In modern, more dense buildings with controlled ventilation
systems, the combustion air must be supplied from the outside so there can not arise a vacuum in the living room. This requires a separate external air supply.

Please note: The simultaneous operation of a stove with solid
fuels and controlled ventilation systems is only permitted, if the stove has an approval under building law as independent of ambient air stove according to DIBT. The stove has to be equipped with a security device e.g. low pressure warning device that has access to the ventilation systems and is able to turn them off in case a vacuum arises.
Please find more information about DIBT in the user manual.
Please check for additional local requirements in your country.