Ventilation And Transmission: HVAC And Adapting To COVID-19
By Adrian Gray
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 (BCO)’s Thoughts on Office Design and Operation After COVID 19 document talks of large droplets, greater than 10 micrometres, “expelled by sneezing and coughing and in still air, typically within about 2 metres 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: faecal to oral whereby particles from the toilet can enter people’s respiritory systems when using WCs.
Counteracting COVID-19 transmission
There are several methods to counteract these routes of transmission. 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.
As for transmissions via surfaces, scientists have emphasized copper’s antibacterial properties, with COVID-19 surviving just a few hours on copper, compared with a number of days for steel or plastic. William Keevil, a senior microbiologist at the University of Southampton, has recently suggested that the UK is behind other countries in using this material on communal areas like handrails and doorknobs. Copper-based nickel would perform better than chrome in certain parts of the office too.
To dilute airborne contamination, the Chartered Institute of Building Service Engineers (CIBSE) 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.
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. The 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
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 mould.” It is also a recommendation that HepVo traps are installed on condensate systems that drain into waste pipework. 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.
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 building 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 faecal-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 motorized doors that are effectively ‘touch free’.
Post-COVID Ventilation Strategies
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 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
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 M&E, 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!
Original Article published on HVACInformed.com (15/07/2020)