Ventilation
Ventilation is the exchange of indoor air with air from outdoors and the distribution of this air within the space.
It is fundamental for ensuring indoor air quality as it helps to:
- dilute pollutants: by introducing outdoor air, ventilation reduces the concentration of pollutants generated indoors;
- remove pollutants: ventilation removes stale air containing pollutants, moisture, and odors;
- control humidity: proper ventilation can help maintain comfortable humidity levels, preventing issues like mold growth and condensation.
Types of ventilation
Ventilation airflow is driven by pressure difference. Ventilation can be classified into different types, depending on how the pressure difference driving the airflow is created: mechanically forced airflow or naturally induced airflow.
Mechanical ventilation
Mechanical ventilation uses fans to control the amount of air supplied to the building.
Mechanical ventilation offers several key benefits. It allows for precise control over the amount of air entering a space, ensuring a consistent and adequate supply of outdoor air. Additionally, the air can be filtered, removing outdoor air pollutants, and the supply air temperature can be adjusted for optimal comfort. Furthermore, heat recovery systems can be integrated to conserve energy by transferring heat from outgoing air to incoming air.
While the initial setup and ongoing maintenance of mechanical ventilation may be more costly, its ability to maintain a reliable ventilation rate and ensure good indoor air quality makes it a worthwhile investment.
Natural ventilation
Natural ventilation is the process of air entering a building through openings like windows, doors, and vents. Natural ventilation is driven by natural forces (wind and buoyancy).
Because the airflow is not well controlled, it can often lead to insufficient ventilation rates. Furthermore, natural ventilation can be problematic in areas with high air pollution or old outdoor temperatures, as the incoming air is neither filtered nor conditioned.
Hybrid ventilation
Hybrid ventilation is a combination of mechanical and natural ventilation methods to manage airflow in buildings.
Ventilation rate
The ventilation rate is the amount of outdoor air that is supplied to a space per unit of time. It is a critical factor in maintaining good IAQ as it directly influences the concentration of pollutants and moisture levels.
The recommended ventilation rate depends on the specific use of the space and the number of occupants. Generally, higher ventilation rates are recommended for spaces with more occupants or where pollutant sources are present.
Air distribution
Air distribution refers to the way that air is circulated and spread within a space. It determines how much how effectively outdoor air reaches all areas of a space and how efficiently stale air is removed.
How to assess ventilation performance
Ventilation performance can be assessed using two key indices: air change efficiency and ventilation effectiveness.
Air exchange efficiency describes how well outdoor air is distributed within a room, while ventilation effectiveness measures how well airborne pollutants are removed.
Air exchange efficiency
This indicates how effectively fresh air is distributed within a room. It is influenced by factors such as the location of supply and exhaust outlets, the layout of the room, and the presence of obstacles to airflow. A higher air exchange efficiency means that outdoor air is more evenly distributed, reducing the risk of stagnant zones where pollutants can accumulate.
Air exchange efficiency can be measured using tracer gas techniques. A tracer gas is released into the room, and its concentration is measured at different locations over time. The data is then used to calculate how quickly and uniformly the tracer gas is mixed with the room air.
Ventilation effectiveness
This measures how efficiently a ventilation system removes airborne pollutants from a space. It takes into account not only the ventilation rate but also the location of pollutant sources and the airflow patterns within the room. A higher ventilation effectiveness means that pollutants are removed more quickly and effectively, leading to better indoor air quality.
Ventilation effectiveness can be measured using tracer gas techniques. In this case, the tracer gas is released at the location of a pollutant source, and its concentration is measured at the exhaust vent. The ventilation effectiveness is then calculated as the ratio of the tracer gas concentration at the exhaust to the concentration at the source.