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Increasing evidence of airborne and aerosol infection of Covid-19 means recirculated air should be cut

Justin Bere

As Covid-19-conscious HVAC associations call for an end to recirculated air in buildings, the system of fresh air with heat recovery for building seems essential

One of the many debates concerning how to combat the Covid-19 virus centres on transmission, which has considerable implications for how we treat air quality in buildings. We already know that the large droplets/particles (up to 10 microns) emitted when sneezing, coughing or talking are mostly transmitted either through the air or via surface contact (hand to hand, hand to surface etc).

Some of the large droplets will be directly inhaled by other people, potentially lethally, but most of these larger particles fall to horizontal surfaces within a few metres of an infected person. From here they can cause hand-to-face infection. There is also a faecal-oral route of infection via bathrooms and faulty or dried-out drain-traps.

The WHO is starting to acknowledge the importance of airborne transmission. The REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, has already called for engineers to stop recirculating air in buildings in areas with a Covid-19 outbreak. It points out that small particles (up to 5 microns), generated by coughing and sneezing, may stay airborne for hours, and can travel long distances. A coronavirus particle is only 0.8 to 0.16 microns diameter so there could be many virus particles in a 5 micron droplet floating around in the air, due to an airborne mechanism that is known for SARS-CoV-1. Recent research which shows the impact of aerosol transmission is summarised in this El Pais article

Unnecessary and unhygenic

Air recirculation is not even necessary. In 2010/11, I acted as an independent client advisor on a large public building designed by others with a client brief that had ‘low energy’ aspirations but was otherwise quite conventional in its services design. I was surprised and concerned at the services engineer’s decision to recirculate air around the building in order to save energy. I argued that air-recirculation was fundamentally unhygienic. I knew by that time that it was possible to design extraordinarily energy efficient buildings with 100% fresh air supply (no recirculation). But the fabric of the building would have needed significant improvement to supply heating economically using 100% fresh air with heat recovery.  The easy solution for the design team was to convince the client that air-recirculation was perfectly normal, and nobody had died from this. The situation has now become more serious.

So why is air recirculated? There is one main objective: increase air flow to allow more heating power. This is different to a very efficient building where the heating power is so low that we can reach the requirement through ‘hygiene ventilation’, ie 100% fresh air ventilation flow. Conventional buildings need way more heating power. For example, a passive house, ventilated at 0.5 air changes per hour (ach), can be heated through hygiene ventilation, whereas you need around 3ach to heat a conventional newbuild home by warm air. When you pump fresh air into a building, the same amount of air must of course go out to avoid it pressurising like a balloon. During the heating season, introducing fresh heated air would involve expelling the same amount of already warmed air. Good heat recovery would be difficult or expensive to achieve for high air volumes, so recirculation is used in conventional buildings to reduce the energy waste that pumping large volumes of warmed indoor air out of the building would entail.

A passive house, ventilated at 0.5 air changes per hour (ach), can be heated through hygiene ventilation, whereas you need around 3ach to heat a conventional newbuild home by warm air

Devised and developed for his own prototype home by Professor Wolfgang Feist of the Passivhaus Institute at Darmstadt, Germany, 100% fresh air heat recovery ventilation is an alternative, and completely hygienic, concept. The technique is now also widely applied to both domestic and large public PH buildings. At the heart of Feist’s concept is the principle that a building can be kept warm ‘passively’ in winter by using the existing internal heat sources and the solar energy entering through the windows as well as by the minimal heating of incoming fresh air. At the gentle flow rates of fresh air needed for hygienic ventilation, a cross-flow heat exchanger with heat recovery of around 90% is usually possible, using negligible fan power.

Recirculation risks

By contrast, to maintain air-quality, HVAC systems that provide heating for conventional buildings often use CO2 sensors on the extract air. During the heating season, if the air from habitable spaces contains more CO2 gas (from people’s breath) than the set point, then that extract air is exhausted and has to be replaced by fresh warmed air. However, it isn’t known if the usual winter set point for CO2 is appropriate in a Covid-19 pandemic. Where an existing HVAC system is recirculating air containing small clusters of SARS-CoV-2 virus, the risks of air-recirculation need to be quantified. In the meantime, REHVA advises avoiding central air-recirculation during Covid-19 episodes and closing the recirculation dampers, even if there are return air filters. As the REHVA guidance says, filters don’t normally block viruses. REHVA’s guidance also recommends that local fan coil systems should be turned off to avoid resuspension of particles at room level.

In view of REHVA’s advice, I suggest that architects that have HVAC systems in their completed buildings should speak with their services engineers about following the REHVA guidance, and should ensure that appropriate advice is passed on to building owners for immediate action.

But what about current designs for conventional buildings with HVAC and other air-recirculating systems that have not yet been built? I believe that design risk assessments should be updated to consider the risk of spreading infection through air-recirculation in the case of a Covid-19 episode. If current and future risks include virus episodes during winter months, it follows that designers of conventional offices, schools, supermarkets, government buildings, galleries, exhibition halls, health facilities, cruise ships etc might now need to consider the risk to occupants of air-recirculation systems, together with the cost of throwing large amounts of heat energy away without heat recovery whenever there is a virus risk, or alternatively risking the comfort of building occupants.

The revised risk assessment should also examine the return-on-investment potential of designing very much better-quality building fabric so that 100% fresh air supply can economically reduce present and future risks of building occupants and owners.

Suggested action list for architects

  • Urgently produce an inventory of completed buildings that recirculate air in order to find out which potentially present a public health risk. At-risk buildings may include, for example, offices, schools, public buildings, exhibition buildings or supermarkets with HVAC systems.
  • Consider modification of buildings or building controls that present a risk according to the REHVA advice, before reoccupiion after lockdown, or until after a satisfactory risk assessment has been approved by a building owner.
  • Consider the risk of spreading the virus by means of lifts and emergency escape staircases of tall buildings.
  • Check the adequacy of designs with regard to water-traps in drainage systems of tall buildings. Drain traps in high-rise buildings are susceptible to being blown out by wind pressure.
  •  In early-stage design of future projects, compare the return on investment of designing structures with excellent quality fabric, such as Passive House buildings which enable supply of 100% fresh air, with the cost of less efficient buildings that cannot supply 100% fresh air economically and are, at times when there is a perceived health risk, not allowed to recirculate air.

More on REHVA’s guidanceCIBSE guidance on ventilation and the reopening of offices.

Justin Bere is founder of bere:architects

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