Frequently Asked Questions

Since the first version of the REHVA Guidance document which was published on 17 March, REHVA has received several questions and requests for clarifications from industry, academia and government experts. The authors of the document in an effort to respond to these requests have added this FAQ section that it will be continuously updated as more frequently asked questions arise. 

What precautions should I take when changing filters?

Question

Should there be any precautions to be taken when changing filters or cleaning supply vents on a maintenance routine? Are the maintenance personnel at risk while carrying out such works? 

Answer

In general, it is wise to assume that filters have active microbiological material on them.  Whether this represents an important infectious disease risk from viruses is not known, but the precautionary principle would suggest that care should be taken. This becomes particularly important in any building (including residential) where there are known or similar cases of any infectious disease including COVID-19. The system should be turned off when changing filters and taking all necessary protective measures such as wearing gloves, including an FFP3 respirator if available, outdoors if possible and disposed of in a sealed bag. (Reference)

 

 

 

 

 

 

 

 

 

 

 

Is disinfection treatment of air ducts necessary after the pandemic?

Question

Do you recommend to clean air ducts (especially exhaust) with disinfection treatments after the pandemic? 

Answer

According to our recommendation this is not necessary, because viruses stay viable no longer than 2-3 days on plastic and stainless steel surfaces. In the ductwork, this time is probably even shorter, as in the airflow they stay viable only for 3 hours.

Can high temperature in fan coil units eliminate viruses?

Question

In the guidance document it is mentioned that the virus is susceptible to temperatures above 30 ºC. In fan coil units, temperatures of 40ºC can be easily reached. Can this be a way to eliminate or reduce viruses in suspension in a building? Would it be advisable to maximise the temperatures of the fan-coil circuit? 

Answer

To inactivate the virus, a temperature increase is needed up to 37 ℃ for one day and 56 ℃ for 30 minutes. Considering a safety margin of 3-4 ℃, it would be advisable to operate at 40 ℃ for one day and 60 ℃ for one hour. These temperatures apply mainly for the heat exchanger surfaces and not the filters where temperature is expected to remain at lower levels. Therefore, a filter replacement might be considered if such heating measure will be taken. (Reference)

Are UV lights and ionisation units recommended?

Question

Would you recommend the installation of UV air treatment systems or ionisation units in an existing air handling unit (AHU), given that they won’t increase pressure losses? 

Answer

Such installations (UV systems, ionisation units etc.) are more relevant for healthcare facilities. REHVA guidance targets common non-residential buildings and in this case, outdoor air is NOT a contamination source. Therefore, UV-treatment of outdoor air in air handling units is not necessary. This is the reason we do not recommend UV-applications. 

Should I suspend fire dampers servicing within ventilation ducts?

Question

A number of engineers have raised concerns regarding the servicing of fire dampers within ventilation ducts. Do you believe we need to suspend this type of servicing during COVID 19 outbreak? 

Answer

HVAC maintenance personnel could be at risk when filters (especially extract air filters) are not changed in line with standard safety procedures. To be on the safe side, always assume that filters have active microbiological material on them, including viable viruses. This is particularly important in any building where there recently has been an infection. Filters should be changed with the system turned off, while wearing gloves, with respiratory protection, and disposed of in a sealed bag. It is possible that some virus particles are in extract ducts. Therefore, similar protection could be recommended.

Can virus particles re-enter to the ventilation system via a heat wheel?

Question

Can virus particles enter to the ventilation system via a heat wheel and in particular through the fins, in the same way as moisture particles enter to the system?

Answer

In the case of heat wheels, the carry over transfer is limited mainly to gaseous pollutants, for instance to tobacco smoke and other smells. In the case of properly operating equipment, heat wheels with purge sector practically do not transfer particles – this applies also for virus particles. The main concern in older equipment is the air leakage from exhaust side to supply side. This depends on pressure difference and condition of seals. If fans create higher pressure on the exhaust air side, the air leakage can increase from a few percent to 15%. If needed, the pressure differences can be corrected by dampers or by other arrangements. Because the leakage does not depend on the rotation speed, it is not needed to switch rotors off, but inspection for older equipment can be recommended.

Can “full fresh air” supply be used for food stores without increasing humidity levels?

Question

In food stores (supermarkets) by switching the ventilation to full fresh air with no recirculation, the heating and cooling plant will struggle to maintain temperature within the sales area. Also, in a supermarket environment “full fresh air” will lead to an increase of humidity levels and this will impact the performance of the refrigeration, inevitably affecting the display temperature of food products which might fall outside accepted limits. In order to protect the supply of food, would it be advisable to allow enough air recirculation in order to maintain humidity levels (assuming the absence of dehumidification equipment)?

Answer

In general, humidity levels in European climates are possible to control without special dryers in 100% outdoor air systems. In climates that are not warm and humid, the capacity of cooling coils of the air handling units should be enough for adequate dehumidification also in 100% outdoor air mode (depending on coolant temperature levels and sizing of coils). If the air handling unit is capable to control the temperature of the store, the relative humidity should not change significantly. In case the pandemic continues during the summer with high temperatures, it is possible that some national guidelines on how to operate the ventilation systems will follow. If recirculation is not banned by authorities, the alternative would be to improve filtering, either by installing better filters or UV-equipment. UVGI-systems kill any bacteria and viruses, but they need good engineering to be correctly designed and installed to be effective.

Are there any additional precautions advisable when carrying out a routine ventilation ductwork cleaning?

Question

Are there any additional precautions advisable when carrying out a routine ventilation ductwork cleaning?

Answer

In the ventilation ductwork cleaning, both organic and inorganic dust emission rates can be very high. Commonly vacuum collector units are used to create necessary pressure and air speed in the ductwork part under cleaning, and the dust is collected to the filter unit of the vacuum collector. These units are typically equipped with HEPA filters which are also needed to capture possible virus particles. Therefore, the equipment and standards for ductwork cleaning are sufficient also for the outbreak situation. Detailed information on the ductwork cleaning work planning and equipment is available in REHVA Guidebook 8. What is different in the COVID-19 outbreak situation, is the need for protection of the cleaning personnel. Please, refer to the first FAQ for more information.

What are the recommended rates of air exchange for different types of workplaces such as offices, shops, factories and so on?

Answer

Indeed there is not yet information that outdoor air ventilation rate will safely reduce the infection risk, but at least existing standards should be followed. Existing standards in non-residential buildings lead to about 10 L/s per person which is equivalent to at least 2 ach (air changes per hour). On the other hand, we know what ventilation rate is too small, some papers about so called superspreading events show that infections have been associated with very low ventilation rates of about 1 L/s per person. Researchers can speculate would 10 L/s per person be enough or should this value increased for instance to 20-25 L/s per person – this information is currently not available Covid-19.

How does this relate to the level of occupancy? Presumably you need faster turnover in crowded places?

Answer

The amount of liters per person fresh air needed is directly related to occupancy. If in an open plan office one has ventilation of 2 L/s per m2 floor area, in the case of high occupancy (5 m2 per person) the ventilation is 2x5=10 L/s per person, and in the case of less occupants (10 m2 per person) the ventilation would be doubled 2x10=20 L/s per person. The fresh (outdoor air) ventilation rate per person matters, so this can be increased by lowering the occupant density.

Is there a recommended maximum CO2 level?

Answer
CO2 level is an indicator for outdoor air ventilation. 10 L/s per person recommended by current standards corresponds to 950 ppm CO2 concentration (including 400 ppm outdoor CO2 concentration). This applies in the case of long time occupancy, so often 800 ppm are considered as an indicator of good ventilation and indoor air quality. But similarly to first question, there is no good Covid-19 evidence available.

What is the recommended relative humidity level for workplaces?

Answer

In REHVA guidance document it is explained how relative humidity and temperature contribute to virus transmission indoors. It is known that SARS-CoV-2 has very high virus stability (viability) at 65% RH and typical indoor temperature of 21-23 °C. Therefore, humidification has no practical effect and is not recommended.

How might all the above be changed to make buildings safer in the context of Covid-19?

Answer

In addition to good ventilation, it has been pointed out that a strong air flow from one person to another might cause infection. Therefore, good air distribution, i.e. providing even ventilation rate at low air velocity within all points in the room is important.

Would increasing air exchange rates be useful?

Answer

Definitely, the outdoor air ventilation rate per person matters.

Is it safe to keep using heat exchangers to recover waste heat?

Answer

Heat recovery equipment properly designed and maintained is a safe solution. Actually, heat recovery allows not to use recirculation and therefore is highly recommended.

Is it safe to recirculate warm air after filtering?

Answer

Recirculation is generally not recommended, because it will spread pollutants to all spaces of building. Typical filters have low efficiency for small virus laden particles.

Can filters actually remove viral particles?

Answer

Common outdoor air filters used in ventilation and air conditioning systems are not effective for viral particles (and they do not have to be, because outdoor air is not a source of viruses). Special high efficiency, HEPA filters are needed to filtrate viral particles. Such filters are used in clean rooms and also in some air cleaners, but not in common ventilation systems.

Relative humidity decreases as temperature rises so would reducing temperatures a little be a benefit?

Answer

This is good advice in winter time in cold climates. If the outbreak would happen in winter, then this would be useful, because nasal systems and mucous membranes are more sensitive to infections at very low RH of 10- 20 %. However, this is not the case in the spring time.

All buildings have areas that are hard to ventilate so are there other ways of cleaning and disinfecting the air going into them?

Answer

Cleaning and disinfecting are essential, but an infected person will contaminate the air – there is no other way than to ventilate or to use a room air cleaner.

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