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How to make HVAC technology Covid19-safe?

Ambient air proportion, running times, air exchange capacities, indoor room humidity, filtration, disinfection, maintenance – stand for a variety of key parameters for ventilation and air-conditioning systems to significantly contribute to reducing the risks of infection with the corona virus SARS-CoV-2.

October 2020

Against the current pandemic background, these were the aspects discussed by Germany-based experts from production, plant construction and technical monitoring in the third Building Technology Experts online event:

  • Christoph Kaup, managing partner, HOWATHERM Klimatechnik
  • Robert Oettl, Managing Director, TÜV SÜD Advimo
  • André Preußler, Head of Design & Technical Management, Caverion
  • Bernhard Steppe, Managing Director Sales, WOLF

They assessed the options for building owners, operators and users of public buildings and infrastructures to minimise the risks of infection in the face of the current corona pandemic. Please read our summary here or watch the video stream.  

Already the introductory round showed how important the differentiated approach to the do's and don'ts of modern air handling units (AHUs) is: the current situation alone – shortly before the start of the heating season and after the technical lockdown of many systems for months – requires a thorough hygiene inspection and testing of existing systems.

According to the experts a "one fits all" recommendation for this, may only be made at one point: With the users of the respective building and their reliable compliance with hygiene rules according to the applicable hygiene formula: Keep your distance – observe hygiene – wear a fabric mask to cover your mouth and nose. Any aspects  concerning technical devices to control the quality and safety of indoor air must be individually adjusted and, if necessary, retrofitted, depending on the building and room conditions as well as the type, age and design of the HVAC system. Here are the most important points in brief:

Air conditioning systems are part of the solution, not part of the problem

Ventilation technology provides a range of set screws to improve and maintain the quality and safety of indoor air:  fresh air flow, running times, indoor room humidity, filtering, disinfection, and maintenance. It all depends on the intelligent interaction.

From opposite poles to a triad

Robert Oettl, from one of the Technical Control Boards of Germany (TÜV), identified the new challenge posed by Corona for the participants: "Up to now, the question of designing and operating air conditioning systems has been caught between two poles – comfort and convenience on the one hand and energy efficiency on the other. In view of the no less acute global challenge to sustainably protect the climate, energy efficiency has increasingly become the determining factor in recent years. The aspects of energy efficiency and indoor air quality have not been given equal priority. Corona would now break this bipolar tension: "hygiene" would be added as a third, equivalent factor, which could possibly run the risk of thwarting the imperative of energy saving. This is a critical factor in the effort to protect the environment, which manifests itself concretely in higher operating costs for operators and users of buildings equipped with air conditioning systems.

Reasonable upgrading and conversion

Air exchange and the proportion of ambient air become a safety factor for indoor air, as scientific findings show. In the most frequent cases, it is now important to upgrade systems in an appropriate, demand-oriented way in order to feed more fresh air into the indoor air and to adjust further parameters for a reduction of virus concentrations.

Aerosols as a successful target for AHUs

It is not the viruses themselves that are the point of attack for air handling units, but the droplets and aerosols carrying the virus. As a device manufacturer, Christoph Kaup pointed out the properties of aerosols in different climatic environments. According to him, indoor humidity, temperature and turbulence influence the risk of spreading in the indoor air differently, as industrial research has also shown: a relatively higher humidity may cause aerosols in the nanometer range to sink to the ground faster, while a high air exchange rate may cause light aerosols to float in the air at risk. This is what makes reducing the density of pathogens in indoor spaces so complex – but that is precisely what the ventilation and AC systems have the right set screws for.

Adjusting screws: indoor air volume and humidity

The increase of the air exchange rate must be considered for all AHU systems. However, with moderation (comfort aspect) and taking into account the risk of excessive air turbulence (keyword: aerosol dispersion). André Preußler advised building owners and planners to take the new conditions into account from the very start when planning and designing capacities, in order to find a safe and cost-effective way forward. To reduce the proportion of recirculated air, to choose filter classes appropriately and to maintain a good indoor humidity in the corridor between 40 and 60%, he said, was the quintessence of keeping the infection rate low, especially in the dry winter air.

But here too, according to Bernhard Steppe from the manufacturers’ side, fine-tuning is necessary in case of doubt: it will be better to control indoor air humidity continuously and readjust if necessary or, for example, retrofit with humidifiers.

Disinfection by diligent filtering and maintenance

All panelists described filtration as an important part of the overall solution for indoor air recirculation units, with which the infective load can be significantly reduced. Double filtration with class F7 and F9 fine particulate air filters proved to be successful methods. According to Christoph Kaup, this conventional mechanical technology is also in comparison to HEPA (High Efficiency Particulate Air Filter) filters, such as class H14, sufficiently close to separate the fine droplets in which the Covid 19 viruses float in the indoor air like in a Trojan horse.

In the filters, Kaup continued, the viruses then remained bound and would not be transported further. The virus material (dead material without its host) can therefore be separated. However, as with all types of pathogens and particles, it must be ensured that the filters are not contaminated – through regular, professional maintenance – in accordance with general hygiene regulations.

According to Kaup, disinfection with UVC radiation can be an additional solution, because the radiation destroys the viral DNA. This is a highly safe process, he says, but in practice it must be measured against economic considerations in view of additional safety precautions and energy intensity.

The experts' "filter conclusion" was: Well maintained, the classic F7-F9 filter combinations make absolute sense and provide 99.9% safety. In addition, they can be retrofitted with mobile accessories. André Preußler commented on ideal maintenance intervals for the part of plant construction and facility management: In principle the annual filter change is sufficient. In the case of systems with a high air flow rate, this should be done every six months. Robert Oettl confirmed this from the Technical Inspectorate and specified the recommendation: not because the virus is the problem, but due to the fact that, once the ventilation unit performance is increased to reduce the spread of the virus, the maintenance cycle must keep pace with it!

Runtimes and "ramp-up 2020”

A major reason for the comprehensive maintenance of the systems should be the restart of operations after weeks and months of inactivity due to the corona lockdown. Here, too, the experts emphasise that testing before restarting is a general, but not a corona-related request.

The ramp-up should be accompanied by generously dimensioned pre-run and post-run phases (from 1-3 h) before and after regular operation in the building. Ideally, one should start with a fresh air component or let the system run through initially. In general, such an extension of the running time (also in regular operation, depending on the frequency, about 1 h pre- and post-run or one night run) reduces the concentration of pathogens and particles of all kinds and improves the air quality. For a good flushing of the rooms, this method could be a proven alternative to increasing the volume flow, which quickly reaches its performance limits.

The effect of excessive air humidity and the turbulence effect on aerosols caused by too high a volume flow was discussed controversially. The carbon dioxide (CO2) tolerance in indoor air was assessed just as differently, as was the question of whether a system and the supply of fresh air should be "run" according to CO2 values. But here, too, the experts agreed in a differentiated consensus: this consideration makes sense, because a low CO2 air content indicates fewer users in the building and, thus, lower ventilation requirements and energy consumption.

The framework of standards and guidelines is appropriate

Under the pandemic conditions, good indoor flushing will take on a new, greater importance than it previously had. Against this background, the standards and guidelines must also be applied and adapted in terms of increased protection against infection and of hygiene requirements. This, according to Robert Oettl, could significantly improve indoor air conditions and safety. In his view, there is no need for new standards.

When asked by the participants whether good ventilation of rooms could make the distance rule dispensable, Oettl asked for relativisation: in his opinion, this junctim depends on many factors that have to be investigated. But: the 1.5m distancing rules are more linked to the protection of a direct speech-sneezing distance than with aerosol behaviour, and in general, air conditioning cannot be a substitute for the distancing and hygienic rules, but an effective supplement.

Conclusion: Air as a vital necessity brings AHUs into the focus of infection protection

All panel experts expect air handling systems to become more important in the future, in line with Christoph Kaup's credo "air is a vital necessity". The fight against the corona pandemic has shown: The hygiene of indoor air is of great importance for the protection against infection – especially if we all spend more time indoors again soon.

The challenge will be a joint one of plant manufacturers, operators, investors and industry.

With this in mind, in September the German Federal Ministry of Economics and Energy (BMWi) launched a funding programme "Corona-compatible conversion and upgrading of air handling systems" for the years 2020 and 2021. The BMWi is motivated by the fact that the support programme is intended to make an important contribution to reducing the risk of corona infection in the cold season, where many and changing people meet every day: in lecture and school halls, in theatres and museums, in municipal assembly rooms and community centres.

Further reading:

World Health Organisation (WHO)

Bundeszentrale für gesundheitliche Aufklärung (BZgA)

Aerosol research at German Hermann-Rietschel-Institute at Berlin Technical University

German Federal Ministry of Economics and Energy (BMWi) on AHU program:

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  • Hygiene, function & design