EBC Annex 35 Control Strategies for Hybrid Ventilation in New and Retrofitted Office Buildings (HYBVENT)


Status: Completed (1998-2002)

Operating Agent: Dr Per Heilselberg, Indoor Environmental Engineering, Aalborg University, Sohngaardsholmvej 57, DK-9000 Aalborg, Denmark.
Tel: +45 9635 8541
Fax: +45 9814 8243
Email: ph[at]civil.aau.dk

Website: www.hybvent.civil.aau.dk


Some Definitions

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


Participants: Australia, Belgium, Canada, China (Hong Kong), Denmark, Finland, France, Germany, Greece, Italy, Japan, Norway, Sweden, the Netherlands, United Kingdom, USA



Hybrid Ventilation, State of the Art Review
edited by Angelo Delsante and Tor Arvid Vik  Download

Principles of Hybrid Ventilation
edited by Per Heiselberg
Hybrid Ventilation Centre, Aalborg University, 2002, 73 pp. + CD ROM.  Order Printed Copy £20.00 (Order Code ANN 35 2002:1)  Download

An Integral Solution for Ventilation, Health and Energy
The Second International One-Day Forum on Hybrid Ventilation
Held at Delft University of Technology, 14th May, 2001 Download

A comprehensive list of technical reports and research papers for this annex is also available at the Hybvent Website