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In this blog, our expert Welat Han, Business Developer at Swegon, explains the two most common principles: mixing ventilation and displacement ventilation.
Fresh air is essential for our well-being. Our brains and cells need oxygen to function properly and polluted air can cause more problems than we often realize. Some substances in the air are harmful on their own, but can be even more so when combined with others. In fact, a human being breathe around 15 kg of air every day, while needing less than 1 kg of food and about 1.5 kg of water. The imbalance between air and food/water makes it possible to compare the breathing system to a building’s ventilation system.
The basic task of a ventilation system is to remove contaminated air such as gases, particles etc. and replace it with clean, tempered air. It can also help create a comfortable indoor climate without draughts or large temperature differences in the occupied zone as long as a proper balance between air velocity, air temperature and radiant temperature is ensured.
Why a good indoor climate?
Imagine how much time is spent indoors. On average, people spend around 90% of their time inside buildings. This makes the indoor climate crucial for people’s health and well-being. In fact, research shows a strong correlation between indoor environmental conditions and human health. Poor air quality and uncomfortable conditions can contribute to illness, increase sick leave and negatively affect concentration, productivity and overall performance.
In recent years, not least due to the Covid-19 pandemic, the awareness of ventilation and air quality has increased. Today, a well-designed indoor climate is recognized not only as a health factor but also as a key contributor to energy efficiency, positive financial performance and reduced global impact.
Ventilation principles
One of the first choices that needs to be made when planning a ventilation system concerns which ventilation principle to use. The two most common principles are: mixing and displacement, the latter also known as thermally controlled ventilation.Ventilation should always be adapted to the room design and its intended use as the choice of ventilation principle depends on several factors:
- Room size
- Ceiling height
- Furnishing
- Type of activities performed in the space
The occupied zone is defined by the blue coloured surface in the picture. For Swedish conditions, the air velocity in the occupied zone should be ≤ 0.2 m/s.
The selected ventilation principle must ensure good air quality, a proper temperature distribution and minimal draught.
Mixing ventilation
For mixing ventilation the air diffuser is placed high in the room, close to the ceiling, and the supply air enters at a relatively high velocity. The air is distributed throughout the entire room and is, in principle, mixed homogeneously with the room air. By choosing supply air diffusers carefully, the result can be completely draught-free ventilation.Two examples of air diffusers from Swegon’s product range that are particularly well suited for mixing ventilation are EAGLE Ceiling and KITE Ceiling.
An illustration of how mixing ventilation works. The blue arrow is supply air, entering the room, and yellow is extract air, leaving the room.
In mixing ventilation, the air exchange efficiency is typically 50 %. This describes how effectively the air in the room is replaced. Higher efficiency indicates fewer stagnation zones and better overall air distribution.
Displacement ventilation
In displacement ventilation, the air diffuser is placed low in the room, at floor level. Cold supply air enters at a relatively low velocity and spreads across the floor surface. As the supply air is warmed by heat sources such as people and equipment, it rises upwards and pushes away the room air, which is evacuated via the extract air diffuser at ceiling level.
An illustration of how displacement ventilation works. The blue arrow is supply air, entering the room, and yellow is extract air, leaving the room.
The air exchange efficiency for displacement ventilation is also very high, ranging from 50 to 100 %.
Which principle to use when?
Mixing ventilation is suitable for spaces with low or medium ceiling heights and high comfort requirements, such as classrooms, small restaurants and offices. The supply air enters at ceiling level and mixes with the room air, creating a stable flow pattern and an even temperature distribution. This ventilation principle offers flexible diffuser placement, does not take up floor space and can provide higher cooling capacity than displacement ventilation.
However, it is less effective in large rooms with high ceilings, where cooling the entire room requires more energy and the supply air may not reach the occupied zone. There is also a risk of draughts at high cooling loads and short-circuiting of heated air. Further, mixing ventilation is not suitable in spaces with strong point-source pollution or where contaminated air needs to be removed rather than mixed.
Displacement ventilation, is best suited for large spaces with medium to high ceiling heights, large heat sources and limited cooling demands. Typical examples include industrial premises, airports and large cafeterias. Since the supply air is introduced at floor level, the occupied zone is ventilated first, while warmer and contaminated air is pushed upward toward the ceiling.
The ceiling height should exceed 3 meters, as rooms with greater height benefit most from displacement ventilation. It is also advantageous when distributing large air volumes and is effective in environments with local heat sources, like restaurant kitchens. When cooling with air, both mixing and displacement ventilation can be used, but displacement ventilation often provides greater benefits in suitable conditions.
However, displacement ventilation should be avoided in rooms with low ceiling heights as there is limited space to allow extract air to rise above the occupied zone. It is also not appropriate for premises with a need of heating, as the supply air is colder than the room air, and in heavily furnished spaces where the airflow can easily be blocked.
To learn more and feel confident in the choice of ventilation principle, learn more in our reference cases, Valle Wood and Malmö Live, or in our technical guide. And remember, the ventilation principle is key to ensure an optimal indoor climate for people to feel good inside.
