Hybrid ventilation systems may consist of a combination of natural ventilation for air supply and a mechanical exhaust system. The quality level of thermal comfort is influenced by e.g. the way of air supply, the heating system at the facade and internal heat loads. This article deals with these parameters and their influence on the indoor climate based on research in climatic chambers. The article presents: a discussion why and when a climatic chamber test is useful, its possiblities and limitations; an example of a climatic chamber test: test setup and test results; conclusions and recommendations for the design and use of hybrid ventilation system, based on a number of different climatic chamber tests.