REMARK:  The answers that follow were developed in November 2020. The recommendations by ASHRAE and REHVA may change over time. More information can be found in the recordings of the webinar "COVID-19 Ventilation related guidance by ASHRAE and REHVA" held on November 20th, 2020, with presentations by ASHRAE, REHVA and a comparison of the recommendations.

REHVA and ASHRAE agree on their main recommendations, but there are some slight differences in the specifics.

Both REHVA and ASHRAE note that long-range aerosol-based transmission is possible and that ventilation plays an important role to limit the risk of transmission. They stress the need to increase ventilation flowrate (more air renewal) for more dilution and reduction of pollutants concentration. However, the specific ventilation rate to most effectively reduce the risk of transmission of airborne particulate matter is unknown. Therefore, the questions below remain open and REHVA and ASHRAE have slightly different answers. Various explanations can be found for those differences including:

  • Typical ventilation systems in Europe and US are different. While Europe relies on outdoor air ventilation and air conditioning that is typically separated from hydronic heating, in the US air systems for air conditioning typically include heating and cooling and air recirculation with minimal outdoor air intake.
  • The impact of thermal conditions is considered to be more important for ASHRAE than for REHVA It may be partly due to a better acceptation of adaptative comfort in Europe than in the US.
  • Regarding filters, REHVA, recommending HEPA and ePM1 80% filters, has a more “safe-side” approach than ASHRAE who recommends MERV 13 filters but with higher flowrate.

According to REHVA and ASHRAE, how much should ventilation level be increased?

REHVA proposes to adopt the principle of ALARA (As Low as Reasonably Achievable) for the concentration of pollutants. REHVA and ASHRAE agree that mechanical ventilation rate should be set at its reasonable maximum, that DCV system should be overruled (by a setpoint at 400ppm for CO2) and that rooms should be “flushed” before and after occupancy. REHVA recommends 800 ppm CO2 (absolute value) as a proxy of good ventilation.

However, while ASHRAE warns against causing thermal stress, and thereby lowering resistance to infection, and estimate that other methods should be considered first when the energy/comfort/IAQ conditions warrant , REHVA recommends to supply as much outdoor air as reasonably possible and use openable windows much more than normal even if this causes thermal discomfort (for buildings without mechanical ventilation).

According to ASHRAE and REHVA, how should recirculation be used?

ASHRAE recommends upgrading recirculation filters to MERV 13 and does not recommend switching off recirculation. When using MERV 13 filters, ASHRAE recommends maximizing flow through the filter to remove as many infected aerosols as possible. In general, ASHRAE notes that dilution, filtration and disinfection all act together and that in many circumstances’ filtration can be more effective than increased ventilation.

REHVA recommends to close recirculation dampers even if they have air filters. If recirculation cannot be avoided, additional measures for return air filtering should be taken with the installation of HEPA filters or at least ePM1 80% filter (MERV 13 are to be compared to ePM1 50% filters).

For room level recirculation, ASHRAE does not have specific recommendations, while REHVA recommends either to switch off the system or run the fan constantly at low speed (to avoid the collection and release of contaminated particles). However, currently REHVA is updating fan coil recommendations in their version 4 guidance with the focus on sufficient outdoor air ventilation and warning for possible high velocity directed airflows.

According to REHVA and ASHRAE, are filters effective to filter or inactivate virus?

While ASHRAE suggests that MERV 13 filters are sufficient to catch infected aerosols, and estimates that high air change rates through moderately high-efficiency filters do help, REHVA implies that only HEPA filters can filter all virus particles effectively, however accepting ePM1 80% filter as a minimum improvement.

Ultraviolet Germicidal Irradiation (UVGI) technologies are recommended by ASHRAE in all kinds of buildings, while REHVA only states that “UVGI” and Germicidal Ultraviolet “GUV” may be used.

According to ASHRAE and REHVA, how should portable air-cleaning devices be used?

Both ASHRAE and REHVA recommend using portable air-cleaning systems, but REHVA implies it is mostly effective if located in the breathing zone. REHVA only recommends air cleaners with HEPA filters efficiency and says that UVGI may be installed in return air ducts in systems with recirculation or in rooms provided that they are correctly sized, installed and maintained. ASHRAE estimates that UVGI upper room systems are more effective than in-duct in terms of CADR (Clean Air Delivery Rate) but are only suitable in some spaces.

According to ASHRAE and REHVA, can heat-recovery sections be used?

ASHRAE provides extensive information to evaluate on-site the risk of unwanted recirculated air in heat recovery sections, and according to the fan arrangement may recommend to by-pass the heat recovery. REHVA does not provide a method to ensure that there is a 100% air separation but recommends to by-pass heat recovery or to adjust the pressure if critical leaks are detected for rotary air to air heat exchanger.


Valérie Leprince, INIVE


  1. REHVA. REHVA COVID‐19 guidance document How to operate HVAC and other building service systems to prevent the spread of the coronavirus (SARS‐CoV‐2) disease (COVID‐19) in workplaces. 3 August, 2020.
  2. ASHRAE. COVID‐19 response resources from ASHRAE and others. ASHRAE COVID 19 website. Retrieved November 2020.
  3. Mingyue Guo, Peng Xu,Tong Xiao, Ruikai He, Mingkun Dai, Shelly L. Miller. Review and comparison of HVAC operation guidelines in different countries during the COVID‐19 pandemic. Building and Environment (2020), doi:
  4. AIVC. AIVC Newsletter Special Issue on COVID-19. November 2020