This post has been updated.
In most office buildings and North American homes, the air coming out of the ducts is mostly recirculated, with a small proportion of fresh air added in office buildings. In homes, the fresh air is often added just through leaks and the occasionally opened window. In an earlier post titled Can building science help reduce the risk of COVID-19? I noted that this probably wasn't too much of a problem in the current crisis, because the evidence at the time was that the droplets carrying the virus were not airborne. I quoted an expert who said, "As far as researchers know now, air is not the vector by which the virus spreads."
But as is happening so often in this pandemic crisis, everyone is learning as we go along. In a long post, Justin Bere and Bere: Architects write that in Europe, REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, is calling for engineers to stop recirculating air in buildings where there has been 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.
REHVA notes that the two-meter rule was based on analysis of large droplets:
However, some recent research indicates that large droplets from sneezing can travel much further than 2 metres, even if there are no air movements. 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.
In most buildings, the key function of the HVAC system is heating or cooling; conventional buildings need a lot of air to do this, so it mostly is recirculating since bringing in fresh air and tempering it would take way too much energy. As buildings get more efficient, the "hygiene ventilation" or fresh air becomes more dominant, as less is needed to heat and cool. As noted in the previous post, in Passive House designs, all the air supplied in ducts is hygiene ventilation, fresh air brought in from the outside. Any heating or cooling required is done separately. This now appears to be a really smart and prescient idea.
If American engineers take the European advice, they will have a serious problem with just about every existing building. The systems might cope in spring or fall when the outside air is close to the same temperature as inside air, but I don't think any system is designed to run on 100 percent fresh air. In our homes, it means that people who have someone who is in quarantine or are sick in the house have to turn off their forced air furnaces or central air conditioning. Most office buildings might have to shut down; you can't just stick a HEPA filter in, it blocks too much air movement. And of course, in most office buildings you can't open the windows.
On the other hand, it might mean that we have to build much more energy-efficient buildings to reduce the amount of air required; as Bere: Architects note:
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.
Bere has a couple of recommendations, including checking existing buildings to see which are a public health risk, and to "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 the 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."
UPDATE: Engineer Ted Kesik, Professor of Building Science, Daniels Faculty of Architecture, Landscape and Design, University of Toronto (my alma mater), has a few comments:
Dedicated outside air systems (DOAS) with energy recovery is the way to go, preferably displacement ventilation systems that introduce fresh air low and exhaust stale air high, so that people get the good stuff in the occupied zone and the rising air carries the contaminants up to the exhaust registers and out.
As for the claim this would increase energy consumed in buildings 4X, the example that was given in one of the reader responses dealt with the business-as-usual, code minimum, energy pig types of buildings that are prevalent in North America. If we build buildings that rely more on passive rather than active systems and design them to high levels of energy efficiency, the conditioning of ventilation air is not such a big deal. The problem with dilution ventilation systems is that you are ventilating whenever heating or cooling is required, even if there is nobody in the building. We provide ventilation for inhabitants and so by providing zoned DOAS with energy recovery and carbon dioxide sensors (DCV - demand controlled ventilation), we only ventilate when the building is occupied. For example, I was involved with the design of an elementary school that had a DOAS with energy recovery serving each classroom and these only operated during classroom hours when occupied (9 AM to 3 PM) instead of 24/7. That is an enormous reduction in ventilation heating/cooling load just through the intelligent application of design and the selection of appropriate equipment. And the teachers found they had never experienced better indoor air quality in a classroom. Getting it right in new buildings is a no-brainer and our codes and standards should prevent unsatisfactory practices.
But we shall be greatly challenged retrofitting our existing buildings to eliminate dilution ventilation systems, yet it must be recognized these pose health and safety risks not just from COVID-19, but also from global warming and climate change. We may suppress the pandemic but climate change is not going away and a big part of our building energy usage in North America results from dilution ventilation systems. The good news is that comprehensive building retrofits represent less carbon footprint than new buildings, not to mention the avoidance of enormous amounts of embodied carbon wasted through demolition. We’ve made a mess and now the responsible thing to do is clean it up, rather than passing on our problems to future generations.