Ventilation and Transmission: HVAC and Adapting to COVID-19
Effective heating, ventilation and air conditioning systems have always been part of maintaining a healthy building environment, and with the impact of COVID-19 and the unique way the virus is spread, it has never been more imperative that HVAC plays a vital role in keeping occupants of buildings safe, especially as people begin to return to the office and other commercial environments.
COVID-19 has three known contamination routes. First of all, there is person-to-person transmission, which could be indirect too, if the virus travels from someone to a surface they have touched, which is then touched by another person. Then there is airborne transmission. The British Council for Offices’ Thoughts on Office Design and Operation After COVID-19 document talks of large droplets, greater than 10 micrometers, “expelled by sneezing and coughing and in still air, typically within about 2 meters of the infected person.” But Dr. Linsey Marr, the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech, speaking to the New Scientist says that people emit thousands of times more smaller droplets than larger ones. She thinks that it is these ones that infect people with COVID-19. Then there’s the third contamination route: fecal to oral whereby particles from the toilet can enter people’s respiratory systems when using bathrooms.
Counteracting COVID-19 Transmission in an Office Environment
First, we should look at the methods of transmission and then evaluate methods to counteract them. The risk of the virus spreading from person-to-person can be lessened where there is a focus on smart technology. This begins upon arrival at a building, with the use of touchless entry systems, for instance harnessing facial recognition technology. Once inside, staff could then be directed to an area of the office that isn’t already occupied via digital signage or an app. And instead of manually pressing a button, information from the employee’s ID pass about which floor they work on can be read by a card reader, activating the elevator.
The risk of the virus spreading from person-to-person can be lessened where there is a focus on smart technology.
To dilute airborne contamination, the Chartered Institute of Building Service Engineers recommends running ventilation systems at a higher flow rate. “This may require changes to C02 set points for both mechanical ventilation and automated windows,” it states in its COVID-19 Ventilation Guidance. In fact, the recent advice from American Society of Heating, Refrigerating and Air-Conditioning Engineers is that there can never be too much clean air, and they have commenced work on recommendations that will quantify the virus related risk in a space relative to the air change rate, with perhaps 10 air changes being good and 2 poor. Interestingly, most of the fan coil and chilled beam systems recently installed provide between 2 and 3 air changes.
Airborne Particles and the Need for Ventilation
Chinese and American academics looking at outbreaks in the Chinese province of Zhejiang found that airborne transmission of the virus may have taken place in 48.3% of people in a badly ventilated office. Essentially to stop the spread of COVID-19, ventilation needs to be increased and more fresh air needs to be brought in. The risk of contamination via recirculated air can be mitigated with a higher level of filtration such as F9. This is a very fine system that will catch nanoparticles of 70nm but does involve greater energy use to overcome the resistance.
An alternative is to keep these systems on for much longer — typically two hours before people arrive and then two hours after they leave. CIBSE’s COVID-19 report also states that, “Recirculation of air within a single room, where this is complemented by an outdoor air supply, is acceptable.”
Getting abundant fresh air in the system is key. This could be as simple as just opening the windows. The BCO’s report goes so far as to say, “Actively use operable windows and openings to boost ventilation to occupied spaces as much as possible, even if this is at the expense of thermal comfort.”
Fan Coils and Chilled Beams
Getting abundant fresh air in the system is key.
The BCO also recommends that fan coils, which recirculate air locally in the occupied space, “should be frequently and thoroughly cleaned and where condensation occurs, drain pans and traps should be maintained frequently to prevent growth of bacteria and mold.” It is also a recommendation that HepVo traps are installed on condensate systems that drain into waste pipework. I have seen some research that questions the safety of fan coils due to the air movement that they generate having the potential to distribute virus particles widely throughout an office environment. Occupiers should consider a CFD study as part of their back to work safely plan. As far as chilled beams are concerned, CIBSE says that active chilled beams can be operated as normal, while with passive chilled beams there should be a good supply of air.
I would be interested to see some further research on the performance of underfloor and low-level air distribution. The lower velocities and laminar air flow associated with these systems causes less air turbulence, particularly in the zone where air is breathed. This would seem to have an obvious advantage in reducing the risk of virus spread in an office environment.
It is also interesting to consider variable air volume which has not been used widely in offices since the 90s, primarily because of the space it requires. But this type of ‘all air’ system can provide a much higher air change rate than fan coil and chilled beam systems. If we can find a way of cleaning the return air, with filtration and other technologies, then a VAV a system would typically provide 8-10 air changes of ‘clean air.’
Based on what we currently know and only in relation to the risk associated with COVID-19 I would rate the safety of currently most popular base building ventilation systems as follows
- Displacement (full fresh air or F9 filters filter providing clean recirculated air)
- VAV (with cleaned return air using a minimum F9 filters)
- Passive chilled beams with under floor fresh air supply.
- Underfloor distribution (i.e. fan tiles)
- Active chilled beams
- Fan coils
Whilst the supply air helps with the dilution of any potential virus particles in a particular space, actually removing them from a space requires a good extract system and it may be that ducted extract systems will become more prevalent. Transmission risk could be substantially reduced by using CFD to position extract points with precision in places where most particles are likely to accumulate.
There are some very encouraging emerging technologies, that might help combat the spread of COVID-19, things to keep a track of include UV-C lighting which can be used to clean spaces or inside ductwork to clean the air, and if used in conjunction with filtration could be extremely effective. You need to take care though as UV-C light is very dangerous if you come in direct contact, quickly burning skin and destroying light sensitive retinas. Japanese scientists have discovered that UV light at a certain frequency can destroy virus particles whilst being safe for humans, whilst this is in the early stages of development, it an encouraging example of where technology is being developed to help the fight against coronavirus.
Something else I see an interest developing in is bipolar ionisation, alternately referred to as cold plasma. Historically this has been used to clean the air in medical and food production applications. Laboratory tests have shown that when ionised air is supplied to a space will eliminate COVID-19 from air and surfaces in a short space of time so has the potential to be used in a wider range of applications. Care should be taken before installing this technology in an office environment as some of the equipment is not designed for this purpose.
While ionisation manufacturers have been concerned about reducing the risk of ozone, some products have been developed in Denmark, and used in occupied spaces for some time, that use minute amounts of ozone to disinfect food processing facilities. These products have been shown to be effective against the SARS and other viruses and this would indicate a potential to eradicate COVID-19 with ozone levels similar to those that occur naturally on a sunny day.
Limits on the amount of ozone that is safe in an occupied space are a new area of research, but a recent standard has been set by the American underwriter’s laboratory with UL2998 setting a limit of 5 parts per billion for certifying air purification products. But this is not yet a standard that has been officially adopted internationally. This is clearly an area that has great potential in the fight against COVID-19 and I would expect that U.K. research organisations such as BSRIA and BRE will be taking an interest and setting standards for using these new technologies.
Mixed Mode Ventilation
The ‘mixed mode’ of ventilation will become more commonplace. When it is not high summer, the cooling can be turned off so windows can be opened. This could even eventually replace the familiar sealed building model. This system can happen automatically with sensors, after all, fresh air is good for people: There are several recent examples of this being done successfully, other buildings, such as London Wall Place, have been designed future proofed for ‘mixed mode’ use to be adopted if this is preferred by a tenant.
Meanwhile, to combat fecal-oral transmission, bathroom extraction fans need to be kept on high and again perhaps running the systems for 24 hours a day. Toilets that automatically shut and touchless flushes can also help to stop the spread of the virus. The same goes for anti-bacterial coatings on bathroom doors. Some of clients are considering motorised doors that are effectively ‘touch free’.
Post-COVID-19 Ventilation Strategies
Toilets that automatically shut and touchless flushes can also help to stop the spread of the virus.
There is definitely set to be more access to outside air moving forward and there is a strong sustainability argument to be made for this method. However, some of the changes to ventilation strategies being deployed for a post-COVID-19 world will inevitably have some compromises for carbon emissions. If systems are run at a higher rate and for longer, if not continuously, throughout the day then that has implications for a larger carbon footprint, as the buildings become less energy efficient. However, in the middle of a global pandemic, it’s a price worth paying. As energy saving methods (thermal wheels and plate heat exchangers) also present a risk, CIBSE recommends that these are bypassed and not used in the current environment. Products are now being developed that could utilise in duct UV-C lighting systems to disinfect thermal wheels, this will help to diminish the environmental cost of keeping us safe in our workplaces.
Of course, some of these solutions are temporary but other, smart office elements like touchless versions of door handles, room/desk booking systems (wayfinding) and reception sign-in procedures look set to be with us for the longer term. These all affect the mechanical and electrical, as well as the architecture and design of buildings. We will overcome COVID-19 but we need to listen to the lessons that we are learning, and some will most certainly become permanent before the next virus that hits the human race comes along.