Hybrid Ventilation - The basic philosophy is to maintain a satisfactory indoor environment by alternating between and combining natural and mechanical systems to avoid the cost, the energy penalty and the consequentially environmental effects of full year-round air conditioning.

Annex 35 HybVent - is a taskshared international research project initiated by the IEA Implementing Agreement "Energy Conservation in Buildings and Community Systems (ECBCS)". Annex 35 is a four-year project running from 1998-2002 with about 30 research institutes, universities and private companies from 15 countries world wide participating.

The project has been followed up by an EU project RESHYVENT, which focuses on hybrid ventilation in residential buildings.


For many years mechanical and natural ventilation systems have developed separately. Naturally, the next step is the development of ventilation concepts that utilize and combine the best features from each system to create a new type of ventilation system – Hybrid Ventilation.

Hybrid ventilation systems provide a comfortable internal environment by combining both natural ventilation and mechanical systems. Hybrid ventilation uses different features of each system depending on the time of the day or the season of the year. The basic philosophy is to maintain a satisfactory indoor environment by alternating between and combining the systems to avoid the cost, the energy penalty and the consequential environmental effects of full year-round air conditioning.

Buildings ventilated by hybrid ventilation often apply other sustainable technologies and an energy optimization requires an integrated approach in the design of the building and its mechanical systems. It is expected that hybrid ventilation offers significant opportunities for improving the indoor environment and reducing energy demand.


  • to develop control strategies for hybrid ventilation systems for new build and retrofit of office and educational buildings
  • to develop methods to predict hybrid ventilation performance in hybrid ventilated buildings
  • to promote energy and cost-effective hybrid ventilation systems in office and educational buildings
  • to select suitable measurement techniques for diagnostic purposes to be used in buildings with hybrid ventilation systems


  • State-of-the-art review of hybrid ventilation technologies and of control strategies and algorithms
  • Principles of Hybrid Ventilation, including solutions for efficient, energy and cost-effective hybrid ventilation
  • Control strategies for hybrid ventilation
  • Analysis tools for performance prediction of hybrid ventilation. Decision tools for hybrid ventilation applications
  • Refinement and recommendations of suitable measurement techniques for diagnostics and commissioning of hybrid ventilation systems
  • Demonstration of principles through pilot studies

Research Programme

Annex 35 is divided in three subtasks each devoted to a specific area of interest.

Subtask A: Development of Control Strategies for Hybrid Ventilation

Integrating a natural and a mechanical ventilation system into a common hybrid ventilation system requires development of new control strategies. Strategies are developed to ensure that demands to the indoor environment are fulfilled in the most energy efficient manner.

Subtask B: Theoretical and experimental studies of performance of hybrid ventilation. Development of analysis methods for hybrid ventilation

The hybrid ventilation air flow process is investigated to achieve a better understanding, which is a prerequisite of a successful application. Analysis methods are developed, that focus partly on combining existing methods and partly on developing new methods for air flow control and thermal conditioning.

Subtask C: Pilot Studies of Hybrid Ventilation

Pilot study buildings incorporating hybrid ventilation systems undergo a full year-round monitoring programme. Data are collected to demonstrate performance (IAQ, thermal comfort and energy consumption) and to evaluate control strategies and analysis methods