In a previous article, we talked about indoor pollution and the fact that, contrary to popular belief, indoor environments can be even more polluted than outdoor ones. This was confirmed, for instance, by a study conducted by the Royal College of Paediatrics and Child Health and the Royal College of Physicians, which showed that the former can be 5 to 13 times more polluted than the latter.

But is there a link between the air we breathe at home or at work and the air that enters our lungs as we walk down the road? According to a recent survey carried out by the Institute of Atmospheric Physics of the Chinese Academy of Sciences, it would seem so. For a whole month, the concentration of PM2.5 in the air was monitored inside an office and in the immediate vicinity outside. During the analysis, the windows were opened and closed several times in order to better understand the mechanism of indoor/outdoor air exchange. The result? Most of the aerosol particles found indoors came from outdoors.

So this study dispels the myth that periodically opening the windows is beneficial, regardless of the type of room and outdoor air conditions. Should the quality be poor, this would only intensify people’s exposure to PM2.5 and other dangerous particles, which are known to have a much higher outdoor concentration.

Natural ventilation versus controlled mechanical ventilation

As we have already seen, controlled mechanical ventilation is a system technology that ensures air purification in indoor environments whilst recovering the thermal energy contained therein. A solution that has enormous advantages and has nothing to do with natural ventilation that is achieved by opening the windows.

The latter option, for example, does not allow inlet air to be filtered, can lead to feelings of discomfort due to the significant temperature difference between the indoor and outdoor environments, and also be completely inefficient in terms of energy.

Controlled mechanical ventilation units, on the other hand, protect people from exposure to particles that are dangerous to their health, through special sanitisation systems, and reduce energy waste by recovering heat from the air.

So let’s look at two components that best represent mechanical ventilation and make it a truly advanced solution: sanitisation systems and the heat recovery unit.

The heat recovery unit: active or passive?
The heat recovery unit: active or passive?

While it is true that controlled mechanical ventilation provides undeniable advantages in terms of improved air quality that positively impacts on people’s health, this technology is also extremely efficient in terms of energy saving. Thanks to a component inside the units called a heat recovery unit (or exchanger), the thermal energy of the exhaust air that is drawn in and expelled outdoors is not lost, but rather contributes to ensuring that the clean, filtered air from the outdoor environment is at the right temperature.

This type of heat recovery is referred to as passive or static and is unlikely to ensure heating and cooling of the air in mid-season, when it is neither too hot nor too cold. On the other hand, active or thermodynamic recovery is when the traditional passive exchanger is combined with a heat pump refrigerant circuit, which starts up when needed, thereby ensuring a second recovery (i.e. heating or cooling the air) and making the system even more efficient.

Here's an example. Clivet’s ELFOFresh EVO VMC unit uses this technology to introduce air that is warmer than the ambient air temperature for heating and cooler air for cooling, thus fulfilling up to 85% of the building’s thermal demand and even achieving 100% in mid-season.

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