Alexandre Burgaud, Laure Toullier, Sofia Chajon Gomar, Mya Jamal Lahjouji
As humans, one of our main concerns is to feel good and healthy when indoor. However, our comfort must deal with other issues such as economy and ecology. In order to have an idea of how we can combine those parameters, we had to analyze two different ways to act on human well-being: natural and mechanical ventilation.
Natural ventilation is an airflow through openings, such as windows or doors, induced by a pressure difference between inside and outside the building. Mechanical ventilation is an airflow controlled by specific devices.
To surround the subject, we question ourselves on factors influencing human condition in buildings, the optimal design for the best ventilation, and current progress on ventilation systems. Natural ventilation suffers from a huge variability but is the best way to save money, energy, and to renew indoor fresh air especially for diseases concerns. As for mechanical ventilation, it provides a better thermal comfort, gives better protection against air polluters, but sacrifices an important amount of energy.
Effect on humans and influencing factors in building
Building ventilation has a major impact on mood and more generally on health. It’s necessary in order to ensure a good indoor air quality by removing contaminants, odors, moisture, to provide a thermal comfort of the indoor environment and to guarantee a good air distribution. Otherwise, low ventilation rates can cause several infections and sick buildings symptoms.
Therefore, we illustrate the effects on humans and factors in building based on a one-year-study [1] in Urumqi, China. In order to collect data, scientists performed several measurements (using sensor packs) and surveys, in 5 apartments with mechanical ventilation: 4 apartments had heat recovery and one was without, and 4 apartments had only natural ventilation. In this region, winter is cold (in January, the average outdoor temperature is -15.2°C) and in summer, temperatures are in average less than 30°C. We will assume NV for natural ventilation and MV for mechanical ventilation.
In this study we can clearly notice the difference between NV and MV: the evaluation of an indoor climate environment by long-term residents gives an average of 6.25/ 10 for NV and 8/10 for MV. Indeed, an advantage of MV lies on the control of the amount of air coming inside and remove moisture or odors. Moreover, people are not affected by outside noise and pollutants can be filtered. By contrast, NV cannot really regulate thermal comfort (heat flow is not constant and depends directly on wind, pressure difference and outside temperature). However, we are aware that NV provides a healthier lifestyle as people have a connection to outside. Indeed, Controlled Mechanical Ventilation (CVM) requires regular maintenance or the air coming from it could contain toxic particles or bacteria that may be harmful to users.
From the study in Urumqi, the four apartments using only NV had in winter a relative humidity of 29.3%. Whereas the one using MV had a relative humidity of 26.1%. During winter, as people usually open less windows, the relative humidity is consequently higher in naturally ventilated apartments. However, relative humidity drops quickly by opening windows. Therefore, relative humidity is lower with MV.
People’s feeling about air-dryness is significantly different in apartments using NV rather than MV. Indeed, in our case-study, in apartments using NV, 3 out of 4 air-dryness evaluations by long-term residents answered no. Whereas in MV, people would rather feel obvious air-dryness (3 out of 5) or feel a weak air-dryness (2 out of 5). Thereby, the use of humidifiers was almost always necessary in MV. Evidently, NV gives a better feeling to users regarding air-dryness.
CO2 concentration should stay beneath 1000ppm, otherwise it would influence human performance. Its concentration varies depending on using NV or MV. In our case study, CO2 concentration levels were lower with MV, due to improved ventilation. Moreover, when opening windows, CO2 concentration drops slowly. Also, if the outdoor air has a significant CO2 concentration, its concentration will also increase indoor if using NV.
All the measurements recorded during the case-study showed that in NV, the higher relative humidity is, the higher the CO2 concentration is. However, measurements did not show this correlation in MV.
Mean temperature in winter appeared to be higher in naturally ventilated apartment : 24.3°C compared to 22.7°C for MV. But it is quite equivalent in summer: 26.3°C for NV and 26.6°C for MV. With MV, temperature stays constant while large temperature variation occurs with NV when opening windows, as the temperature drops quickly.
Design recommendations
NV uses wind forces and stack effects, due to air density differences between inside and outside the building, to continually provide fresh air in buildings. It therefore strongly depends on the design of the building. Indeed, it is more complicated to naturally ventilate a tower than a ground-floor house. We must therefore consider the design of our structure and adapt the ventilation.
For natural ventilated buildings we must maximize wind-induced ventilation. We are able to do it by positioning the building ridge so that it is perpendicular to wind direction. Also, naturally ventilated buildings should both be narrow and have several openings (windows, doors …) oriented in a way that does not block the passing air flow. Finally, some structural details such as staircases or attics allow the use of passive systems such as crossed, induced or vertical ventilation. Indeed, higher the ceiling is, lower the resistance to flow is. Consequently, cross ventilation is improved.
Advances in Mechanical Ventilation
While NV seems like a good way to clean the air in your home, it cannot guarantee you an adequate air flow. This type of ventilation is no longer preferred and has been replaced by MV.
Unfortunately, MV requires energy consumption. Being a global concern, energy conservation has therefore influenced the types of MV and has given rise to many advances.
Indeed, depending on our choice, CMV allows significant energy savings. Among them we can find single or double flow systems, humidity sensitive ones, but also hybrid ventilation systems.
Unfortunately, some of these systems are not that ecological. Dual-flow CMVs, for instance, are not much more environmentally friendly than single-flow ones because even if the energy consumption is limited, the carbon footprint is not reduced. Still, advances make it possible to couple ventilation systems with geothermal heat exchangers with low energy consumption to reduce the carbon footprint.
To conclude, we see that mechanical ventilation is globally the best way to provide a good comfort, with a better control on many aspects such as thermal constancy or pollutants filtering. Also, it has some important drawbacks, especially environmental and economical ones whereas an adequate building design can efficiently reduce energy consumption. As innovations in mechanical ventilations still produce a great amount of carbon even if environmental considerations rise more and more, we can think that our ventilation will not remain entirely mechanical but would better be a mix of both solutions.
References
[1] Effect of mechanical ventilation and natural ventilation on indoor climates in Urumqi residential buildings, 2018, by Yi Zhao, Hejiang Sun and Daixin Tu, available on : https://www.sciencedirect.com/science/article/pii/S0360132318304906
[2] Natural Ventilation, National Renewable Energy Laboratory, by Andy Walker, 08.02.2016, available on : https://www.wbdg.org/resources/natural-ventilation
[3] Advantages and disadvantages of mechanical ventilation systems, Hunker, by Julie Bawden-Davis, 05.10.2015, available on : https://www.hunker.com/12556797/advantages-disadvantages-of-mechanical-ventilation-systems
[4] Ventilation croisée, effet de cheminée et autres concepts de ventilation naturelle, Build-Green, 11.11.2019, available on : https://www.build-green.fr/ventilation-croisee-effet-de-cheminee-et-autres-concepts-de-ventilation-naturelle/
[5] Natural or mechanical ventilation? What should you opt for ?, Airovient Engineering Green air, 06.21.2016, available on : http://www.airovient.com/natural-or-mechanical-ventilation-what-should-you-opt-for/
[6] Basics of natural ventilation, CoolVent (MIT), 2020, available on: http://coolvent.mit.edu/intro-to-natural-ventilation/basics-of-natural-ventilation/