Correct ventilation

Centralised or decentralised domestic ventilation units with heat recovery provide good air quality independently of window ventilation. In addition and unlike with window ventilation, the heat contained in the “used” outgoing air is returned to the incoming air.

Ventilate briefly but thoroughly…

  • The moisture in the room’s air increases when you use the kitchen or the bathroom and from house plants. Insufficient ventilation can result in condensation damage and mould. The “used” breathing air must also be replaced. The following applies as a guideline: Ventilate for five minutes approximately every two hours. To do this, open the window wide and close it again fully after a short time. It is essential to avoid continuous ventilation with a tilted window!
  • The radiator thermostats should be switched off during ventilation.
  • Seal old windows well. It is more economical in terms of saving energy to install new, tightly-closing windows in conjunction with a domestic ventilation system.

Window ventilation

With window ventilation, how long it takes for the used room air to be completely replaced by fresh air from outside depends on both the type of ventilation (through-ventilation, intermittent ventilation, tilt ventilation) and the wind and temperature conditions.

Tilt ventilation (to avoid)

With windows tilted, it takes about 60 minutes for the used room air to be completely replaced. However, the windows usually remain tilted for even longer, resulting in an unnecessarily high rate of air exchange. Another disadvantage: The window reveals, the surrounding components and the internal fittings cool down significantly. This increases the amount of time taken to heat up the room again, and there is a risk of moisture condensing on the cooled surfaces.

If the continuous ventilation is also done with the thermostat valves open, much of the rising warm air from radiators is unused and flows outside. Tilted windows are a welcome “entry opportunity” for intruders. Large amounts of water from driving rain can enter the living areas. Continuous ventilation with tilted windows comes with many disadvantages. For energy reasons, this type of ventilation is at most recommended for periods when the heating is not switched on, from May to September.

Intermittent ventilation

With the window open, it takes between 5 and 10 minutes in the winter months for the used room air to be completely replaced.

Rule of thumb: The colder it is outside, the shorter the ventilation time.


A room can be ventilated very quickly with the window and door – preferably located opposite each other – fully open. This is the most effective and – in terms of energy – economical method of ventilation if the time is strictly limited to a few minutes.

Closed windows

In older buildings, basic ventilation takes place via leaks and draughty windows. Even without opening windows, there is an uncontrolled exchange of air in the heated rooms which is often associated with high energy losses.

Modern buildings are made as airtight as possible. For this reason, user behaviour in new builds and in houses that have undergone energy-efficient modernisation must be changed so as to consciously replace used air with fresh air and to prevent warm, moist room air from causing structural damage and mould.

Windowless inner rooms should have a possibility for ventilation (shaft ventilation, fan ventilation). If this is not provided, the used and malodorous air can reach adjoining living areas. This unpleasant effect can be further exacerbated by unfavourable wind conditions.

Moisture damage: In interior kitchens and bathrooms, there is also the risk of moist air condensing on the cold surfaces of components and causing moisture damage.