To minimise the risk of catching Covid-19, the UK government advises us to wash our hands and to keep two metres apart from people that we don’t live with. The distance rule is of course a crude ‘rule-of-thumb’, because air is not static, either outdoors or inside buildings. So, should the rules change if you are downwind of someone outdoors, or should they change if your office building is recirculating used air back into occupied spaces as has been common practice for years in the design of Heating Ventilation and Air Conditioning (HVAC) systems in offices, public buildings, supermarkets, and perhaps also cruise ships, trains and aircraft?


REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, has just called for engineers to stop recirculating air in buildings in areas with a Covid-19 outbreak. According to CIBSE Journal, April 2020, REHVA’s interim guidance on the operation and use of building services with a coronavirus disease, states that virus particles in return ducts can re-enter a building if centralised air handling units have recirculation. It recommends avoiding central recirculation during SARS CoV-2 episodes and closing the recirculation dampers, even if there are return air filters. As the REHVA guidance says, these don’t normally filter out viruses. REHVA’s guidance also recommends local fan coil systems should be turned off to avoid resuspension of particles at room level.


It seems reasonable, appropriate and urgent to consider the risk that recirculation of air is a potential link between an infected person and other occupants or passengers of a building or cruise ship or aircraft. If REHVA’s assessment of the risk of air-recirculation is correct, the implications are serious. Their advice, which was published on April 3rd 2020, came after Covid-19 had become a worldwide pandemic and cost tens of thousands of lives. It’s not yet clear how serious the risk of air-recirculation is, nor now how many engineers, building managers and owners are yet considering the health-risk of air-recirculation in a Covid-19 pandemic, let alone if they have taken action to change the ventilation controls on their buildings. Governments and the construction industry are so far silent on this topic too.


The air recirculation principle has 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 heating power needed only through ‘hygiene ventilation’, ie 100% fresh air ventilation flow. Common buildings need way more heating power. For example, a Passive House home, ventilated at 0.5ach can be heated through hygiene ventilation, whereas you need around 3ach to heat a common new build by warm air. When you pump fresh air into a building, the same amount of air must of course go out of the building to avoid it pressurising like a balloon. Where a lot of heated air is required, bringing this in as fresh air would involve losing the same amount of warmed air out of the building. Good heat recovery would be difficult or expensive to achieve for high air volumes, so air-recirculation is used to reduce the energy wastage of pumping large volumes of indoor air out of the building.


CIBSE Journal reports that there are two main transmission routes of the Covid-19 virus:

  • Via large droplets/particles emitted when sneezing coughing or talking
  • Via surface contact (hand to hand, hand to surface etc)


These two transmission routes appear to be the drivers of the government advice to keep two metres apart from other people, and to wash hands thoroughly.


But there are other factors raised by REHVA and highlighted by the CIBSE Journal:


  1. The World Health Organisation (WHO) recognises the risk of dried out floor drains in buildings. In the SARS 2003-04 outbreak, the disease was spread in a Hong Kong apartment building by a dried-out floor drain. Drain traps in high-rise buildings are susceptible to being blown out by wind pressure.
  2. The two-metre rule is based on large droplets (>10microns) that may travel 1-2 metres from an infected person before falling on surfaces such as desks and tables. However, some recent research indicates that large droplets from sneezing can travel much further than 2 metres, even if there are no air movements.
  3. Small particles (<5 microns), generated by coughing and sneezing, may stay airborne for hours according to the REHVA guidance, 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.


Anyone who has tried to stop builder’s dust spreading from one part of a building to other parts of a building, will know how difficult it is to stop the spread of particles. Outside air pressure causes internal air movements across a building. REHVA point out that toilets with open windows may cause contaminated airflow from toilets to other spaces, even where extractor fans are installed.


But what about deliberate recirculation of used and potentially contaminated air, back into the building from which it was extracted?  I expect that most people will not realise that this is a relatively common design feature on land, sea and air; wherever there are conventional air handling units.


But why do building services designers recirculate air in buildings? The answer is that they do this to save energy in buildings where the heat demand is too great for 100% fresh-air cross-flow heat exchangers. But this practice must now stop according to REHVA, at least until the Covid-19 pandemic is completely over. So, until the virus is completely stamped out, world-wide energy use due to buildings is set to climb steeply if air-recirculation is not allowed in conventional buildings.


So, what is the technical explanation for the recirculation of air in buildings, and perhaps also in cruise ships and aircraft? Peak heat losses and peak heat gains are severe in poorly insulated accommodation, or where there are large areas of glass. Mechanical and electrical services engineers are tasked with creating comfortable conditions for occupants in all conditions and at all times of the year. Services engineers can overcome fabric efficiency problems with powerful heating and cooling equipment, but this comes with a large energy cost. To try to avoid runaway energy costs, CO2 sensors are fitted, and if the extracted air isn’t regarded as too stale, then it’s commonly piped back into the building. Some designers of conventional buildings heated in this way even call this an energy efficient approach and may even label it as ‘heat recovery ventilation’ which seems odd to say the least.


I believe that it is common practice to try to mitigate the health risks of this approach to design by passing recirculated air through filters before it returns to the occupied spaces. But even the highest grade of filter that I believe is generally recommended for this purpose, an F7 ‘pollen’ grade of filter, while quite effective at filtering particles of 1-10 microns out of the air (eg pollen and diesel particulates), cannot capture a single virus particle or a small cluster of virus particles. A Coronavirus particle is only 0.8 to 0.16 microns diameter, so even if a high quality F7 filter is used, the virus will slip through the filter unless it is part of a cluster of particles larger than 1 micron. This is why REHVA have effectively banned recirculation of air during a Coronavirus pandemic.


So, is there another way of keeping buildings comfortable and energy efficient without recirculating air?


Fortunately, building designers can play a part in more effectively protecting communities from the spread of disease by specifying a tried and tested alternative approach for domestic and commercial buildings to save energy without air-recirculation. It is by specifying a very energy-efficient building fabric in combination with 100% fresh air Heat Recovery Ventilation (also known as MVHR).


There are two technologies for delivering heat recovery ventilation with up to 90% efficiency:

  1. Via a cross-flow heat exchanger
  2. Via a rotary heat exchanger


A cross flow heat exchanger is the best solution of the two, because it will never allow the fresh air supply to be contaminated by the stale, extracted air. A rotary heat exchanger presents the risk of contamination of the fresh air supply by droplets of condensation formed out of the extracted air. The risk is normally considered very small, but where a deadly virus is concerned, perhaps even this small risk is too much. We should therefore use the cross-flow system if at all possible.


So Covid-19 should be a wake-up call for the building industry, for governments and for statutory regulators of the construction industry. It’s only when building heat load is very small (such as in a Passive House building) that ‘hygiene ventilation’ is possible. Hygiene ventilation is a term used to describe 100% fresh air supply (no recirculation) that is provided in the quantity needed for optimal human health. In a building with very high-quality fabric designed for low heat losses, and without excessive window areas, and with summer night natural air-cooling, then a cross-flow heat exchanger is the perfect solution for comfort, economy and ample fresh air through the heating season.


We would also recommend regulator act urgently on the following:


  1. Urgent production and publication of an inventory of buildings that recirculate air in order to find out which buildings potentially present a public health risk. This includes air-recirculation in offices, public buildings, exhibition buildings, (hopefully not those converted to hospitals), supermarkets and perhaps even the various forms of transport on land, sea and air.
  2. Consider mandatory modification of buildings that present a risk according to the REHVA advice, before they are reoccupied, or a satisfactory risk assessment has been approved by a regulatory authority.
  3. Set up a task force to compare the return on investment of designing buildings 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. Purpose of the taskforce being to consider a much tougher mandatory fabric efficiency standard instead of air-recirculation in the design of all new buildings.
  4. Check the adequacy of regulations with regard to water-traps in drainage systems, especially tall buildings, and consider regulating for the use of waterless traps.



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