REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, released its guidance for the use and operation of building services in areas affected by COVID-19. REHVA's recommendations are intended for commercial and public buildings and are consistent with the information provided for employers by WHO.

[Read about the COVID-19 guidance from the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) here.]

Professor Jarek Kurnitski, Estonian Research Council. Source: TalTechProfessor Jarek Kurnitski, Estonian Research Council. Source: TalTechThe document addresses the transmission of the virus via small particles, less than 5 microns, which are generated by talking or coughing and can stay airborne for hours and travel long distances, unlike larger droplets that evaporate and desiccate within milliseconds, according to REHVA Research and Technology Committee chair, professor Jarek Kurnitski from Tallinn University of Technology. "The size of a coronavirus particle is 80-160 nanometres and it remains active at common indoor conditions up to 3 hours in indoor air and 2-3 days on the room's surfaces." He added that increased ventilation can remove more particles and that surface contact is another transmission route, while toilet flushing creates a special risk because by creating plumes containing droplets and droplet residue.

Avoid recirculation of air

The document recommends ventilation system operation times, use of window airing and safe use of heat recovery sections. Recommendations also address the importance of restricting recirculation in central ventilation systems because virus particles in return ducts can re-enter a building through the recirculation portions of centralized air handling units. Specifically, REHVA recommends closing dampers and, when possible, turning off decentralized systems such as fan coil units that use recirculation. In addition, heat recovery devices may carry over the virus attached to particles from the exhaust side to the supply side. In rotary heat exchangers, particles deposit on the return side and can then be resuspended when the heat exchanger turns to the supply side. Therefore turning off rotary heat exchangers temporarily, during SARS-CoV-2 episodes, is recommended.

Extend the operation of ventilation systems

REHVA guidance recommends, if possible, to keep the ventilation system running 24/7, possibly lowering the ventilation rates when the building is empty, rather than turning it off. At a minimum, it advises turning ventilation systems on a couple of hours earlier than usual and extending operation at the end of the day.

Operable windows should be used in buildings without mechanical ventilation systems, specifically opening windows for 15 minutes upon entering the rooms. This is especially important if the room had been occupied by others. Windows can boost ventilation in buildings with mechanical ventilation as well, but should not be opened in toilets to avoid contaminated airflow to other rooms.

REHVA recommendations do not include environmental changes because the SARS-CoV-2 virus is resistant to changes to temperature and humidity levels, other than extremes that are either not attainable in most buildings or are unacceptable for comfort or other reasons. REHVA also advises that replacing outdoor air filters and duct cleaning are not effective against room-to-room infection if the guidance regarding heat recovery and recirculation is followed.

For more information, review the COVID-19 — REHVA Guidance document. Additional information is available from Professor Jarek Kurnitski at

REHVA represents more than 120,000 engineers from 27 European countries.