lessons learned from the previous SARS and influenza outbreaks.
“Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating ventilation, and air conditioning systems, can reduce airborne exposures.” In addition, the association also reported that “Ventilation and filtration provided by heating, ventilating, and air conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air. Unconditioned spaces can cause thermal stress to people that may be directly life-threatening and that may also lower resistance to infection. In general, disabling of heating, not a recommended measure to reduce the transmission of the virus.”
So those are our marching orders troops— let’s see how we can help out.
Information from ASHRAE and others provide us with practical weapons we can utilize. Transmission via inhalation is increased in poorly ventilated spaces where there are many people. Community “super spreader” events clearly indicate aerosol transfer of this pathogen. The degree to which HVAC systems might contribute to spreading is unclear, however the consensus among industry practitioners is that we should take ASHRAE’s advice. Remember, none of these strategies provide 100 per cent protection. There is no prescriptive route to achieving a viral free environment other than to remove the threat completely by preventing infectious persons or objects from entering a space in the first place.
Further information on ASHRAE’s response to the pandemic can be found at: https://www.ashrae.org/technicalresources/resources.
To reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air, ASHRAE suggests the following strategies be employed in HVAC systems:
Let the fresh air in
Increase outdoor air delivered into rooms (which requires the simultaneous exhausting of an equal amount of stale indoor air). Most ventilation air strategies balance the need for fresh air against the cost of heating or cooling it. Although costs will go up, increasing the amount of fresh air by opening dampers and running fans longer is one of the easiest ways to enhance dilution. Cold Canadian winters limit how far we can go with this strategy and supplemental heat may be required.
Use multi-pronged air exhaust strategies
Exhausting indoor air removes some of the viral particles from the occupied spaces, but its effectiveness varies with system design. Most exhaust fans remove a portion of the fresh air along with stale air, so it can take longer than you might think to get rid of most of the bad stuff. Exhaust methods depend on existing structural details, mechanical equipment, and facility usage.
Spot ventilation to remove contaminants at source is popular, while general exhaust and ventilation has become common with the advent of HRV and ERV devices. In most cases, people living in single-family residences are already in the safest environments for avoiding COVID-19 and don’t need to take any extra precautions. In multi-family or commercial/institutional structures, extended use of exhaust fans will be helpful. Running HVAC systems continually, or for extended pre- and post-purge unoccupied periods is recommended. Be cognizant of preventing component freeze-up and maintain comfortable indoor air temperature during occupied times, and always provide equal volumes of fresh air make-up to maintain a neutral building pressure.
Beware of airflow
Control airflow velocities, direction, and patterns. Airborne particles become entrained in moving air based on particle size and weight vs. air velocity. Keeping air velocities lowest in breathing zones should reduce dispersal of respiratory droplets between occupants. Room air distribution should be examined closely. To avoid blowing a stream of droplets or aerosolized viruses onto occupants, take note of how diffusers are directing air streams and try to determine airflow patterns and adjust accordingly or otherwise divert. Accurately mapping room air currents is very difficult, but a common sense evaluation of diffuser and return air placement may identify areas of concern. From the beginning of this mess, I have been trying to stay out of poorly ventilated spaces and avoid being downwind of other persons. My neck is sore from looking up at ceilings to see where the diffusers are, but I think it helps.
Filtration gold standard
Improve filtration/treatment of recycled air. During heating and cooling seasons, most conditioned spaces require indoor air recirculation. For central system filtration, fine media filters can capture many of the viral particles, further increasing dilution. MERV 13 or better fibrous filters are recommended provided they do not cause unacceptable restriction in airflow, and they should be inspected regularly for blockage. The gold standard is HEPA filtration, but most heating and cooling equipment cannot handle the high pressure drops of these filters. System bypass HEPA filters or in-room stand-alone units have their own fans to drive air through the filter and are very effective in capturing viral sized particles. Sub-HEPA in-room units using MERV 13 or better media appear to be effective as well.
Germicidal ultraviolet light devices can be effective in helping to stop viral growth, but they are not a magic bullet and should only form one part of your attack. Experts in their utilization should be consulted. Manual disinfection procedures can kill viruses inside ductwork and on coil surfaces, but duct systems need to be designed for these procedures and it is unlikely you will see this strategy used outside of hospitals.
Timing is everything
Increase duration of system operation beyond occupied hours (pre-purge and post-purge of indoor air from the space). This is a strategy that is best utilized in commercial and institutional settings where there are defined occupied/unoccupied periods. Many of these facilities have building automation systems that can be re-programmed to open outdoor air dampers and operate fans while monitoring for minimum temperature protection.
Around 40 to 60 per cent relative humidity is recommended to decrease risk of infection, but be cognizant of outdoor temperature and building envelope dewpoint to avoid condensation and potential structural damage and mould growth.
Here we go, out of space yet again and I haven’t really gotten into the nuts-and-bolts suggestions I intended to explore. Look for Part Two in the March issue.