Guidance of the European Federation for Heating, Ventilation and Air Conditioning (REHVA) on COVID-19.

Guidance of the European Federation for Heating, Ventilation and Air Conditioning (REHVA) on COVID-19,
April 3, 2020

(this document updates the version from March 17; further updates will follow if necessary.)

 

 How to operate and use HVAC systems in buildings to prevent the spread of coronavirus in workplaces.

(COVID-19) (SARS-CoV-2)

Translation into Bulgarian and publication on the REHVA website – company “TANGRA – AV” Ltd.

https://www.rehva.eu/activities/covid-19-guidance

 

Introduction

 

In this document, the European Federation for Heating, Ventilation and Air Conditioning (REHVA) summarizes advice regarding the operation and use of building services in areas with a coronavirus (COVID-19) epidemic to prevent the spread of COVID-19, depending on factors related to HVAC systems.

 

 

The proposals below are intended as a supplement to the general guidelines for employers and building owners presented in the WHO document “Getting workplaces ready for COVID-19″ The text below is primarily aimed at HVAC specialists and facility managers (building managers), but it may also be useful for occupational health and public health professionals.

 

 

Below are the building-related precautionary measures covered and some general responses explained. The scope is limited to commercial and public buildings (for example, offices, schools, retail spaces, sports facilities, etc.), where only occasional presence of infected individuals is expected; hospitals and healthcare facilities (usually with a higher concentration of infected people) are excluded.

 

The focus of the guidelines is on temporary, easy-to-organize measures that can be applied in existing buildings that are still being used at normal occupancy. The advice is intended for a short period of time depending on the duration of local epidemics.

 

PATHWAYS OF TRANSMISSION

 

Important for every epidemic are the pathways of transmission of the infectious agent. Regarding COVID-19, the standard assumption is that the following two modes of transmission are dominant: through large droplets (droplets/particles emitted when sneezing, coughing, or talking) and through surface contact (carrier, hand-to-hand, hand-to-surface, etc.). A third mode of transmission, which is attracting increasing attention from the scientific community, is the fecal-oral route.

 

The fecal-oral route of transmission of the SARS-CoV-2 infection is tacitly acknowledged by the WHO. In this document, they recommend flushing toilets with the lid closed as a precautionary measure. Additionally, they suggest avoiding dried-out floor drains and other sanitary devices by regularly adding water (every 3 weeks, depending on the climate) so that the water trap functions properly. This is related to observations during the SARS epidemic of 2002-2003: open connections to sewer systems appeared to be the route of spread in a residential building in Hong Kong (Amoy Gardens). It is known that flushing toilets create jets containing droplets and droplet residues when toilets are flushed with the lids open. Furthermore, SARS-CoV-2 viruses have been detected in stool samples (reported in recent scientific studies and by Chinese authorities). Recently, a similar incident was reported in an apartment complex (Mei House). Therefore, the conclusion is that fecal-oral transmission routes cannot be excluded as a path for virus spread.

 

There are two mechanisms of airborne exposure vi, vii:

1. Transmission through close contact via large droplets (>10 microns), which are released and fall onto surfaces no further than about 1–2 meters from the infected person. Droplets are produced by coughing and sneezing (sneezing usually produces many more particles). Most of these large droplets fall onto nearby surfaces and objects—such as desks and tables. People can become infected by touching these contaminated surfaces or objects and then touching their eyes, nose, or mouth. If people stay within 1–2 meters of an infected person, they can become infected directly by inhaling droplets that are sneezed, coughed, or exhaled by that person.
2.  Airborne transmission through small particles (<5 microns), which can remain suspended in the air for hours and can be transported over long distances. These are also generated by coughing, sneezing, and speaking. Small particles (droplet nuclei or residue) form from droplets that evaporate (10-micron droplets evaporate in 0.2 seconds) and dry out. The size of the coronavirus particle is 80–160 nanometers^2,VIII  and it remains active for many hours or several days (unless specific cleaning is performed)^IX,X,XI. SARS-CoV-2 remains active for up to 3 hours indoors and 2–3 days on surfaces in a room under standard indoor conditions. Such small viral particles stay suspended in the air and can travel long distances, carried by indoor air currents or suction air ducts of ventilation systems.

 

One piece of evidence for this: the SARS-CoV-2 coronavirus has been isolated from swabs taken from suction ventilation openings in rooms occupied by infected patients. This mechanism suggests that maintaining a 1–2 meter distance from infected individuals may not be sufficient, and increasing ventilation is beneficial due to the removal of more particles³.

Figure 1. WHO reports mechanisms of exposure to COVID-19 SARS-CoV-2 droplets (dark blue color). Light blue color: airborne transmission mechanism known from SARS-CoV-1 and other influenza viruses; currently, there is no reported evidence specifically for SARS-CoV-2 (figure: courtesy of Francesco Franchimon).

 

Airborne transmission of SARS-CoV-2—infection through exposure to droplet nuclei particles—is currently recognized by the WHO during hospital procedures and indirectly through guidelines for increasing ventilation^XV. It may occur under certain conditions according to the Chinese National Health Commission (unpublished result). Airborne transmission may be possible according to Japanese authorities under certain circumstances, such as when talking to many people at close range in an enclosed space, where there is a risk of infection spread even without coughing or sneezing^XVI. The latest study^XVII concludes that aerosol transmission is plausible because the virus can remain viable in aerosols for several hours. Another recent study^XVIII analyzing super-spreading events shows that enclosed environments with minimal ventilation strongly contribute to the characteristically high number of secondary infections. The draft manuscript discussing airborne transmission concludes that evidence is emerging indicating that SARS-CoV-2 is also transmitted via airborne particles^XIX.

 

Practical recommendations for building services management

 

Increase the supply rate of fresh outdoor air and the exhaust ventilation.

 

In buildings with mechanical ventilation systems, it is recommended to extend the operating time of these systems. Adjust the system timer settings to start ventilation at nominal speed at least 2 hours before the building’s occupancy time and switch to a lower speed 2 hours after the occupancy period ends. In ventilation systems controlled by air quality sensors, lower the CO₂ setpoint to 400 ppm to ensure operation at nominal speed. Maintain ventilation 24/7, with a reduced (but not turned off) ventilation rate when the building is unoccupied. In buildings that are closed due to the pandemic (such as some offices or educational institutions), it is not recommended to turn off ventilation; instead, it should operate continuously at a reduced speed. Considering that heating and cooling demands are low during spring, the above recommendations involve limited energy costs while helping to remove viral particles from the building and eliminate those deposited on various surfaces.

 

The general recommendation is to supply as much outdoor air as reasonably and practically possible. The key aspect is the amount of fresh air delivered per person. If, due to smart workplace management, the number of employees is reduced, do not concentrate the remaining employees in smaller areas; instead, maintain or increase the social distance between them (the minimum physical distance between people should ideally be 2–3 meters) to enhance the cleaning effect of ventilation.

 

Exhaust ventilation systems in toilets should always operate 24/7 (around the clock), and ensure that negative pressure is maintained, especially to prevent fecal-oral transmission.

 

 

VENTILATE MORE OFTEN

 

A general recommendation is to stay away from crowded and poorly ventilated spaces. In buildings without mechanical ventilation systems, it is recommended to actively use operable windows (much more than usual, even if this causes some thermal discomfort). Ventilation through windows is the only way to increase air exchange in buildings without mechanical ventilation. When a person enters a room, they could open the windows for about 15 minutes (especially if others have been in the room before). Additionally, in buildings with mechanical ventilation, window ventilation can be used to further enhance ventilation.

 

Opening windows in toilets with passive ventilation or mechanical exhaust systems can cause contaminated air flow from the toilet to other rooms, meaning the ventilation starts to work in reverse. Therefore, opening windows in toilets should be avoided. If there is no adequate exhaust ventilation in the toilets and window ventilation in the toilets cannot be avoided, it is important to keep windows open in other rooms as well to create cross-ventilation (drafts) throughout the building.

 

Humidification and air conditioning have no practical effect

 

Relative humidity (RH) and temperature contribute to the transmission of viruses indoors by affecting the virus’s viability, the formation of droplet nuclei, and the sensitivity of occupants’ mucous membranes. Transmission of some viruses in buildings can be limited by altering air temperature and humidity levels. In the case of COVID-19, unfortunately, this is not an option because coronaviruses are quite resistant to environmental changes and are only sensitive at very high relative humidity above 80% and temperatures above 30˚C^IX,X,XI, which are neither achievable nor acceptable in buildings for other reasons (e.g., thermal comfort and microbial growth). It has been found that SARS-CoV-2 is highly stable—remaining viable for 14 days at 4℃; inactivation requires 37℃ for one day or 56℃ for 30 minutes. The stability (viability) of SARS-CoV-2 has been tested at typical indoor temperatures of 21-23℃ and relative humidity of 65%, showing very high viral stability at this RH XXII. Along with previous evidence for MERS-CoV, it is well documented that humidification up to 65% may have very limited or no effect on the stability of the SARS-CoV-2 virus. Therefore, the evidence does not support that moderate humidity (RH 40-60%) will be helpful in reducing the viability of SARS-CoV-2; consequently, humidification IS NOT A METHOD to reduce the viability of SARS-CoV-2.

 

The small droplets of interest to us (0.5 – 10 microns) will evaporate quickly at any level of relative humidity (RH) XXII. The nasal systems and mucous membranes are more sensitive to infections at very low relative humidity levels of 10-20%^XXIII,XXIV , which is why some humidification is sometimes recommended during winter (up to levels of 20-30%). However, this indirect need for humidification in winter in the case of COVID-19 is not relevant given the upcoming climatic conditions (from March onward, indoor humidity is expected to be above 30% in all climate zones in Europe without humidification).

 

Thus, in buildings equipped with centralized humidification, it is not necessary to change the set points of the humidification systems (usually 25 or 30% ^XXV). In any case, given the upcoming spring season, these systems should not be operating. Heating and cooling systems can operate normally, as they do not have a direct impact on the spread of COVID-19. Generally, there is no need to adjust the set points for heating or cooling systems.

 

SAFE USE OF HEAT RECOVERY UNITS

 

Under certain conditions, viral particles from the extracted air can be reintroduced into the building. Heat recovery units can transfer the virus attached to particles from the exhaust air zone back to the supply air zone through leaks. Air-to-air heat exchangers (i.e., rotary regenerative or enthalpy wheels) can be sensitive to significant leakage due to poor design and maintenance. For proper operation of rotary heat exchangers equipped with purge sectors and correctly adjusted, the leakage rate should be approximately the same as that of plate heat exchangers, in the range of up to 1–2%. For existing systems, leakage should be below 5% and compensated by increasing outdoor air ventilation according to EN 16798-3:2017. However, many rotary heat exchangers may not be properly installed. A common error is that fans are mounted in such a way that higher pressure is created on the exhaust air side, leading to leakage of extracted air into the supply air zone. The rate of uncontrolled transfer of contaminated extracted air in such cases can be as high as 20%^XXVI, which is unacceptable. It has been found that rotary heat exchangers that are properly designed, installed, and maintained have near-zero transfer of particle-bound contaminants (including airborne bacteria, viruses, and fungi), with transfer limited to gaseous pollutants such as tobacco smoke and other odors ^XXVII. Therefore, there is no evidence that virus-carrying particles of 0.1 microns and larger can be transmitted through leaks. Since leakage rate does not depend on the rotational speed of rotary heat exchangers, it is not necessary to turn off these devices. Normal operation of mechanical ventilation with rotary heat recovery facilitates maintaining a cleaner environment and higher supply of fresh air indoors. It is known that transfer via leaks is greatest at low airflow rates, so a higher ventilation rate is recommended.

 

If there is suspicion of leaks in the heat recovery sections, the pressure of the supply and exhaust fans should be adjusted to avoid a situation where higher pressure on the exhaust air side causes air leakage into the supply air side. Pressure differences can be corrected by dampers or other appropriate solutions. In conclusion, it is recommended to inspect the heat recovery equipment, including measuring the pressure difference, to ensure the absence of leaks and proper system operation. During these activities, maintenance personnel must strictly follow standard safety procedures when working in dusty and potentially contaminated environments, including wearing gloves and respiratory protection.

 

The transmission of viral particles through plate heat exchangers or systems with an intermediate heat carrier is not a problem when the HVAC system ensures no cross-contamination between the air streams and 100% separation of air between the exhaust and supply sides ^XXVIII.

 

Do not use recirculation.

 

Virus particles in the return air ducts can also re-enter the building when centralized air handling units are equipped with recirculation sections. It is recommended to avoid central recirculation during a SARS-CoV-2 epidemic: close the recirculation dampers (either through the building management system or manually). If this causes problems with cooling or heating capacity, it should be accepted, as preventing contamination and protecting public health is more important than ensuring thermal comfort.

 

Sometimes air handling units and recirculation sections are equipped with air filters for the recirculated air. However, this should not be a reason to keep the recirculation dampers open, as these filters typically do not effectively filter virus-carrying particles since they have standard efficiency ratings (G4/M5 or ISO coarse/ePM10 filter class)^XXIX rather than HEPA-level efficiency.

 

Some systems (such as fan coil units and induction modules) operate with local air circulation (at room level). If possible (i.e., if there is no significant cooling demand), it is recommended to switch off these units to avoid the re-dispersion of viral particles within the room—especially when the space is typically occupied by more than one person. Fan coil units are equipped with coarse filters that do not effectively capture small particles but can still accumulate them. To deactivate viruses on the surface of the fan coil unit’s heat exchanger, it is possible to heat the unit to 60°C for one hour or 40°C for one day.

 

Cleaning air ducts has no practical effect.

 

There have been exaggerated statements recommending the cleaning of ventilation ducts to prevent the transmission of SARS-CoV-2 through ventilation systems. However, cleaning air ducts is not effective in preventing infection within a room, since the ventilation system is not the source of contamination—provided that the above guidelines for heat recovery and air recirculation are followed. The virus, when attached to small particles, does not easily settle in ventilation ducts and is typically expelled by the airflow to the outside. Therefore, no changes to the normal duct cleaning and maintenance procedures are necessary. It is much more important to increase the supply of fresh air and to prevent air recirculation, as outlined in the recommendations above.

 

No replacement of outdoor air filters is necessary

 

In the context of COVID-19, the question has been raised as to whether filters should be replaced and what level of protection they offer in rare cases of external viral contamination — for example, if the exhaust air zone is close to the fresh air intake zone. Modern ventilation systems (air handling units) are equipped with fine outdoor air filters located directly after the intake of fresh air (filter classes F7 or F8, or ISO ePM2.5 or ePM1), which effectively filter fine dust particles from the outside air. The size of a bare coronavirus particle is approximately 80–160 nm ^VIII (within the ePM1.0 range), which is smaller than the main capture range of F8 filters (which have a capture efficiency of 65–90% for PM1). However, many of these small particles will still deposit on the filter fibers through diffusion. Additionally, SARS-CoV-2 particles often attach themselves to larger particles, which fall well within the capture range of these filters. This means that in rare cases of virus-contaminated outdoor air, standard fine outdoor air filters provide reasonable protection against low concentrations and incidental airborne viruses in the outside air.

 

The heat recovery and recirculation sections are equipped with less efficient exhaust air filters (G4 / M5 or ISO coarse / ePM10), which are intended primarily to protect the equipment from dust. These filters are not designed to filter small particles, as the virus-carrying particles will be removed through the exhaust air stream.

 

Regarding filter replacement, standard maintenance procedures can be applied. Clogged filters are not a source of contamination in this context, but they do reduce the supply airflow, which can negatively impact indoor air quality. Therefore, filters should be replaced according to standard procedures—when pressure, time limits are exceeded, or according to the planned maintenance schedule. In conclusion, it is not recommended to replace the existing outdoor air filters with a different type of filter, nor to replace them earlier than usual.

 

 

HVAC maintenance personnel may be at risk when filters (especially exhaust air filters) are not replaced following standard safety procedures. To stay protected, always assume that filters contain active microbiological material, including viable viruses. This is especially important in any building where a recent infection has occurred. Filters should be replaced with extreme caution, using gloves, respiratory protection, and disposed of in a sealed bag.

 

Room air purifiers can be useful in specific situations

 

Room air purifiers effectively remove particles from the air, providing a similar effect as ventilation. To be effective, air purifiers must have HEPA filter efficiency. Unfortunately, most attractively priced room air purifiers are not sufficiently effective. Devices that use electrostatic filtration principles (not the same as room ionizers!) often also work quite well. Since the airflow through air purifiers is limited, the floor area they can effectively serve is usually quite small, typically less than 10 m². If someone decides to use an air purifier (again: increasing continuous ventilation is often much more effective), it is recommended that the device be placed close to the breathing zone. Special UV equipment, which must be installed for cleaning the supply air or the air in the room, is also effective as it kills bacteria and viruses, but this does not guarantee killing coronavirus particles.

 

SUMMARY OF PRACTICAL MEASURES FOR MANAGING SERVICES IN BUILDINGS

 

1. Provide ventilation with fresh outdoor air in all rooms.
2. Switch the ventilation to the nominal speed at least 2 hours before the building’s occupancy time, and reduce to a lower speed 2 hours after the occupancy time.
3. Do not turn off the ventilation during the night and weekends; instead, keep the systems running at a lower speed.
4. Ensure regular window ventilation (even in buildings with mechanical ventilation).
5. Maintain the toilet ventilation running 24/7.
6. Avoid opening windows in toilets to ensure the proper ventilation direction.
7. Instruct building occupants to flush toilets with the lid closed.
8. Switch air handling units with recirculation to 100% fresh outdoor air.
9. Inspect the heat recovery equipment to ensure that leaks are under control.
10. Either switch off the fan coil units or operate them so that the fans remain continuously on.
11. Do not change the settings for heating, cooling, or any possible humidification settings.
12. Do not schedule duct cleaning during this period.
13. Replace the central outdoor air filter and the exhaust air filters as usual, according to the maintenance schedule.
14. Regular filter replacement and maintenance activities should be carried out using general protective measures, including respiratory protection.

 

To view the full version, please click “here”