In the apartment ventilation vents are present above the windows.
They can be opened by pulling gently on them.
To close the ventilation vents they can gently be pushed.

There are 2 or 3 positions for the vents, depending on the need for ventilation of the apartment. We recommend keeping these open at any time to have the cleanest air possible in the apartment. You don’t have to worry about overconsumption in heating since the amount of extracted air has been computed by engineers and will have a marginal effect.