Let´s get the facts straight regarding ventilation terms in relation to COVID19
Toine van den Boomen addresses the confusion in the market when it comes to ventilation terms in the context of the pandemic
COVID19 has trained the spotlight on the importance of ventilation and indoor air quality, following increasing global awareness on its impact on the health of building inhabitants and rate of infection. However, there is some confusion in the market when it comes to certain terms used to discuss ventilation in the context of the pandemic.
What is the purpose of ventilation?
Ventilation is different from circulation and internal leakage is different from recirculation. Ventilating by opening the windows is an emergency measure and, overall, there is a need to clarify the different terms to avoid being used interchangeably.
Ventilation is the intentional replacement of used indoor air with fresh, outdoor air. The air is used by the people inhabiting the space, who inhale oxygen (O2) and breathe out carbon dioxide (CO2). The concentration of CO2 serves as a measure of the quality and freshness of indoor air. However, the concentration of virus particles in the air also depends on how many sick people are inside.
A ventilation system facilitates the introduction of fresh outside air in an intentional and controlled manner.
The drawbacks of opening windows
Fresh outside air can typically enter the building through inﬁltration, which is considered unconscious or non-intentional ventilation. A way to temporarily increase ventilation through infiltration is to manually open a window or door. By consciously making larger openings in the facade, more fresh outside air will ﬂow in, allowing for better ventilation. More fresh air indoor leads to lower CO2- concentration and reduces the possible concentration of harmful virus particles.
However, there are major drawbacks to opening the windows, including reduced energy efficiency because of increased heat requirement and pollution in terms of outside particles, insects and even noise. It also leads to drier air, especially during the winter, which makes inhabitants more susceptible to respiratory infections including COVID19. We recommend maintaining indoor relative humidity between 40% and 60%.
What does recirculation refer to?
In the reporting surrounding COVID19 and ventilation, internal leakage in a central ventilation unit is often confused with recirculation. Following greater awareness on the Sick Building Syndrome (SBS) of the 1990s, we no longer use recirculation systems. SBS was caused by often too little ventilation and too much recirculation to save energy. Together with inadequate maintenance of the systems, this resulted in buildings where inhabitants became ill.
Recirculation is largely 50%-90% reduction of "used" air. Essentially, the air that is extracted from a room is returned to the same or other rooms. This is done consciously in new systems for clean rooms, surgery rooms and operating theatres, where the recirculated air passes through a very good filter, such as HEPA filters, so that the supply air to the room will be rinsed clean.
If there is a concern that virus particles could be spread again through the exhaust air from a certain room throughout the building, it is, in principle, possible to upgrade recirculation systems with virus-killing UV lamps or photocatalysis filters.
An important condition for the application of UV and photocatalysis is to make sure that there is no ozone (O3), which is carcinogenic, or other residues dangerous for humans are formed in the supply air. That could make the remedy worse than the disease. By installing a balanced air handling “fresh air ventilation” unit, a good and healthy indoor climate can be guaranteed.
The table below gives a weighting of the various ventilation options.
|Open window||Natural ventilation||Hybrid ventilation||Circulation||Recirculation||Balance ventilation|
|Refreshment regarding COVID 19||++||−||+||−−||−−||±||++||++|
|Sound of Outside||−−||−||−||++||++||++||++||++|
|Pollution of Outside||−−||−||−||++||++||++||++||++|
Weighting according to system performance in comparison: ++ good + sufficient ± neutral − insufficient −− poor
Weighting criteria explanation
|Refreshment regarding COVID 19||How much fresh air is coming in? With natural ventilation this depends on the wind. You can choose this yourself with mechanical ventilation. An air conditioner is not ventilation. Central recirculation is undesirable because of the possible spread of COVID 19 through the building.|
|Tour||Controlled introduction of treated ventialtion air produces less draft.|
|Sound fans||If fans are closer to the user, the chances of noise disturbance increase.|
|Sound of Outside||With more openings in the facade, more outside noise will enter.|
|Pollution of Outside||With more openings in the facade, more particulate matter will enter from the outside.|
|Controllability||Balanced ventilation allows temperature control of the ventilation air (including cooling)and humidification (40%<RH<60%)|
|Energy heating||More fans use relatively more electrical energy.|
|Energy fans||With balanced ventilation, heat and moisture recovery is possible / mandatory.|
|Compatibility||Local systems are often better suited to existing buildings than central systems.|
|Installation space/budget||More installation takes up more space and requires a higher budget.|