The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) has published a guidance document for building services in areas with a coronavirus disease outbreak.
The document is targeted at HVAC professionals and facility managers working in commercial and public buildings. It provides practical recommendations for preventing the spread of coronavirus through HVAC and plumbing systems.
Although some of the points in the document have been seen before, there is some interesting new advice.
For HVAC professionals, one of the key questions has been around airborne transmission of COVID-19. Although authorities such as the World Health Organization (WHO) have confirmed that it is mainly spread through contact with larger respiratory droplets (droplet nuclei), research continues around whether smaller particles can remain airborne for long periods and also spread the virus.
The REHVA document notes that although there is no reported evidence yet specifically for COVID-19 infection via this second airborne-particle route, no reported data or studies have ruled it out.
Accordingly, REHVA’s guidance document addresses the risk of airborne transmission through small particles (<5 microns), which may stay airborne for hours and be transported over long distances.
“Such small particles are generated by coughing and talking when larger droplets evaporate usually within milliseconds and desiccate,” says REHVA Research and Technology Committee chair, Professor Jarek Kurnitski.
According to REHVA, the size of a coronavirus particle is 80-160 nanometres (0.08-0.16 microns) and it remains active at common indoor conditions up to 3 hours in indoor air and 2-3 days on surfaces.
Increasing air supply
In line with other industry bodies, REHVA recommends supplying as much fresh outside air as possible.
For buildings with mechanical ventilation systems, REHVA advises changing system timers to start ventilation a couple of hours earlier and switch off later than usual. “[A] better solution is even to keep the ventilation on 24/7, possibly with lowered (but not switched off) ventilation rates when people are absent.”
For buildings with operable windows, REHVA recommends using them much more than usual, “even when this causes some thermal discomfort”.
“Humidification and AC have no practical effect“
REHVA’s document warns about the limitations of humidification and air conditioning in curbing the spread of COVID-19.
It notes that although the spread of some viruses can be limited by changing air temperatures and humidity levels, this is unfortunately not an option for COVID-19.
“The SARS-CoV-2 virus is quite resistant to environmental changes and is susceptible only for a very high relative humidity above 80 per cent and a temperature above 30 ̊C, which are not attainable and acceptable in buildings for other reasons (e.g., thermal comfort).” the document says.
REHVA advises that there is no need to change set-points for humidification systems.
Heat recovery and recirculation
REHVA recommends both turning off rotary heat exchangers and avoiding central recirculation during SARS-CoV-2 episodes.
The guidance says that heat recovery devices may carry over virus attached to particles from the exhaust air side to the supply air side via leaks.
Virus particles in return ducts, it says, may also re-enter a building when centralised air handling units are equipped with recirculation sectors.
Duct cleaning not recommended
REHVA notes that there have been “overreactive statements” recommending to clean ventilation ducts in order to avoid SARS-CoV-2 transmission via ventilation systems.
According to the document, if the guidance about heat recovery and recirculation is followed, this will not be an issue.
Advice on outdoor air filters
According to REHVA, although the size of a coronavirus particle is smaller than the capture area of F8 filters, these small particles will tend to settle on fibres of the filter via diffusion. SARS-CoV-2 particles also aggregate with larger particles which are already within the capture area of filters.
“This implies that in rare cases of virus contaminated outdoor air, fine outdoor air filters provide a reasonable protection for a low concentration and occasionally spread viruses in outdoor air,” says the document.
REHVA represents a network of more than 120,000 engineers from 27 European countries. The REHVA guidance complements the general COVID-19 guidance for employers and building owners provided by the World Health Organization (WHO).
The COVID-19 REHVA guidance document is available here.