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Ventilation system – comfort & protection of the building structure

Photo: © Suriya KK /

A ventilation system primarily provides required air changes in living spaces. This is necessary for several reasons: On the one hand, this is to maintain the air quality at all times and, on the other hand, to increase comfort. After all, air quality is also a measure of quality of life. Besides the health aspect, ventilation has another important background. In everyday life, moisture is created in a building in different ways. This must be removed to prevent moisture damage and mould formation.

Statutory regulations for ventilation in buildings

Stricter regulations for new build of houses and apartments have led to residential buildings making significant progress in terms of energy savings and efficiency in recent years. While an existing building still requires an average of over 200  kilowatt hours  (kWh) per square metre for its annual energy demand, the figure for new buildings is less than a third. The annual energy demand refers to the heating energy that must be provided, taking into account the losses caused by transmission and ventilation.

The function of mechanical ventilation explained

Experts distinguish primarily between natural and mechanical or controlled mechanical ventilation (CVC). Window ventilation is one of the best-known and oldest forms of natural or free ventilation. There is also gap ventilation, rooftop ventilation and shaft ventilation. The ventilation process is made possible by natural forces such as the pressure differences between the indoor and outdoor air, thermal updraught and, last but not least, the wind. In many buildings, however, natural ventilation is not sufficient to ensure moisture protection.

This is where mechanical ventilation comes in, which operates independently of external influences and users. In this context, four terms that play an important role in the function need to be explained:

  • Outdoor air: This refers to the fresh outdoor air that flows to the appliance or is drawn in.
  • Supply air: After the outdoor air has passed through the appliance, it is passed on to the rooms as supply air.
  • Extract air: This is the volume of air that is in the room and flows to the appliance or is actively drawn in during the ventilation process.
  • Exhaust air: Once the extract air has passed through the ventilation unit and arrived outdoors, it is called exhaust air.

In controlled mechanical ventilation, a general distinction is made between central and decentralised systems. Depending on the area of application, both systems have their individual advantages. Both are able to recover heat and thus significantly reduce the ventilation heat demand. A fundamental distinction between ventilation systems lies in their individual design. Which ventilation system is ultimately best suited to which building can be determined through expert consultation. 

How does a central ventilation system work?

In the case of central mechanical ventilation, a single appliance supplies the rooms with fresh air. For this purpose, a central ventilation unit is installed in the wall or ceiling. Ventilation is provided via a connected air distribution system, which is usually concealed in the floor or ceiling structure. In the simplest design, such a ventilation system only has an extract air system that discharges stale air to outdoors. Fresh air only enters the house passively through leaky building envelopes, door slits, window gaps or outdoor air passages.

Alternatively, supply air enters supply air areas (living rooms, children's rooms or bedrooms) on one side, and extract air is drawn from extract air areas (kitchen, bathroom or WC) on the other side. Central systems are mainly used in new build, as a higher planning effort is required. But they can also be used in older buildings. In the course of modernisation, for example, the system can be accommodated in a suspended ceiling.  

Ventilation with summer bypass

The summer bypass enables temperate heating during warm summer nights even without opening windows. This involves bypassing the heat exchanger depending on the room temperature and the outside temperature. This allows fresh and cool outdoor air to flow into the living space.

How does a decentralised ventilation system work?

Unlike a central system, the entire living area is not supplied by a single appliance. Instead, several ventilation units are used to supply individual rooms with fresh air. The units are usually installed directly in the external wall of the rooms to be supplied. There is no need to install air ducts. This is why decentralised systems are particularly suitable for modernisation, as all that is needed is a drilled hole in the external wall and a power supply. However, decentralised mechanical ventilation can also be installed in a new build. This option is particularly interesting for apartment buildings (only certain rooms) or in granny flats. 

Depending on the design, a distinction is made between continuous operation and push-pull operation. A ventilation system with push-pull operation works alternately in extract air or supply air mode. Two units with two wall openings are required for the installation of such a system. The two units communicate via a common controller. In systems with continuous operation, both air streams flow through one unit without interruption. For the installation of such a system, only a wall opening or drilled hole is required.

Detailed information on the differences and similarities between central and decentralised ventilation can be found in the section Comparison of central and decentralised mechanical ventilation.

Mechanical ventilation with heat recovery  

Heat recovery is a special feature of controlled mechanical ventilation. Whether in a central or decentralised system, a built-in heat exchanger extracts heat from the already heated extract air and transfers it to the usually colder outdoor air flowing in from outside. The outdoor air does not have to be heated to the same extent, which significantly reduces the building's ventilation heat demand. 

The heart of a ventilation system with heat recovery is the heat exchanger. Viessmann central ventilation units use cross-countercurrent heat exchangers. These are constructed in such a way that extract and supply air –– separated from each other by a number of polymer layers –– flow past each other. There is no mixing of the two air flows and only heat is transferred. For many homeowners who want to purchase a central mechanical ventilation system, this function is very important.

The enthalpy heat exchanger is a cross-countercurrent heat exchanger that can recover moisture from the extract air in addition to heat. This is achieved with the help of a semi-permeable membrane. The membrane only allows moisture and heat to pass through, while odours and dirt are retained. The use of an enthalpy heat exchanger is particularly useful in winter when the outdoor air is relatively dry.

What are the advantages of controlled mechanical ventilation?

Irrespective of the building regulations for living spaces set out in the Buildings Energy Act, the advantages of mechanical ventilation are numerous.

For constantly high air quality, the entire air in all rooms must be exchanged every two to three hours. As already mentioned, the airtight building envelope of new and modernised buildings prevents this from occurring naturally. The required air change rate cannot be guaranteed via window ventilation, especially when users are absent. Mechanical ventilation, however, is able to supply rooms with sufficient fresh air at any time of the day –– independently of the user.

Classic window ventilation is considered inefficient, particularly if employed incorrectly. Many people tend to leave windows tilted in the belief that heat will not escape from the room as a result. However, the opposite is the case. Warm indoor air escapes through the window unchecked. This results in higher heating costs. Controlled mechanical ventilation with heat recovery supplies just as much fresh air as required and also recovers valuable heating energy.

An underestimated risk of incorrect ventilation of living spaces relates to the insufficient removal of moisture. In the course of a day, several litres of water are released into the indoor air in the form of water vapour. In a 4-person household, up to 15  litres are produced daily by cooking, showering, drying laundry or watering flowers. This increases the relative humidity. If it is not removed, this moisture can condense on wall surfaces and thus lead to mould formation –– in the worst case causing damage to the fabric of the building. Controlled mechanical ventilation removes moisture independently of the user, thus protecting the building fabric and ensuring that the property retains its value.

The picture shows a cooker, a shower, a washing machine and a plant to illustrate what and how much water vapour is produced that must be removed by the ventilation system.

Other advantages of ventilation include the fact that filters are built into the ventilation systems. On the one hand, these protect the system against contamination, on the other hand they filter particles from the outdoor air and thus protect the occupants. Pollen filters are available for allergy sufferers.

A ventilation system brings fresh outdoor air into supply air areas and at the same time removes stale air from extract air areas. In addition to constant air changes, unpleasant odours from extract air areas do not enter the living space.

Another advantage of controlled mechanical ventilation is particularly evident next to a busy road where there is a lot of noise and air pollution. If windows remain closed, the noise level drops. Outdoor air is cleaned by the filters before it enters the rooms.

For natural ventilation, windows must remain open. But even if they are only tilted, there is a significantly higher risk of burglary. A mechanical ventilation system, significantly reduces this risk. Windows can remain closed. A properly installed and adjusted ventilation system also prevents draughts.

FAQ on controlled mechanical ventilation

We have compiled the most frequently asked questions and answers on this topic for you in our guide to mechanical ventilation.

The Buildings Energy Act does not prohibit natural ventilation such as window ventilation. In practice, however, the use of ventilation systems shows that manual ventilation is no longer necessary. If you want to feel a fresh breeze now and again, you can of course open windows at any time.

Depending on where the ventilation unit is installed and where the air diffusers are placed, ventilation systems can generate noise. However, noise is significantly reduced by proper planning and adjustment of the mechanical ventilation. This is also ensured by integral flow- and sound-optimised components. Also available are silencers that can be installed in the individual air ducts. A central mechanical ventilation system is usually somewhat quieter than a decentralised system. But that does not automatically make it better.  

As outdoor air is not free of dust and impurities, all Viessmann ventilation systems have built-in outdoor air filters. Depending on the area of application, filters of different grades are used. As a rule, they are either coarse filters or fine filters. The latter are also called pollen filters. To protect the air distribution system on the extract air side, extract air filters are inserted into the vents in bathrooms, kitchen and WC. These reliably prevent dirt deposits from entering the ventilation unit and air distribution system. The ventilation units have a filter change indicator that indicates when a filter change is required. If necessary, the air distribution system can also be cleaned.  

In an energy efficient house, fresh air hardly ever enters passively. Ventilation systems are designed to continuously bring only as much air as necessary into the building. Therefore, a ventilation system should run 24  hours a day. When residents are absent, however, the system can be operated at a reduced level.

Downloads and links for Viessmann mechanical ventilation

In the brochure below you will find everything you need to know about mechanical ventilation. Learn more about the special features and functions of central and decentralised mechanical ventilation systems. On the last pages you will find the technical data of individual units clearly and comprehensibly summarised.

Does the individual appliance meet the requirements for use in a residential building? How large are the adjustment ranges of air flows, and what energy efficiency class does the appliance have? Find the information relevant to you in the following data sheets for each of the products.     

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