COVID-19 | VENTILATION GUIDANCE WINTER PROTECTION The cold season brings new challenges for building managers trying to minimise the Covid-19 risk while maintaining occupant comfort. Chris Iddon introduces the latest CIBSE guidance on optimum HVAC strategies for Covid-secure buildings Winter weather will pose a ventilation challenge A t the beginning of the coronavirus pandemic earlier this year, evidence began to emerge of super-spreading events in poorly ventilated indoor spaces that were suggestive of aerosol transmission of the SARS-CoV-2 virus. This led to building services bodies around the globe providing recommendations to improve ventilation. In April, CIBSE issued a statement acknowledging the potential risk of SARS-CoV-2 being transmitted via the aerosol route and, in early May, released guidance for building owners, operators and those who maintain them, including the CIBSE Covid-19 Ventilation Guidance. The principle of the guidance was to ensure that as much outside air as reasonably possible was provided to indoor spaces, to dilute any potential airborne, aerosol-encapsulated virus. With the UK entering spring and summer, there was, in many cases, the ability to improve outside air provision by opening more vents and windows, and bypassing recirculation systems to ensure mechanical systems were working on full outside air. Government Covid-secure guidance has also noted the importance of good indoor ventilation in reducing transmission of the virus, while HSE guidance specifically references the CIBSE Covid ventilation guidance.1 More recently, Public Health England has acknowledged that SARS-CoV-2 is primarily transmitted between people through respiratory (droplet and aerosol) and contact routes Airborne transmission may also occur in poorly ventilated indoor spaces, particularly if individuals are in the same room together for an extended period of time.2 There has always been an acknowledgement that what is deemed reasonable in the summer may not be reasonable as the weather cools. Evidence has continued to mount to demonstrate that airborne aerosol transmission is most likely in poorly ventilated (and crowded) indoor spaces. It is also acknowledged that ventilation should be weighed against other factors, particularly thermal comfort and energy use. There is a balance of benefits to be considered when determining what may be reasonable with respect to providing as much outside air as possible. It will certainly be different depending on the season. If there is too much thermal discomfort for the occupants, there is a risk of a behavioural response to close or block off ventilation provision, which would be a highly undesirable outcome. The latest version of the CIBSE Covid-19 ventilation guidance explores this further. It emphasises that at the least the ventilation strategy should achieve the equivalent minimum provision of outside air ventilation rate for the space over the occupied period, as defined in current standards, and ventilation rates beyond this should ensure thermal comfort is not significantly compromised. The updated guidance also considers recirculation of exhaust air, advice for modulating natural ventilation openings, the use of CO2 sensors as an indicator of poorly ventilated spaces, and the use of UV air-cleaning systems. Recirculation It is preferable not to recirculate air from one space to another. However, in certain weather conditions, closure of the recirculation dampers in some systems can lead to unsatisfactory temperature conditions of the supply air and, consequently, a reduction in the rate of supply of outside air to the occupied spaces to levels below what is deemed adequate (10Ls-1 per person for typical offices see CIBSE Guide A). In these instances, there is a balance of risks to be considered with recirculation: the increased risk of cross-contamination between rooms or zones and the reduced risk of contaminant build-up as a result of maintaining adequate levels of provision 26 November 2020 www.cibsejournal.com CIBSE Nov 2020 pp26-27 Ventilation guidance.indd 26 23/10/2020 16:27