On the way to the solution of our energy-problems, the conditioning of buildings is the most importantsector. We have to create new buildings, that produce more energy than they need in the course of ayear, so called plus-energy buildings and we have to perform large-scale retrofitting of existing buildings. Priority must be given to increasing energy-efficiency. Having fulfilled this, the implementation of renewable energy has to be undertaken. To achieve this, new techniques have to be developed, but also the political regulations and the conscience and behaviour of the users have to be seriously taken into account.In the past a lot of developments have been undertaken to reduce the transmission losses. In futurewe have to focus on the ventilation and infiltration losses because of the increase in their relative importance.Even in existing buildings the implementation of central ventilation systems with heat recoveryis easily possible by using a new system, presented in this paper.