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How HVAC Systems Help Mitigate Pathogen Transmission

To say the last year has changed the world would be a large understatement, it’s made us appreciate the small things like hugs and high-fives, but it’s also made us take a closer look at how we, as engineers, can better each building we design. There has been a major focus on how HVAC systems can contribute to or prevent pathogen transmission, including filtration, air flow rate, fresh air circulation, and air exchanges in any given space.

Obernel has been closely following research produced by ASHRAE over the last year and we want to share some important insights with you. This information has come from case studies of pathogen transmission through the COVID-19 pandemic. Some of these are things that we think about inside a healthcare system setting already but aren’t typically introduced in a commercial or educational setting. Or they’re things that ASHRAE has taken a deeper dive into to help us all understand our systems better for healthier indoor air quality.

Don’t worry – we’re not going to try to make you understand intense engineering jargon or overwhelm you with a lot of data to make your head spin. We’ve broken down the information studied over the last year and turned it into applicable solutions for existing buildings and things to think about when building new!

Existing Building Considerations

Ventilation, Filtration, + Air Cleaning

  • Maintain the recommended amount of outdoor air flow for the ventilation system – outdoor air mixed with recirculated air help reduce the pathogens in the air, but you don’t want to overwhelm your ventilation system with too much fresh air
  • Use a combination of filters and air cleaners to achieve an optimal indoor air quality benchmark – like most things in life, they work better together so both filters and air cleaners are best. Pssst… remember to only use proven effective air cleaners
  • Select your set controls options to reduce risk while minimizing energy use – this is tricky because many solutions to mitigating pathogen transmission increases your energy cost, but it’s important to work with your system to find the balance of reducing risk of transmission and not adding a large increase to your energy costs

Air Distribution

  • Promote mixing of air within a space, but avoid strong air currents – stagnant air isn’t good because pathogens can linger in the air within a space, but too strong of air currents can also increase direct transmission from person to person

HVAC System Operation

  Example: Relative Humidity

  • Maintain temperature and humidity set points – we know that there is an optimal air temperature comfort range within an indoor environment, but did you know there is optimal humidity too? It’s a little bit like Goldilocks and the Three Bears. Too low increases the risk of respiratory infections and bacteria. Too high promotes the growth of mold and fungi. You need it to be just right for overall health.
  • Flush spaces between high occupancy uses – generally it takes 3 full air exchanges to properly flush your space of pathogens. It’s preferred, if you can increase the amount of outside air when flushing the space.
  • Limit the amount of contaminated air that may re-enter the building through systems such as heat recovery – it’s nearly impossible to not have reused air, but it’s important to limit or allow the smallest amount to re-enter your buildings

System Commissioning

  • Verify that HVAC systems are functioning as designed – this is a big one! When your building / space is shiny and new, you know everything is working at its highest level, but as time goes by, you need to have your system re-commissioned and tested.
  • Walk the building with your building techs – use the resources at hand to check on your systems and spaces for functionality
  • Call a mechanical engineer – Obernel has commissioning services in house and we are HVAC gurus. We are happy to work with building owners to test and come up with solutions to get your HVAC system performing like new again for improved indoor air quality for the occupants

New Building Considerations

Example: Induction Displacement

Induction Displacement (What?!) – Simply put – it’s a strategy of using the science behind cold air sinks and hot air rises.

  • Displacement Ventilation discharges chilled air at floor level around the occupant’s feet, but not too cold that it is noticeable or uncomfortable
  • The air warms as it comes in contact with occupants in the space and begins to rise.
  • Since the air is warmed as it goes up, it stays at the top of the room and is drawn out by the return air or exhaust at the ceiling – there is no forced air circulation just air moving up from floor to ceiling

Underfloor Air Circulation – Exactly what it sounds like

  • Air system in the floor that pushes air up through vents in the floor
  • The air is then drawn out of the space with vents in the ceiling
  • This can be seen often in casinos that allow smoking to keep the air clear

Radiant Heating with Underfloor Air Circulation

  • Underfloor air circulation (see above) + radiant heat from above to heat occupants

There are a few other new system ideas for consideration when working on a new building, but we’re not as keen about them or they’re a bit more particular than we want to dive in to. But moral of the story is, if you can start from scratch in a new building or space there are ways for you to set your occupants up with an active approach to reducing pathogen transmission through your HVAC system.

A key factor in both existing buildings and new buildings is your filter effectiveness. Most pathogens are very small, smaller than what most filters effectively filter out of the air – but the good news is that they are rarely in the air alone. They attach themselves to larger air particles, like dust, that are effectively filtered out of the air making air filters a great defense in pathogen transmission. It’s important to regularly change your filters and make sure you have the most effective filter that can fit your system. Not sure what that is? Check with your facilities team, check with your system’s rep or ask an MEP engineer!

Improved and healthier indoor air quality is a hot topic right now, but it’s a permanent focus across all markets for Obernel. The considerations shared are ever evolving and Obernel consistently follows the data so we can provide the best service and solutions to our clients and their spaces.

Obernel is continually looking for ways we can go Beyond the Plan, and client resources and education is one of them. If you want more information or have questions about HVAC systems and indoor air quality, please reach out! We’re here to help!


Ned Rector, PE, LEED BD+C, CEM

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