Control Strategies for Hybrid Ventilation in New and Retorfitted Office Buildings (HybVent)

Completed (1998 - 2002)


Definition of Hybrid Ventilation
Hybrid Ventilation is a two-mode system which is controlled to minimise energy consumption while maintaining acceptable indoor air quality and thermal comfort. The two modes refer to natural and mechanical driving forces.

The Purpose of Ventilation
All hybrid systems have to provide air for indoor air quality purposes, but some in addition also provide air for thermal conditioning and thermal comfort during working hours.

The Purpose of the Control System
The purpose of the control system is to establish the desired air flow rate and air flow pattern at the lowest energy consumption possible.


Hybrid ventilation systems can be described as systems providing a comfortable internal environment using both natural ventilation and mechanical systems, but using different features of the systems at different times of the day or season of the year. They are ventilation systems where mechanical and natural forces are combined in a two mode system. The basic philosophy is to maintain a satisfactory internal environment by alternating between these two modes to avoid the cost, energy penalty and consequent environmental effects of all-year-round air conditioning. The operating mode changes with the seasons, and within individual days, such that at any point in time the current mode reflects the external environment and takes maximum advantage of ambient conditions. The main difference between conventional ventilation systems and hybrid systems is that the latter are intelligent systems with control systems that can automatically switch between natural and mechanical mode to minimise energy consumption.

Hybrid ventilation should depend on building design, internal loads, natural driving forces, outdoor conditions and season and fulfil the immediate demands to the indoor environment in the most energy-efficient manner. The control strategies for hybrid ventilation systems in office buildings should maximise the use of ambient energy with an effective balance between the use of advanced automatic control of passive devices and the opportunity for users of the building to exercise direct control of their environment. The control strategies should also establish the desired air flow rates and air flow patterns at the lowest energy consumption possible.


The objectives of Annex 35 were:

  • 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 syste

The annex included three research areas:

Development of Control Strategies

  • Survey of existing strategies
  • Local versus central control
  • Definition of requirements and evaluation criteria for control strategies
  • Development of strategies for switching between ventilation modes
  • Development of strategies for combination of automatic and manual individual control
  • Control system design
  • Demonstration and evaluation of control strategies

Development of Analysis Methods

  • Survey of available analysis methods Achieve better understanding of hybrid ventilation, air flow control
  • Integration of air flow and thermal simulation models
  • Development of probabilistic analysis method
  • Development of decision tool
  • Application and evaluation of analysis methods
  • Application and evaluation of decision tool

Pilot Studies

  • Survey of existing systems and solutions to specific problems
  • Market survey on components
  • Survey on building codes
  • Analysis of hybrid ventilation components and systems
  • Analysis of barriers for hybrid ventilation application
  • Cost-benefit analysis of hybrid ventilation
  • Demonstration buildings
  • Technology transfer


  • State-of-the-Art Review of hybrid ventilation technologies and of control strategies and algorithms. Assessment of potential for hybrid ventilation retrofit
  • Principles of Hybrid Ventilation, including solutions for efficient, energy and cost-effective hybrid ventilation. Recommendations on control strategies
  • 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

Operating Agent

Per Heiselberg
Aalborg University
Department of the Built Environment
Thomas Manns Vej 23
DK 9220 Aalborg
Tel: +45 9940 8541


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