Oh wow, the thread has a small time jump.
Still, on the topic of [Dunstabzugshaube]+[Kontrollierte-Wohnraumlüftung]: Ok, the [Kontrollierte-Wohnraumlüftung] may not run optimally, wildly, or at all during these 20 minutes. Ok, maybe energy is being "wasted." But opening the window or turning on the [Dunstabzugshaube] shouldn’t be a problem at all in connection with [Kontrollierte-Wohnraumlüftung]...