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Decentralized ventilation system – features, function and funding

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Photo: © jovana veljkovic /

The modernisation of energy systems in existing buildings is constantly being driven by rising energy costs in the housing industry. Old housing stock is being modernised comprehensively and now offers its residents comforts such as energy efficient windows and doors, wall insulation or a new heating system with convenient domestic hot water supply.

Modernisation also results in older apartments being so airtight that no adequate fresh air change can take place. The consequence is high humidity levels in the interior, which can lead to mould growth, especially in buildings with insulated external walls. Areas that are particularly at risk are the corners of rooms next to external walls, as this is where humidity condenses. A decentralised ventilation system with heat recovery is the most suitable solution in this case.

What distinguishes decentralised mechanical ventilation?

Decentralised ventilation with heat recovery not only ensures the necessary minimum air change rate. By recovering heat from the extract air and transferring it to the cold incoming outdoor air, less heating energy is required. A heat recovery level of up to 90 percent is achieved. This reduces heating costs and protects the environment. Energy is used in a very efficient way.

One of the special features of this technology is the relatively simple installation. Decentralised ventilation systems can be specifically installed in individual rooms. There is no need to install an air distribution system. All that is required for straightforward installation is a wall diffuser or a hole drilled through the external wall of the respective room and an electrical connection. Residential units can be equipped with several units that operate independently of each other. A combination of central and decentralised ventilation systems is also possible. The section on ventilation systems shows which system is most suitable for which property.

How decentralised ventilation works

To explain how decentralised mechanical ventilation works, a distinction must first be made. This is because decentralised ventilation systems with heat recovery are principally divided into two groups: On the one hand, there are units with continuous operation and, on the other, those with a reverse operation. Both types can be controlled manually or according to demand via CO₂ or humidity sensors. They are installed in an external wall.

Decentralised ventilation systems with continuous operation are also called permanent fans. They are a complete ventilation unit with a built-in cross-countercurrent heat exchanger. The units have two fans that convey air simultaneously. This results in two air flows. The outdoor and supply air flow brings fresh air into the room, while the extract and exhaust air flow removes stale air from the room to the outside. The cross-countercurrent heat exchanger is positioned between the two air flows. In it, the heat is recovered from the indoor air and transferred to the supply air. This means that a large part of the heat remains in the room despite continuous ventilation.

Due to integral filters and the corresponding design of the unit, neither dirt particles nor noise from outside can penetrate through to the rooms. Permanent fans can supply air to and vent individual rooms and can be installed in the external wall both in supply air areas (living room, bedrooms or children's rooms) and in extract air areas (kitchen, bathroom or WC). One appliance that uses this operating mode is the Vitovent  200-D decentralised ventilation system.

There are also other designations for decentralised ventilation systems in reversing operation. They are called push-pull units or pendulum fans. Unlike their continuous operation counterparts, pendulum fans have only one fan. Two constantly communicating units are responsible for the required air change rate. While one ensures the removal of stale air, the other ensures that fresh air flows in. A thermal store is used for heat recovery, which temporarily stores the heat from the extract air and transfers it to the supply air.

Other aspects of the operating principle of decentralised ventilation with reversing operation: both units are connected to each other via a controller. This ensures that the direction of rotation of the fans is reversed after approximately 70  seconds and the air flow is thus conveyed in the opposite direction. These units also have filters against dirt from outside and protect against road noise. With a pair of pendulum fans, two supply air areas can be supplied with air and vented simultaneously if there is an air connection between the two rooms. As a rule, these units are not installed in extract air areas. The Vitovent 100-D ventilation unit is a pendulum fan.

You can find more tips and information about this in the mechanical ventilation section on

Tips for the purchase of decentralised ventilation systems

When modernising older buildings, a ventilation concept is essential. After all, once modernised, they often achieve similar energy values to new buildings. Decentralised ventilation systems have proven themselves in practice to be the most suitable solution for existing buildings. When buying, you should pay attention to the following points.

When looking for the right ventilation system, the scope of the modernisation is a significant factor. After all, a core modernisation achieves very different results from a partial modernisation. Depending on the building characteristics, a central ventilation system may also be considered for a core modernisation. It is also possible to use a decentralised ventilation system in a new build. Which ventilation system ultimately suits which building depends on many factors. Among other things, the building characteristics of the living space, usage patterns and the number of occupants must be taken into account.  

As already mentioned, decentralised mechanical ventilation is primarily characterised by its room-related use. Each individual ventilation unit requires its own electrical connection. Depending on the ventilation unit, this may be a 230  volt connection, as with the Vitovent  200-D, or a data cable, as with the Vitovent  100-D. Since the units are installed in an external wall, homeowners must also take the building type into account when purchasing a decentralised mechanical ventilation unit. Finally, for use in a new build, there are special wall blocks into which ventilation units can be inserted. Retrofitting of the units, on the other hand, is done with the help of a drilled hole.

Since stale air accumulates at the top of the room, ventilation units should be positioned there if possible. Although the units are quiet, very sensitive people may feel disturbed by the airflow noise, especially in sleeping areas. To avoid this, positioning and design of ventilation units should be well planned in advance.

Compared to central mechanical ventilation, the planning effort for a decentralised ventilation system is lower. However, the expertise of a contractor is essential for efficient and quiet operation. An experienced installer quickly determines how to size the ventilation system and where the optimal placement for the air diffusers is. They will also help with the selection of the appropriate type of unit. Depending on the available space, either a ventilation unit with a built-in cross-countercurrent heat exchanger or one with a single fan and a ceramic thermal store should be considered.

Cost and subsidies for decentralised ventilation

A decentralised ventilation system contributes to better air quality and better building protection. The costs for this are within reasonable limits. In most cases, decentralised ventilation units are cheaper to purchase than a central ventilation system. However, the exact costs can only be determined on a building-by-building basis. 

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