Proper functioning and operation of ventilation systems during the Covid-19 epidemic

date: 12.11.2020

category: Sporočila za javnost

 

It is important for every epidemic to understand the transmission routes of the infectious agent. For COVID-19 and for many other respiratory viruses three transmission routes are dominant: (1) combined droplet and airborne transmission in 1–2m close contact region arising from droplets and aerosols emitted when sneezing, coughing, singing, shouting, talking and breathing; (2) long-range airborne (aerosol-based) transmission; and (3) surface (fomite) contact through hand–hand, hand–surface, etc. contacts.
Ventilation systems provide a possibility of long-range aerosol-based transmission. An airborne virus is contained inside expelled respiratory fluid droplets. Large droplets fall down, but small droplets stay airborne and can travel long distances carried by airflows in the rooms and in extract air ducts of ventilation systems, as well as in the supply ducts when air is circulated.

The size of a coronavirus particle is 80-160 nanometre, making filters generally an unsuitable solution. The most important is to ensure proper ventilation of spaces with outdoor air, which requires switching air handling units with recirculation to 100% outdoor air. Viral material in extract (return) air ducts may re-enter a building when centralised air handling units are equipped with recirculation sectors. The general recommendation is to avoid central recirculation during SARS-CoV-2 episodes: recirculation dampers should be closed either using the Building Management System or manually. Despite mechanical ventilation regular window opening is essential, especially in buildings without mechanical ventilation systems where window opening is the only way to boost air exchange rates. Windows should be opened for 15 minutes or more when entering the room (especially when the room was occupied by others beforehand).

The Laboratory for Heating, Sanitary, Solar and Air Conditioning Engineering of the University of Ljubljana’s Faculty of Mechanical Engineering has cooperated in the preparation of the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) guidance and recommendations on how to operate HVAC systems; the document was last updated in August 2020 in response to the Covid-19 outbreak.


Below are some of the essential practical measures and recommendations.

PRACTICAL MEASURES: 15 main items, as illustrated in the figure below:

1. Provide adequate ventilation of spaces with outdoor air.
2. Switch ventilation on at nominal speed at least 2 hours before the building opening time and set it to lower speed 2 hours after the building usage time.
3. At nights and weekends, do not switch ventilation off, but keep systems running at a lower speed.
4. Open windows regularly (even in mechanically ventilated buildings).
5. Keep toilet ventilation in operation 24/7.
6. Avoid open windows in toilets to maintain the right direction of ventilation.
7. Instruct building occupants to flush toilets with closed lid.
8. Switch air handling units with recirculation to 100% outdoor air.
9. Inspect heat recovery equipment to be sure that leakages are under control.
10. Adjust fan coil settings to operate so that fans are continuously on.
11. Do not change heating, cooling and possible humidification setpoints.
12. Carry out scheduled duct cleaning as normal (additional cleaning is not required).
13. Replace central outdoor air and extract air filters as normal, according to the maintenance schedule.
14. Regular filter replacement and maintenance works shall be performed with common protective measures including respiratory protection.
15. Introduce an IAQ sensor network that allows occupants and facility managers to monitor that ventilation is operating adequately.
 

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RECOMMENDATIONS:
- Increase air supply and exhaust ventilation
General advice: supply as much outside air as reasonably possible. In buildings with mechanical ventilation systems, extended operation times are recommended for these systems. Adjust the clock times of system timers to start ventilation at the nominal speed at least 2 hours before the building opening time and switch to a lower speed 2 hours after the building usage time.

- Use openable windows more
General recommendation: avoid crowded and poorly ventilated spaces. In buildings without mechanical ventilation systems, it is recommended to actively use openable windows (much more than normal, even when this causes some thermal discomfort). Window opening is thus the only way to boost air exchange rates. Windows should be opened for 15 min or so when entering the room (especially when the room was occupied by others beforehand).

- No use of central recirculation
Viral material in extract (return) air ducts may re-enter a building when centralised air handling units are equipped with recirculation sectors. The general recommendation is to avoid central recirculation during SARS-CoV-2 episodes: close the recirculation dampers either using the Building Management System or manually.

- IAQ monitoring
The risk of indoor cross-contamination via aerosols is very high when rooms are not ventilated well; therefore, it is recommended to install CO2 sensors at the occupied zone that warn against underventilation especially in spaces that are often used for one hour or more by groups of people, such as classrooms, meeting rooms, restaurants. During an epidemic it is recommended to temporarily change the default settings of the traffic light indicator by lowering the point where ventilation is activated in spaces (already at lower CO2 concentrations as usual).


More information about the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) guidance on how to operate HVAC systems is available on:
https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID-19_guidance_document_V3_03082020.pdf
http://web.fs.uni-lj.si/sithok/wp-content/uploads/2020/10/REHVA-COVID-19-smernica_V3.pdf

The solution does not involve complex interventions in building structure, but proper functioning and operation of ventilation systems. The solution is not to turn off ventilation systems, as the amount of virus-laden particles in the breathing zone of a room ventilated with mixing ventilation system is much lower than when the ventilation system is off.

Aerosol concentrations and cross-infection from 1.5m or more from an infected person can be controlled with adequate ventilation and air distribution solutions. The effect of ventilation is illustrated in the following Figure.

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Illustration of how an infected person (the person speaking on the right) leads to aerosol exposure (red spikes) in breathing zone of another (the person on the left in this case). Large droplet exhalation is marked with purple spikes. When the room is ventilated with mixing ventilation system, the amount of virus-laden particles in the breathing zone is much lower than when the ventilation system is off. Left figure: ventilation system on, right figure: ventilation system off.
 

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