The energy performance of two hybrid residential ventilation systems have been investigated and compared to an all mechanical system. The hybrid systems were; a manual control based on fixed dates, and an intelligent control which chooses the energy optimal mode on an hourly basis. Three different climates were investigated. Results from the thermal simulations show that the energy demand of the hybrid systems is less than the all mechanical system. The reduction with the manual control is in the range of 1.3 – 1.7 kWh/m², while the intelligent control has a larger potential in the range of 2.7 – 4.7 kWh/m². Achieving the potential of the intelligent control requires a control system and automatically openable windows. Presently, no such control systems for residential buildings is widely available at a low cost. The system should combine the control of a mechanical and natural ventilation system with simple installation without the need for custom configuration. Such systems would provide a reduced energy demand at a competitive cost compared to alternative measures.