CEN/TS 18351:2026

• Abstract:

  This document gives guidance on the design of natural ventilation systems and hybrid ventilation systems in non-residential buildings to achieve acceptable indoor air quality. It is applicable to spaces intended for human occupancy in non-residential buildings.; The guidance for design in this document is applicable to natural ventilation systems and hybrid ventilation systems, encompassing the natural ventilation part of the hybrid ventilation systems, only. The type of control and the design process of hybrid ventilation systems is covered by this document.; This document does not set the requirements for the indoor air quality, but utilizes these as input parameters when designing natural ventilation systems and hybrid ventilation systems.; This document does not apply to:; - thermal comfort and overheating;; - health risk from exposure to tobacco smoke;; - air cleaning;; - design of mechanical ventilation systems.

• Status: Published
• Publication Date: 27-Oct-2026
• ICS: 91.140.30 - Ventilation and air-conditioning systems
• Technical Committee: CEN/TC 156 - Ventilation for buildings
• Drafting Committee: CEN/TC 156/WG 20 - Revision of EN 13779
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 28-Oct-2026
• Due Date: 08-Apr-2026
• Completion Date: 28-Oct-2026
• Ref Project: SIST-TS CEN/TS 18351:2027 - Ventilation for buildings - Natural ventilation and hybrid ventilation systems in non-residential buildings - Design

CEN/TS 18351:2026 is a CEN Technical Specification for the design of natural ventilation systems and hybrid ventilation systems in non-residential buildings. It gives guidance for achieving acceptable indoor air quality in spaces intended for human occupancy, and it covers the natural ventilation part of hybrid systems as well as the control concept and design process for hybrid ventilation. It is aimed at designers, engineers, and project teams who need a practical framework for selecting, sizing, and checking ventilation solutions.

What does CEN/TS 18351:2026 specify?

CEN/TS 18351:2026 specifies guidance for designing natural ventilation and hybrid ventilation systems for non-residential buildings. It is focused on spaces intended for human occupancy, and it uses indoor air quality targets as design inputs rather than setting those targets itself.

The document is organised from general principles to detailed design:

• Clauses 1 to 4 cover scope, references, terms, and abbreviated terms.
• Clause 5 explains the design process and the principles of natural ventilation and hybrid ventilation.
• Clause 6 sets out the design requirements that form the basis for the system design.
• Clause 7 gives the design procedure for natural ventilation systems.
• Clause 8 gives the design procedure for hybrid ventilation systems.

It does not apply to thermal comfort and overheating, health risk from tobacco smoke, air cleaning, or the design of mechanical ventilation systems.

Annex	What it covers
Annex A	Graphical method for estimating opening area for natural ventilation, including a worked example for single-sided ventilation
Annex B	Example detailed requirements for natural ventilation systems
Annex C	Example of project stages across the building lifecycle
Annex D	Example of handover, operation, and maintenance of natural ventilation systems
Annex E	Example commissioning checklist for natural and hybrid ventilation systems
Annex F	Control factors for natural ventilation systems
Annex G	Occupied zone guidance
Annex H	Ventilation effectiveness guidance for natural ventilation systems
Annex I	Typology of hybrid ventilation systems

What are the key requirements of CEN/TS 18351:2026?

CEN/TS 18351:2026 is built around one central idea: design the ventilation system so that the occupied zone gets acceptable indoor air quality, while also considering the practical limits of the building, the controls, and the system type. The document separates the design of natural ventilation from the design of hybrid ventilation, because the design logic and control strategy are different.

Natural ventilation: principle, feasibility, and sizing

Clause 5 explains that natural ventilation is driven by wind and stack effect, without fans. The document distinguishes single-sided ventilation, cross ventilation, and stack ventilation, because each one uses the building differently and affects vent placement, airflow path, and room depth.

Clause 7 sets out a step-by-step design route for natural ventilation. The design starts by checking whether natural ventilation is feasible, then moves to the required opening area, vent location, ventilation principle, and the final evaluation of the system design. In practice, this means the designer must decide early whether natural ventilation can stand alone or whether it should become part of a hybrid solution.

Hybrid ventilation: system type and control concept

Clause 5 also defines hybrid ventilation as a combination of natural and mechanical ventilation. The document distinguishes assistance mode and dual-mode systems, and it recognises both alternating operation and simultaneous operation. That matters in practice because the designer has to decide which system is the basic system, which components act as support, and how the switch or balance between modes will be controlled.

Clause 8 requires the hybrid design to be integrated from the start, not treated as two separate systems bolted together later. The design process includes deciding the basic and supporting system, developing the control concept, selecting and detailing the natural and mechanical parts, and evaluating the combined system. The document also notes that a building with separate, unconnected natural and mechanical zones is not hybrid ventilation.

Design inputs and occupied-zone requirements

Clause 6 says that the primary ventilation requirements and the additional design aspects must be applied to the occupied zone. The primary requirements cover air flow rates and indoor air characteristics, while the additional design aspects include energy use, acoustics, draught, pollutants, safety, smoke control systems, and cleaning and maintenance.

This is important in practice because the document does not treat airflow alone as enough. The designer must balance air quality with operational issues such as noise, discomfort, fire strategy, and maintainability.

Useful design outputs and checks

The design procedures in Clauses 7 and 8 are not just conceptual. They lead to practical outputs such as the required opening area, vent position, control strategy, and component selection. Annex A gives a graphic way to estimate opening area, and the document highlights the difference between a required opening area and the actual opening area of selected vents.

The annexes also support checking and delivery. Annex E gives commissioning checkpoints, Annex D covers handover and maintenance, and Annex G and Annex H help the designer judge occupied-zone limits and ventilation effectiveness. In practice, these annexes help translate the design into a system that can be checked, handed over, and operated.

What terms does CEN/TS 18351:2026 define?

CEN/TS 18351:2026 defines the terms used to describe ventilation type, airflow path, and design targets.

• Ventilation - air replacement by purpose-provided means.
• Natural ventilation - ventilation driven only by wind and stack effect.
• Hybrid ventilation - mixed mode ventilation using both natural ventilation and mechanical ventilation, either alternately or at the same time.
• Stack effect - the pressure difference created by indoor-outdoor temperature differences and the resulting density difference in the air.
• Occupied zone - the part of a room where people normally occupy the space and where the indoor environmental requirements apply.
• Ventilation effectiveness - the degree to which the ventilation replaces air, often expressed by contaminant removal effectiveness.
• Primary ventilation requirements - the requirement used to set the needed indoor air quality level or air replacement amount.
• IAQ - Indoor Air Quality, the air quality objective the design is intended to achieve.
• BMS - Building Management System, used for automatic control and monitoring of ventilation components.

Who uses CEN/TS 18351:2026?

CEN/TS 18351:2026 is used by the people who design, specify, review, commission, and operate ventilation systems in non-residential buildings. That includes ventilation engineers, building services designers, architects, project managers, commissioning teams, and maintenance teams.

It is relevant in building sectors such as education, offices, industry, retail, leisure, and healthcare, because the document defines non-residential buildings broadly. The practical tasks covered include selecting the ventilation principle, sizing vents or openings, defining control logic, checking the occupied zone, preparing handover documentation, and commissioning the system.

What changed in CEN/TS 18351:2026 from the previous edition?

CEN/TS 18351:2026 is the first edition of the document. The introduction describes it as a new structured framework for the design of natural ventilation and hybrid ventilation systems in non-residential buildings.

Which standards are used with CEN/TS 18351:2026?

CEN/TS 18351:2026 is linked in its introduction and bibliography to several standards that support the design process:

• EN 16798-1 - provides possible levels of indoor air quality requirements used as design input.
• EN 16798-3 - covers the design and performance of mechanical ventilation, air conditioning, and room-conditioning systems for non-residential buildings, and is complementary to this document.
• EN 16798-7 - provides calculation methods for air flow rates in buildings, and Annex A uses its graphical method.
• EN 16798-17 - gives guidance for inspection of ventilation and air conditioning systems.
• CEN/TS 18335 - covers ventilative cooling systems to achieve acceptable thermal comfort.
• FprEN 15665 - applies to ventilation system design for residential buildings.
• EN 12599 - gives test procedures and measurement methods for handover of air conditioning and ventilation systems.
• EN 16211 - gives methods for measuring air flow rates on site.
• EN 16798-5-1 and EN 16798-5-2 - provide calculation methods for energy requirements and are referenced for control-factor guidance.
• EN ISO 7730 - supports thermal comfort assessment.
• EN ISO 52000-1 - gives the overarching EPB framework.
• EN IEC 60335-2-103 - addresses safety requirements for drives for gates, doors, and windows.
• ISO 16813 - provides general principles for building environment design.

What does the CEN/TS 18351:2026 document contain?

CEN/TS 18351:2026 contains a mix of design principles, procedures, figures, tables, and worked examples. The main text uses figures to show the overall structure of the document and to illustrate natural ventilation and hybrid ventilation principles, while Clauses 7 and 8 present flow-like design pathways for the two system types.

Annex A gives a graphical method for estimating opening area and includes a worked example for single-sided ventilation. Annex B lists example detailed requirements for automatic control, operational strategies, energy use, accessibility, materials, vents, ductwork, and sensors. Annex E provides a commissioning checklist, Annex F gives control factors, Annex G defines the occupied zone, Annex H explains ventilation effectiveness and typical airflow patterns, and Annex I classifies hybrid ventilation system types.

Together, these parts support design, review, commissioning, handover, and operation. The document is therefore useful not only for initial sizing, but also for checking whether the chosen system can be controlled, maintained, and verified in practice.

SLOVENSKI STANDARD
01-januar-2027
Prezračevanje stavb - Naravni in hibridni prezračevalni sistemi v nestanovanjskih
stavbah - Projektiranje
Ventilation for buildings - Natural ventilation and hybrid ventilation systems in non-
residential buildings - Design
Lüftung von Gebäuden - Natürliche und hybride Lüftungssysteme in Nicht-
Wohngebäuden - Planung
Ventilation des bâtiment - Systèmes de ventilation naturelle et hybride dans les
bâtiments non-résidentiels - Conception
Ta slovenski standard je istoveten z: CEN/TS 18351:2026
ICS:
91.140.30 Prezračevalni in klimatski Ventilation and air-
sistemi conditioning systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TS 18351
TECHNICAL SPECIFICATION
SPÉCIFICATION TECHNIQUE
October 2026
TECHNISCHE SPEZIFIKATION
ICS 91.140.30
English Version
Ventilation for buildings - Natural ventilation and hybrid
ventilation systems in non-residential buildings - Design
Ventilation des bâtiment - Systèmes de ventilation Lüftung von Gebäuden - Natürliche und hybride
naturelle et hybride dans les bâtiments non- Lüftungssysteme in Nicht-Wohngebäuden - Planung
résidentiels - Conception
This Technical Specification (CEN/TS) was approved by CEN on 31 August 2026 for provisional application.

The period of validity of this CEN/TS is limited initially to three years. After two years the members of CEN will be requested to
submit their comments, particularly on the question whether the CEN/TS can be converted into a European Standard.

CEN members are required to announce the existence of this CEN/TS in the same way as for an EN and to make the CEN/TS
available promptly at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in
parallel to the CEN/TS) until the final decision about the possible conversion of the CEN/TS into an EN is reached.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TS 18351:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 5
Introduction . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Abbreviated terms . 12
5 Design process and principles of natural ventilation systems and hybrid ventilation
systems . 12
5.1 General. 12
5.2 Primary purpose of ventilation . 12
5.3 Natural ventilation principles . 13
5.4 Hybrid ventilation principles . 14
6 Design requirements . 16
6.1 General. 16
6.2 Primary ventilation requirements . 16
6.2.1 General. 16
6.2.2 Air flow rates . 16
6.2.3 Indoor air characteristics . 17
6.2.4 Setting the primary ventilation requirements . 17
6.3 Additional design aspects . 17
6.3.1 General. 17
6.3.2 Energy use . 17
6.3.3 Acoustic . 17
6.3.4 Draught . 17
6.3.5 Pollutants . 18
6.3.6 Safety . 18
6.3.7 Smoke control systems . 18
6.3.8 Cleaning and maintenance . 18
7 Natural ventilation system design procedure . 18
7.1 General. 18
7.2 Conceptual design approach . 20
7.2.1 General. 20
7.2.2 Natural ventilation design steps. 20
7.2.3 Apply the primary ventilation requirements . 20
7.2.4 Apply additional design aspects . 20
7.2.5 Evaluation of building location and design . 20
7.2.6 Establish driving forces and ventilation principle . 21
7.2.7 Check the effectiveness to a depth of the room . 22
7.2.8 Initial estimation of required opening area . 23
7.2.9 Evaluate ventilation system design . 27
7.3 Basic and detailed design approach . 27
7.3.1 General. 27
7.3.2 Design steps . 28
7.3.3 Apply the primary ventilation requirements . 28
7.3.4 Apply additional design aspects . 28
7.3.5 Evaluate the building location and design. 28
7.3.6 Establish driving forces and ventilation principle . 30
7.3.7 Check the effectiveness to a depth of the room . 30
7.3.8 Find the required opening area . 30
7.3.9 Evaluate ventilation system design. 34
8 Hybrid ventilation system design procedure . 34
8.1 General . 34
8.2 Basic and supporting system . 36
8.3 Conceptual design approach . 36
8.3.1 General . 36
8.3.2 Investigate the feasibility of natural ventilation . 37
8.3.3 Decide on the basic and supporting system . 37
8.3.4 Control operation of the driving forces . 38
8.3.5 Evaluate initial hybrid ventilation system design . 39
8.4 Basic and detailed design approach . 39
8.4.1 General . 39
8.4.2 Design steps . 39
8.4.3 Apply the primary ventilation requirements and additional design aspects. 39
8.4.4 Selection and detailing the natural ventilation system . 39
8.4.5 Selection and detailing the mechanical ventilation system . 40
8.4.6 Develop hybrid ventilation system . 40
8.4.7 Determine the basic and supporting system . 41
8.4.8 Develop control concept . 41
8.4.9 Evaluate the system design . 43
Annex A (informative) Method 1 — graphic representation of EN 16798-7 . 44
A.1 General . 44
A.2 Process. 44
A.3 Example for single-sided ventilation . 44
Annex B (informative) Example of detailed requirements for natural ventilation systems . 46
B.1 General . 46
B.2 Automatic control . 46
B.3 Operational strategies . 46
B.4 Energy consumption . 47
B.5 Accessibility . 47
B.6 Materials and component requirements . 47
B.7 Vents . 48
B.8 Ductwork including structural ducts . 50
B.9 Sensors . 50
Annex C (informative) Example of project stages . 51
Annex D (informative) Example of handover, operation and maintenance of natural
ventilation systems . 53
D.1 General . 53
D.2 Handover . 53
D.3 Handover documentation . 53
D.4 Inspection, service and maintenance . 54
Annex E (informative) Example of commissioning checklist for natural ventilation systems
and hybrid ventilation systems . 55
E.1 General. 55
E.2 Functional testing . 55
E.3 Natural ventilation system . 55
E.4 Hybrid ventilation system . 56
Annex F (informative) Control factors for natural ventilation systems . 57
Annex G (informative) Occupied zone . 58
Annex H (informative) Guidance on the ventilation effectiveness for natural ventilation
systems . 59
H.1 General. 59
H.2 Ventilations effectiveness . 59
H.3 Typical air flow patterns with natural ventilation . 59
Annex I (informative) Typology of hybrid ventilation systems . 61
I.1 General. 61
I.2 Typology of hybrid ventilation systems . 61
Bibliography . 64

European foreword
This document (CEN/TS 18351:2026) has been prepared by Technical Committee CEN/TC 156
“Ventilation for buildings”, the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN/CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to announce this Technical Specification: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Introduction
This is the first edition of the document, making a significant step forward in addressing a topic that, until
now, has not been properly standardized in any CEN publications. Natural ventilation and hybrid
ventilation have been discussed in various research and design contexts, but there has been a lack of
unified guidance specific to their practical application in building design. This document fills that gap by
providing a dedicated and structured framework for the design of natural ventilation systems and hybrid
ventilation systems. It reflects the growing recognition of the need for clear, actionable guidance in this
emerging area of ventilation design.
This document provides an introduction and a comprehensive guidance on the ventilation principles and
the design aspects that may be considered in the design of natural ventilation systems and hybrid
ventilation systems in non-residential buildings to achieve acceptable indoor air quality (IAQ).
Natural ventilation systems can either be designed as a stand-alone system or integrated with a
mechanical ventilation system (or vice versa) offering the strengths of the two systems resulting in a
hybrid ventilation system.
Traditionally, ventilation needs have been met by natural ventilation in which the flow process is driven
by wind and differences in temperature. Making natural ventilation the oldest form of ventilation. Natural
ventilation has developed from being largely uncontrolled systems to a demand-controlled ventilation
system with cooling and ventilation capabilities. Hybrid ventilation was introduced much later at the end
of the 20th century and was described as systems using both natural ventilation and mechanical
ventilation. This shift also calls for a corresponding transition for those involved in the ventilation system
design. Hence, this document is to support the design of these ventilation systems.
The primary role of ventilation is to achieve acceptable IAQ for the occupants by replacing the indoor air,
but can also be applied to achieve acceptable thermal comfort. This document focusing on IAQ, only.
Besides ventilation, the IAQ can be controlled by source control, filtration and air cleaning. All important
aspects and considerations when designing a ventilation system. The design of ventilation systems
commonly considers other aspects like energy, acoustics, maintenance, and durability. These aspects are
only partly covered in this document. The requirements to fulfil the acceptable IAQ can either be set
formulated as performance-based or in a prescriptive manner, both methods are covered in this
document.
IAQ is a fundamental component and plays an important role achieving overall indoor environmental
quality (IEQ), which encompasses all aspects of the indoor environment that affect occupant health,
comfort and well-being. IEQ includes not only air quality, but also thermal comfort, lighting, acoustics,
and other environmental conditions.
Resilient ventilation system design is also an important part to ensure future robustness of the ventilation
solution and how this can cope with various external impacts like future weather conditions.
This document has a link to the following CEN publications:
— EN 16798-1 which includes possible levels of indoor air quality requirements.
— EN 16798-3 applies to the design, energy performance of buildings and implementation of
mechanical ventilation, air conditioning and room conditioning systems for non-residential buildings
and can be seen as complementary to this document.
— EN 16798-7 which gives information on calculation of air flow rates in buildings.
— CEN/TS 18335 applies to the design of ventilative cooling systems to achieve acceptable thermal
comfort.
— FprEN 15665 “Ventilation for buildings - Ventilation systems in residential buildings - Design”
applies to the design of ventilation systems for residential buildings to achieve acceptable indoor air
quality.
1 Scope
This document gives guidance on the design of natural ventilation systems and hybrid ventilation
systems in non-residential buildings to achieve acceptable indoor air quality. It is applicable to spaces
intended for human occupancy in non-residential buildings.
The guidance for design in this document is applicable to natural ventilation systems and hybrid
ventilation systems, encompassing the natural ventilation part of the hybrid ventilation systems, only.
The type of control and the design process of hybrid ventilation systems is covered by this document.
This document does not set the requirements for the indoor air quality, but utilizes these as input
parameters when designing natural ventilation systems and hybrid ventilation systems.
This document does not apply to:
— thermal comfort and overheating;
— health risk from exposure to tobacco smoke;
— air cleaning;
— design of mechanical ventilation systems.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
ventilation
air replacement by purpose-provided means
Note 1 to entry: Prescriptive requirements or performance-based requirements on ventilation are not a matter for
this definition.
Note 2 to entry: Ventilation can be combined with air treatment.
3.2
purpose-provided
provided for the purpose based on characteristics that fit the requirements
3.3
air replacement
mean of dealing with IAQ which replaces the air within an enclosure with air coming directly from
outdoors or passing through a transfer zone or both
Note 1 to entry: Air replacement can be done through ventilation, air leakage and airing.
3.4
natural ventilation
ventilation whose operating principle is based solely on the effect of wind and the stack effect
Note 1 to entry: Open windows can be used for natural ventilation if they are purposed-provided.
3.5
hybrid ventilation
mixed mode ventilation
ventilation whose operating principle is based on the combination of natural ventilation and mechanical
ventilation
Note 1 to entry: Natural ventilation and mechanical ventilation can either work simultaneously or alternating.
3.6
mechanical ventilation
ventilation whose operating principle is based solely on the operation of fans
3.7
airing
air replacement by means that are not purpose-provided
Note 1 to entry: Usually done through manual opening of doors or windows or both.
3.8
stack effect
thermal buoyancy
pressure difference caused by the difference in density between indoor air and outdoor air due to their
temperature difference
3.9
stack ventilation
natural ventilation which relies on vents located at different heights in the sides of a building or room
Note 1 to entry: The concept of side includes facade and roof.
Note 2 to entry: Stack ventilation relies mainly on the stack effect.
3.10
cross ventilation
natural ventilation which relies on vents located on different sides of a building or room
Note 1 to entry: The concept of side includes façades and roofs.
Note 2 to entry: Cross ventilation mainly relies on the effect of the wind.
3.11
single sided ventilation
natural ventilation which relies on one or several vents located on the same side of a room
Note 1 to entry: The concept of side includes façades and roofs.
Note 2 to entry: Single sided ventilation relies on the fluctuation of the wind and on the stack effect at room level.
3.12
vent
opening in the building envelope that is intended for the passage of air
3.13
standard ventilation
ventilation to achieve acceptable air quality under normal occupancy
Note 1 to entry: Acceptable indoor air quality can be linked with occupants or building needs.
3.14
intensive ventilation
boost ventilation
ventilation at increased air replacement rate for a specific purpose limited in time
3.15
non-residential
relating to a building, etc. that is not used for people to live in
Note 1 to entry: One can make the distinction between 6 different groups:
1) educational – e.g. building for education and research;
2) office – e.g. office, trade, inventory, incl. public administration;
3) industry – e.g. warehouses, factories;
4) retail – e.g. retail stores, shopping centres, shops;
5) leisure – e.g. pubs, restaurants, cafes, sport facilities;
6) healthcare – e.g. medical centres, hospitals, clinics.
3.16
intake vent
vent through which air is taken from outside the building
3.17
exhaust vent
vent through which air is discharged to outside the building
3.18
supply air
air that is provided inside the building by a ventilation system or by airing
3.19
extract air
air that is removed from inside the building by a ventilation system or by airing
3.20
occupied zone
part of a room that people occupy under normal conditions of occupation
3.21
discharge coefficient
square of the quotient of the dynamic pressure of a moving fluid upstream of a local device by the
pressure loss generated by this device on the fluid
Note 1 to entry: For an air terminal device, it is equivalent to the quotient of the actual air flow rate flowing through
the air terminal device by the flow rate that would flow through the opening of a free straight duct of constant cross
section (loss coefficient = 1) without that device.
3.22
ventilation effectiveness
degree to which ventilation is effective in replacing the air
Note 1 to entry: It is usually expressed by the contaminant removal effectiveness.
3.23
contaminant removal effectiveness
ventilation effectiveness in terms of removing air contaminants from a breathing zone
Note 1 to entry: It is expressed as the ratio of the air contaminant concentration in the extract air to the average air
contaminant concentration in the breathing zone.
3.24
fan assisted hybrid ventilation
hybrid ventilation based on natural ventilation that can be supported by fans
3.25
stack and wind assisted hybrid ventilation
hybrid ventilation based on mechanical ventilation that can be supported by the effect of wind and the
stack effect
3.26
zone
one or more rooms
3.27
primary ventilation requirements
requirement aiming to quantify a level of indoor air quality to be achieved or an amount of air
replacement to be achieved
3.28
descriptive requirement
requirement in the form of a specified technical solution
EXAMPLE Minimum opening area.
3.29
ventilation system
combination of a ventilation installation and the enclosure it serves
4 Abbreviated terms
For the purposes of this document, the abbreviations given in Table 1 apply.
Table 1 — Abbreviated terms
Abbreviated terms Designation
BMS Building Management System
CO Carbon dioxide
IAQ Indoor Air Quality
IEQ Indoor Environmental Quality
RH Relative Humidity
VOC Volatile Organic Compounds
5 Design process and principles of natural ventilation systems and hybrid
ventilation systems
5.1 General
Figure 1 shows the overall structure of this document and the design route. Design requirements and
criteria are dealt with in Clause 6. The design of the natural ventilation system and hybrid ventilation
system can be evaluated following the pathway in Clause 7 and Clause 8, respectively.

Figure 1 — Overall structure of the document
5.2 Primary purpose of ventilation
The primary purpose of ventilation is to provide acceptable IAQ by diluting or removing air contaminants
within a space. This can be done by natural ventilation, mechanical ventilation, or a combination of the
two called hybrid ventilation.
Natural ventilation and hybrid ventilation shall be designed to meet the requirement, e.g. achieving the
ventilation requirements. Ventilation rate may occasionally be increased or decreased for a specific
purpose during a designated timeframe. The increased part is referred to as intensive ventilation which
can be handled by the ventilation system or airing. The latter can supplement the ventilation system to
increase the air replacement rate.
5.3 Natural ventilation principles
Natural ventilation is replacing the indoor air in the building relying on the natural driving forces, without
the use of fans.
A natural ventilation system is a ventilation system designed to achieve the selected IAQ target values
using vents which relies on pressure differences created by thermal forces or wind forces to ventilate the
space inside a building.
The design of a natural ventilation system requires the establishment of the location and size of purpose
provided vents which are intentional designed vents in the building envelope.
It is important to distinguish between different types of natural ventilation, as the term covers systems
ranging from simple vents kept constantly open to fully automated natural ventilation systems capable
of automatically regulating system components to achieve acceptable IAQ.
Natural ventilation can be divided into three main principles:
— single-sided ventilation;
— cross ventilation;
— stack ventilation (or stack effect).
Figure 2 illustrates a cross section of a building utilizing the three types of natural ventilation principles
in the different rooms.
Key
supply air
extract air
direction of the air
1 single sided ventilation
2 cross ventilation
3 stack ventilation
Figure 2 — Illustration of different natural ventilation principles
5.4 Hybrid ventilation principles
Hybrid ventilation is based on the combination of natural ventilation and mechanical ventilation. Hybrid
ventilation systems are characterized by their switching between ventilation principles or adjusting the
driving forces with the purpose of optimizing the IAQ by utilizing the strengths of both ventilation
systems. The characteristic feature of a building provided with hybrid ventilation is the integration of the
building design and the ventilation installation into the design to ensure effective utilization of both
natural and mechanical driving forces. A hybrid ventilation system is designed to regulating system
components based on a control concept maintaining acceptable IAQ.
Hybrid ventilation can be divided into three main types:
a) A. Assistance mode
— Fan assisted hybrid ventilation that is based on natural ventilation that can be supported by fans;
— Stack and wind assisted hybrid ventilation that is based on mechanical ventilation that can be
supported by the effect of wind and the stack effect;
b) B. Dual-mode
— Dual-mode hybrid ventilation that is based on the alternation in time (e.g. day/night or
summer/winter), or the simultaneous operation, of fully independent natural ventilation and
mechanical ventilation.
Figure 3 illustrates a cross section of a building utilizing the three types of hybrid ventilation in the
different rooms.
Key
supply air
extract air
direction of the air
fan
mechanical ventilation with heat exchanger

1 fan assisted hybrid ventilation
2 dual-mode hybrid ventilation: Natural ventilation and mechanical ventilation
3 stack and wind assisted hybrid ventilation
Figure 3 — Illustration of different hybrid ventilation principles
NOTE In case dual-mode hybrid ventilation, Figure 3 illustrates natural ventilation and mechanical ventilation,
as two fully independent systems in the same room.
6 Design requirements
6.1 General
The primary ventilation requirements and additional design requirements to be fulfilled in the occupied
zone shall be applied for the design of the ventilation systems. These lay the foundation and the
boundaries for the design of the natural ventilation system and hybrid ventilation system.
NOTE See Annex G for guidance on determine the occupied zone.
6.2 Primary ventilation requirements
6.2.1 General
Standard ventilation is the ventilation to achieve acceptable air quality under normal occupancy which
may be based on IAQ parameters like CO , water vapour, etc.
Primary ventilation requirements are used as the basis for the design of ventilation systems. Primary
ventilation requirements consist of a set of requirements. This document does not set the primary
ventilation requirements but gives guidance for the expression and use of these requirements.
Primary ventilation requirements aim to quantify the level of IAQ or the amount of air replacement to be
achieved.
A performance-based approach provides the possibility to assess the design of a given ventilation system.
This approach includes primary ventilation requirements and a corresponding performance assessment
method which may be applied to assess the performance of a ventilation system.
Primary ventilation requirements can be set at the design stage or imposed by external constraints (e.g.
regulation or contract) or both.
It is possible to use the performance-based approach to check the performance of descriptive
requirements (e.g. minimum opening area) from a prescriptive approach.
The prescriptive approach includes primary ventilation requirements expressed in the form of air flow
rates and corresponding descriptive requirements.
Ventilation effectiveness has an impact on the required ventilation rate. The ventilation effectiveness of
1 (complete mixing) is normally applied but shall be adjusted if it differs from complete mixing.
NOTE 1 Standard ventilation could include moisture protection but not include all IAQ criteria.
NOTE 2 EN 16798-1 provides information on possible primary ventilation requirements.
NOTE 3 EN 16798-1 provides information on possible occupancy profiles.
NOTE 4 Different calculation models can be used to assess the different KPIs.
NOTE 5 Annex H provides guidance on the ventilation effectiveness for natural ventilation.
6.2.2 Air flow rates
The primary ventilation requirements can be expressed in the form of air flow rate (e.g. l/s, l/s.m ,
l/s.pers, air replacement rate per room). Air flow rates can be described by descriptive requirements, e.g.
in the form of opening areas.
6.2.3 Indoor air characteristics
Primary ventilation requirements can be expressed on the basis of IAQ criteria. This can be expressed by
a level of performance including:
— a parameter (e.g. CO , RH, VOC);
— a performance indicator (e.g. RH, VOC, CO target level);
— type of exposure (e.g. average concentration, cumulative exposure);
— acceptable deviations expressed e.g. percentage of time below or above a target value, maximum
(peak) limit.
6.2.4 Setting the primary ventilation requirements
The following shall be specified at room level during occupied and unoccupied periods:
— standard ventilation air flow rate;
— minimum air flow rate;
— acceptable deviation.
Air flow rates such as reduced, increased or intensive ventilation air flow rates may be specified.
Primary ventilation requirements based on IAQ shall be specified.
6.3 Additional design aspects
6.3.1 General
The design of a natural ventilation system and hybrid ventilation system is normally not limited to the
standard ventilation or primary ventilation requirements only, but incorporates additional design
aspects like energy use, acoustics, draughts, etc.
6.3.2 Energy use
Energy use related to ventilation is an important factor to be considered when designing the ventilation
system. Including possible energy use for: transport of the ventilation air (fan electrical energy), heating
and cooling, humification or de-humification, controlling system.
NOTE EN ISO 52000-1 establishes a systematic, comprehensive and modular structure for assessing the
energy performance of buildings.
6.3.3 Acoustic
Acoustics related to the noise generated from the internal- and external environment including the
ventilation system itself play an important role when designing the ventilation system.
NOTE 1 EN 16798-1 provides possible criteria on acoustics.
NOTE 2 Noise generated from the internal environment can also include privacy e.g. when sound travels from
one room to another.
6.3.4 Draught
Draught caused by the ventilation is a relevant design aspect when designing the ventilation system.
NOTE EN ISO 7730 and EN 16798-1 provide possible criteria on draught.
6.3.5 Pollutants
Indoor and outdoor concentration of contaminants of concern, where valid data are available, is relevant
to consider when designing the ventilation system.
NOTE EN 16798-1 provides possible criteria on pollutants.
6.3.6 Safety
Opening of vents in the building envelope e.g. during the night-time can compromise the safety. Hence, it
can be relevant to consider this when designing the ventilation system.
6.3.7 Smoke control systems
Smoke control systems is a key consideration when designing a ventilation system.
NOTE The fire safety strategy can be integrated in the design for a ventilation system as smoke potentially can
follow the ventilation air flow paths.
6.3.8 Cleaning and maintenance
The cleaning and maintenance of the natural ventilation and hybrid ventilation components is important
when designing the ventilation system. This includes parameters such as accessibility of the components
that need to be replaced or cleaned.
NOTE Annex D and Annex E include examples of handover, operation and maintenance documentation as well
as a checklist to be considering for commissioning of natural ventilation systems and hybrid ventilation systems.
7 Natural ventilation system design procedure
7.1 General
The primary objective is to identify if natural ventilation is feasible as standalone or if it should be
supported – be an element within the hybrid ventilation approach.
The main goal is to identify the required effective opening area, location of vents, control strategy, and
the main components to develop a design. A resilient ventilation system design is an aspect which can be
included e.g. by looking at different future projection scenarios from weather data to ensure performance.
One important aspect to be considered is the differentiation between the required opening area when
sizing and the actual opening area of the vents that have been selected as well as the control of the
components.
Each design phase (conceptual, basic and detail) is represented by different methods and level of details
to assess the design of the natural ventilation system.
The conceptual design approach gives a rough estimate of the openings area and the feasibility of natural
ventilation. The conceptual design approach described in 7.2 cannot be used as the final design of the
natural ventilation system, only.
The basic and detailed design approach can take its basis from the output of the feasibility study in the
conceptual design approach. The main difference between the basic and detailed design approach is the
level of details given which might reflect the calculations tools and the resolution of the calculations.
Figure 4 illustrates the design procedure of the natural ventilation system design.
Figure 4 — Design procedure of a natural ventilation system
The conceptual design phase and the basic and detailed design approach include some overall
considerations and generic steps which shall be applied when designing a natural ventilation system.
The steps which shall be considered are:
1) apply the primary ventilation requirements;
2) apply additional desi
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