In HVAC systems there are some issues that are considered as secondary importance, but they are instead essential for the correct functioning of the system. In technical rooms and buildings it is essential to maintain a certain overpressure so that no dangerous substance can enter in the buildings. Europair has studied, developed and tested a particular damper for a definite and accurate regulation of the ambient overpressure. The same damper can be used as a non-return damper for all systems where several fans are required for safety, not all operating at the same time.