COVID-19 guidance from REHVA

Written by Eurac Research

As a response to the coronavirus pandemic, REHVA (Federation of European Heating, Ventilation and Air Conditioning Associations) have developed a guidance document on how to use building services in order to prevent the spread of COVID-19 through HVAC or plumbing systems.

In order to provide with effective measures on building services, it is important, first of all, to identify the transmission routes of an infectious agent. The two main transmission routes of COVID-19 are: via large droplets (e.g. through sneezing, coughing and talking) and via surface contact.

Via air there are two exposure mechanism. The first one may happen with close contact transmission through large droplets (> 10 microns) which can fall on surfaces or objects within 1-2 meters from the infected person. In this case, it is possible to get in contact with the virus by touching the contaminated surfaces or objects. It is also possible to get in contact with these large droplets when standing in front of an infected person within 1-2 meters.

The second mechanism may happen with airborne transmission through small particles (< 5 microns) which can last in the air for hours and move for long distances. SARS-CoV-2 remains active for up to 3 hours in indoor air and 2-3 days on surfaces in common indoor conditions. Hence, these particles can travel long distances in rooms and in the extract air ducts of ventilation systems. Several recent studies and the WHO have acknowledged this airborne transmission of SARS-CoV-2 under certain circumstances, such as when talking to people in a closed environment, or through aerosol transmission.

Taking into account these considerations regarding the transmission routes of COVID-19, REHVA have identified several practical recommendations for building services operation which will be reported below.

In buildings with mechanical ventilation systems extended operation times are recommended. In particular, it is suggested to start ventilation at nominal speed at least 2 hours before the actual use of the building and change to lower speed 2 hours after the use of the building. It is also recommended to keep ventilation 24/7 (at low speed) even in buildings which are currently empty due to the pandemic. The aim is therefore to supply as much outdoor air as possible. On the other hand, in case of buildings with no mechanical ventilation systems, they recommend using operable windows as much as possible.

Although transmission of some viruses may be slowed down or halted by changing indoor humidity and temperature, COVID-19 has resulted to be quite resistant and thus very difficult to inactivate through this procedure in indoor environments. Hence, humidification is not a method to reduce the viability of SARS-CoV-2.

Heat recovery devices my carry over viruses attached to particles from the exhaust air side to the supply air side via leaks, particularly when the rotary heat exchangers have not properly been installed. It is recommended that high ventilation rates are set up in order to decrease carry-over leakage. The guidance also adds that virus particles transmission via heat recovery device is not a problem when HVAC systems are equipped with twin coil unit.

Virus particles in return ducts may get back inside buildings when centralized air handling units have recirculation sectors. Hence, it is recommended to avoid central recirculation during SARS-CoV-2 episodes. It is also highlighted that duct cleaning is not effective in case of room-to-room infection because the ventilation system is not a contamination source, as long as heat recovery and recirculation recommendations are followed.

REHVA experts also state that there should not be an extraordinary filter replacement for COVID-19 apart from normal maintenance as clogged filters are not a contamination source, although they may decrease the supply airflow thus having a negative effect on indoor contamination.