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In deciding what to do it is important to think about the infection mechanism, what is known and what is not yet certain.
There are things that can be done with commercial built-up HVAC systems in both the base building system and in tenancy fit outs that will improve their role in mitigating the transmission of the virus. In this it is also important to remember that anything done with the HVAC system to provide a healthier indoor environment is generally a positive outcome for occupants.
The primary objectives are to dilute potential viral aerosol concentrations and move these aerosols away from people.
Increasing ventilation rates, increasing outside air rates and increasing run times will all have an impact on energy usage, both fan energy and thermal energy. If not balanced with energy efficiency measures this will likely materially increase building energy consumption and compromise sustainability ratings.
Air Filtration or Air Cleaning
The objective is to remove contaminated particles and or cleanse the recirculating air.
If aerosols are caught up in recirculating HVAC systems can they be pushed back into an occupied space and infect people? In theory this is possible and whilst there is not substantive evidence to date to suggest this is a significant transmission risk, improving filtration performance will generally improve indoor air quality.
What can be done to improve the air filtration of recirculated air?
Increasing the filter standard may also require an increase in filter replacement frequencies. Generally, air filters in commercial HVAC systems are disposable and not cleaned and reused. Replacement and or cleaning of air filters should only be undertaken by experienced service providers using appropriate Personnel Protective Equipment and approved processes.
– Electrostatic Precipitators that use electrically charged screens to attract particles were common when smoking was permitted in buildings. These can have a filtration efficiency close to HEPA filters with less air flow resistance. They have particular spatial and maintenance requirements and need a power supply. These filters also typically create ozone, potentially at a level that requires the air to be treated with an activated Carbon filter or similar, introducing further resistance into the airflow and introducing an element that requires regular replacement.
In relation to disinfecting or purifying the recirculating air, there are a number of technologies that can be used, to date usually in healthcare or laboratory situations.
Ultra-violet germicidal irradiation (UVGI) can be applied to kill viruses and bacteria. The effectiveness of this depends on the strength of the radiation and the period of time in which it has to impact on particles. This technology can be applied in ductwork and air handing plant, in rooms above the occupied zone, and stand alone in-room recirculating units are also available.
When installed in air handling units the risk of UV exposure to maintenance personnel must be managed with safety interlocks, signage and UV safe viewing portals. UV light can also cause premature degradation of some materials used in air handling units.
The potential effectiveness of UVGI technology will depend on the circumstance of the particular building and its systems, and the method of application. (For a more detailed explanation of UVGI systems refer A.G. Coombs Advisory Note; Ultraviolet Germicidal Irradiation for HVAC Applications)
Ionisation technology can be employed to charge airborne particles including aerosols, causing them to clump together to facilitate filtration or precipitation. Some ionization systems produce ozone as a by-product at levels considered harmful to human health. These systems should be avoided.
The potential effectiveness of ionisation technology will depend on the circumstance of the particular building and its systems, along with the method of application.
Air disinfection or cleansing methods should be considered as part of a broader strategy that includes enhanced ventilation and improved filtration. Consideration should also be given to their ongoing operational and maintenance requirements and associated costs.
The objective is to create conditions that limit the formation of aerosols from larger particles.
Research indicates that particles emitted by breathing can rapidly evaporate and shrink in size when emitted creating smaller and more problematic aerosols. The relative humidity of the air in a space is believed to have an effect on the rate of evaporation. Current advice suggests indoor relative humidity be kept in the range of 40-60%.
Commercial air conditioning systems typically do not directly control relative humidity and could require additional control functions and or adjustment to better assure this range is maintained.
The installation of humidification devices may also be considered.
Improved humidity control should be considered as part of a broader strategy that includes the initiatives described previously, some of which may detrimentally affect humidity.
(For a more detailed explanation of the role of humidity in HVAC refer A.G. Coombs Advisory Note; Humidity Control – Using HVAC to Help Reduce Infection Spread).
This is also an important factor in how HVAC systems can be used to help to potentially mitigate the transmission of the virus. The objective is to improve occupant knowledge and understanding of the role that HVAC plays and provide confidence in the system’s performance.
There are a number of useful initiatives that can be considered to assure and/or improve the ability of HVAC systems to assist in the mitigation of viral transmission:
Many of these initiatives will also increase energy usage and affect building energy and potentially water ratings. Likely implications should be determined ahead of changes along with other efficiency measures that can balance these negative effects.
The best approach for a setting will depend very much on the design and operation of the particular building, its systems and its occupancy. An audit of current conditions and a well-informed approach can deliver high value improvements whilst minimising any effects on energy or water usage.
Matthew Peacock, A.G. Coombs Advisory, Melbourne
P: +61 3 9248 2700 | E: mpeacock@agcoombs.com.au
Andrew Bagnall, A.G. Coombs Advisory, Sydney
P: +61 2 8020 6000 | E: abagnall@agcoombs.com.au
John Bourne, A.G. Coombs Advisory, Brisbane
P: +61 7 3648 0500 | E: jbourne@agcoombs.com.au
Rakesh Ravichandran, A.G. Coombs Advisory, Canberra
P: +61 2 6217 5600 | E: rravichandran@agcoombs.com.au
AIRAH – Information about Coronavirus Disease (COVID-19):
www.airah.org.au/AIRAH/Navigation/Industry_leadership/Coronavirus_ information/Coronavirus_information.aspx
ASHRAE – Coronavirus (COVID-19) Response Resources:
www.ashrae.org/technical-resources/resources
CIBSE – Coronavirus (COVID-19) Advise:
www.cibse.org/Coronavirus-(COVID-19)
McKinsey – Can HVAC systems help prevent transmission of COVID-19?:
www.mckinsey.com/industries/advanced-electronics/our-insights/can-hvac-systems-help-prevent-transmission-of-covid-19
McKinsey – Airflow considerations for businesses during the time of COVID-19:
https://covid-tracker.mckinsey.com/hvac?cid=other-eml-alt-mip-mck&hlkid=7c4573f6283c460f8e58160e2bcc62a9&hctky=12214436&hdpid=8339c23a-cd9b-4bfe-8780-14b5e1a6087f
Morawska, Lidia; Milton, Donald K – It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19):
https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa939/5867798
REHVA – COVID-19 Guidance:
https://www.rehva.eu/activities/covid-19-guidance
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