There continues to be emerging findings on the COVID19 risks associated with Heating, Ventilation and Air Conditioning (HVAC) systems. The HSE provide some guidance on the matter, they find that ‘The risk of air conditioning spreading coronavirus (COVID-19) in the workplace is extremely low. You can continue using most types of air conditioning system as normal.

But, if you use a centralised ventilation system that removes and recirculates air to different rooms it is recommended that you turn off the recirculation and use a fresh air supply, but if you’re unsure, ask the advice of your heating ventilation and air conditioning (HVAC) engineer or adviser.’ (sound advice indeed!)

However, there are other sources and governing bodies that have explored the risks further and continue to monitor the latest findings from the most recent studies carried out. Needless to say HVAC systems themselves and individual system configurations are not that straight forward and need exploring further to really understand each businesses level of risk, or the occupied building COVID-19 risks when it comes to HVAC systems.

But ‘in a nutshell’…

  • Avoid stagnant air, keep air moving
  • Fresh air is best
  • Recirculated air in one room is not great, and recirculated air between 2 or more rooms is the worst place to be ‘hell’
  • Mixing fresh air with recirculated air is better, as much as you can possibly supply
  • But switching to a 100% fresh air supply is the ‘golden ticket’ 
  • Open vents, open windows, and keep humidity levels at least at 40%
  • Turn on a ceiling fan (the HSE find that desk fans are fine but we’re not entirely convinced based the trajectory of the air flow)

If you want to read more about this, and you really should, just to ensure that none of this is misunderstood or taken out of context, so here you go… brace yourselves!

Switch off recirculating ventilation systems

  • Virus particles in return ducts can re-enter a building when centralized air handling units are designed to or capable of recirculating air back into the building, e.g. some commercial kitchens that have no option of external ventilation with a route to atmosphere.
  • Recirculation air filters are not a reason to keep recirculation dampers open as these filters do not filter out particles with viruses effectively since they have standard efficiencies and not HEPA efficiencies

Filtration and air cleaners

  • Outdoor air filters will typically not capture COVID-19 as the virus is smaller than the capture area of most filters
  • Outdoor air is not a source of viruses, thus no need to replace filters
  • If you do replace any air filters, an FFP3 mask, protective safety glasses or visor and nitrile gloves are ideal articles of PPE that should be worn and good personal hygiene adopted, because filters may have active microbiological material on them

Increase air flow

  • It can be done by increasing the fan speed of your HVAC system
  • Open air vents
  • Turn on ceiling fans
  • Be careful to keep the pressure difference in the individual rooms unchanged between the air supply and the exhaust air
  • Care must be taken that the actual fan motor power input do not exceed the maximum allowed power input.

Force dampers to introduce outdoor air only

  • For the sole purpose of increasing the external air flow, it is advisable to close the recirculation damper and at the same time open the outdoor air and exhaust air dampers, taking care not to alter the pre-existing overpressure conditions
  • For systems designed to be able to operate with all external air, for example free-cooling (Scheme n.1), only external air mode is recommended, providing for total closure of the recirculation damper and simultaneous opening of both outdoor and exhaust air dampers.
  • For systems that do not provide free-cooling (Scheme n.1), it is still advisable to close recirculation dampers and simultaneously open both outdoor and exhaust air dampers. The fan flow rate will be reduced, but it will consist of only outdoor air. Care must be taken avoiding that fan is going to work at points of instability. In such case, fan speed must be lowered, either by acting on inverter frequency, if present, or by varying the pulleys diameter.

Keep relative humidity set point above 40%

  • Low relative humidity values make mucous membranes dry, reducing their barrier function against viruses.
  • Therefore, in winter operation, air must be kept at least 40% relative humidity. If humidification is needed and the system is not equipped with a humidification system, use of local humidifiers must be evaluated taking into account the climatic conditions.
  • In summer, the problem of low relative humidity should never arise. Should this occur, it is advisable to act by increasing the minimum saturation temperature, that is, the temperature set-point of cooling coil outlet fluid.

Ventilation continuous operation (24hrs)

  • Although there is no evidence that introducing outdoor air even during off-hours helps reduce the risk of contracting the virus, the precautionary principle suggests doing so. Continuous operation on a daily basis ensures that indoor air is at outdoor air conditions when the premises are reopened

Summary of practical measures for HVAC operation

  1. The best action to limit any risk of COVID-19 infection by air is to ventilate indoor environments with outdoor air as much as possible;
  2. Keep air moving, prevent stagnant air from forming
  3. Non recirculating mechanical ventilation systems and air conditioning systems, which also provide ventilation, can perform this function more effectively than simply opening the windows, also because they improve the quality of the outdoor air with filtration;
  4. Switch off recirculating air systems or close dampers and switch to outdoor air ventilation only – 100% outdoor air
  5. Secure ventilation of spaces with outdoor air
  6. Switch ventilation to nominal speed at least 2 hours before the building usage time and switch to lower speed 2 hours after the building usage time
  7. At nights and weekends, do not switch ventilation off, but keep systems running at lower speed
  8. Ensure regular airing with windows (even in mechanically ventilated buildings)
  9. Keep toilet ventilation 24/7 in operation
  10. Avoid open windows in toilets to assure the right direction of ventilation
  11. Instruct building occupants to flush toilets with closed lid – due to the risk of spreading the virus via air borne faecal particulates
  12. Inspect heat recovery equipment to be sure that leakages are under control
  13. Switch fan coils either off or operate so that fans are continuously on
  14. Do not change heating, cooling and possible humidification set points
  15. Do not plan duct cleaning for this period
  16. Replace central outdoor air and extract air filters regularly
  17. Regular filter replacement and maintenance works shall be performed with common protective measures including respiratory protection
  18. Hot and Cold system mixing air between individual rooms due to recirculation will result in spreading the virus within the rooms, example; A single Air Handling Unit, AHU suppling air to a number of individual rooms and recirculating all extracted air back into the rooms

HVAC – Heating, Ventilation, Air Conditioning

The different types of HVAC systems explained and the risk associated with each one for the spread of air borne COVID19

A. Hot and Cold system recirculating extracted air serving one room only such as radiant heaters (blown hot air), air conditioning units and fans in individual rooms will enable the virus to spread within that room. Filtered systems are fairly ineffective in preventing the spread of the virus, any virus captured in the filter can remain active within the filter fibres and dispersed back into the atmosphere when dislodged – High Risk

B. Hot and Cold system recirculating extracted air from the rooms (mixing) between individual rooms will enable the virus to spread, example; A single Air Handling Unit, AHU suppling air to a number of individual rooms and recirculating all extracted air back into the rooms – Very High Risk

C. A mechanical ventilation system serving any number of rooms will force fresh outdoor air into the rooms via the air intake and ventilation supply air ducts, this will not result in the virus passing between one room to another. Air in the rooms is extracted via the ventilation extract air ducts and forced outside. Any virus concentration in the room is diluted by the presence of outdoor fresh air – Low Risk

D. All-Air HVAC system which mixes outdoor air with recirculated air extracted from the rooms has the potential to spread the virus between the rooms, however the outdoor air will dilute the virus concentration. To reduce the risk further the recirculation dampers should be closed – Medium Risk – Low Risk

E. A single zone ALL-Air HVAC system used in large open commercial spaces are typically designed to provide fresh air only, and to improve the effectiveness of this equipment to reduce the presence of the air borne virus the recirculating damper should be switched off – Medium Risk – Low Risk


The two sources of the most up to date information this blog was taken from was the Chartered Institution of Building Services Engineers (CIBSE) and REHVA, the European Federation of Heating and Ventilation Engineers.

For the unedited versions of all guidance provided by both bodies, links are provided below.

Source of information (accessed 30th June 2020) (accessed 30th June 2020)

Author: beeiit

All comments welcomed, thank you

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