SIST-TS CEN/TS 18351:2027

• 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: 11-Nov-2026
• ICS: 91.140.30 - Ventilation and air-conditioning systems
• Technical Committee: OGS - Heating for buildings
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 11-Nov-2026
• Due Date: 16-Jan-2027
• Completion Date: 12-Nov-2026
• Ref Project: CEN/TS 18351:2026 - Ventilation for buildings - Natural ventilation and hybrid ventilation systems in non-residential buildings - Design

SIST-TS CEN/TS 18351:2027 is a SIST technical specification that adopts CEN/TS 18351:2026 for Slovenia. It gives guidance from CEN on designing natural ventilation systems and hybrid ventilation systems in non-residential buildings. The document focuses on achieving acceptable indoor air quality (IAQ) for spaces intended for human occupancy, with the design process for hybrid ventilation and the natural ventilation part of hybrid systems included.

What does SIST-TS CEN/TS 18351:2027 specify?

SIST-TS CEN/TS 18351:2027 gives design guidance for natural ventilation and hybrid ventilation in non-residential buildings. It is meant for spaces used by people, and it uses IAQ targets as design input rather than setting IAQ limits itself.

The scope is intentionally narrow in a practical sense. The document does not cover thermal comfort and overheating, tobacco smoke risk, air cleaning, or the design of mechanical ventilation systems. That matters because it tells a designer when this specification is the right starting point, and when another document is needed.

The document is organized into eight clauses plus informative annexes A to I. Clause 5 explains the ventilation principles, Clause 6 covers design requirements, Clause 7 gives the natural ventilation design procedure, and Clause 8 gives the hybrid ventilation design procedure.

Annex	What it covers
A	Graphic method for opening area estimates, with an example for single-sided ventilation
B	Example detailed requirements for natural ventilation systems
C	Example project stages
D	Example handover, operation and maintenance for natural ventilation systems
E	Example commissioning checklist for natural ventilation and hybrid ventilation systems
F	Control factors for natural ventilation systems
G	Occupied zone
H	Guidance on ventilation effectiveness for natural ventilation systems
I	Typology of hybrid ventilation systems

What are the key requirements of SIST-TS CEN/TS 18351:2027?

SIST-TS CEN/TS 18351:2027 is built around a design logic: define the IAQ goal, choose the ventilation principle, then check whether the building and controls can deliver it. For a project team, the main task is to turn ventilation intent into a workable opening layout, control concept, and system arrangement.

1. Start from IAQ and the purpose of ventilation

Clause 5 states that the primary purpose of ventilation is to provide acceptable IAQ by diluting or removing contaminants. The document treats natural ventilation, mechanical ventilation, and hybrid ventilation as alternative or combined ways to do that. In practice, this means the designer must first decide what the ventilation must achieve before selecting components.

2. Distinguish natural ventilation from hybrid ventilation

Clause 5 separates natural ventilation, which relies on wind and stack effect, from hybrid ventilation, which combines natural and mechanical ventilation. It also identifies three natural ventilation principles - single-sided, cross, and stack ventilation - and two hybrid patterns - assistance mode and dual-mode. This matters because the chosen principle determines the airflow route, the opening positions, and the control approach.

3. Design for the occupied zone and more than air flow alone

Clause 6 says the primary ventilation requirements and additional design requirements shall be applied in the occupied zone. Those additional aspects include energy use, acoustics, draught, pollutants, safety, smoke control systems, and cleaning and maintenance. In practice, the document pushes designers to check whether the ventilation works where people actually are, and whether it fits the building's operational constraints.

4. Use a staged design process for natural ventilation

Clause 7 sets out a conceptual design approach and a basic and detailed design approach. The process includes applying the primary requirements, evaluating the building location and design, establishing the driving forces and ventilation principle, checking effectiveness to a depth of the room, estimating opening area, and evaluating the design. This is important because the document distinguishes between early estimates and the final design calculation, so an opening area used for concept work is not automatically the final component size.

5. Check feasibility before committing to a natural ventilation solution

Clause 7 makes feasibility a central step. The designer is asked to judge whether natural ventilation can work as a stand-alone solution or whether it should be part of a hybrid approach. For project teams, that means testing the building form, site conditions, and room depth early enough to avoid a design that cannot deliver airflow in practice.

6. Design hybrid ventilation as an integrated system

Clause 8 requires hybrid ventilation to be designed from the start as an integrated natural and mechanical solution. It distinguishes the basic system from the supporting system, and it defines control operation as alternating or simultaneous. In practice, the designer must specify not only the hardware, but also when and how each mode takes over.

7. Define the control concept and system type clearly

Clause 8 and Annex I classify hybrid systems by basic and supporting system, control operation, spatial level, and operating time. Annex I gives a typology that helps name the system consistently. That helps during design reviews, tendering, and handover, because everyone can refer to the same system type instead of using informal labels.

8. Treat control, commissioning, and maintenance as part of design

Annex B, Annex D, and Annex E show that the document expects control, handover, maintenance, and commissioning to be planned with the design. Annex B covers demand control, sensor placement, operational strategies, accessibility, and vents, while Annex E gives commissioning checks for vents, sensors, overrides, and switching between modes. In practice, this means the design must be maintainable and testable, not only theoretically correct.

What terms does SIST-TS CEN/TS 18351:2027 define?

SIST-TS CEN/TS 18351:2027 defines the terms that shape the whole document, so users can distinguish system type, operating principle, and performance target.

• Ventilation - air replacement by purpose-provided means.
• Natural ventilation - ventilation whose operating principle is based solely on wind and stack effect.
• Hybrid ventilation - mixed mode ventilation that combines natural ventilation and mechanical ventilation.
• Mechanical ventilation - ventilation whose operating principle is based solely on fans.
• Occupied zone - the part of a room that people occupy under normal conditions.
• Ventilation effectiveness - the degree to which ventilation replaces the air, usually expressed by contaminant removal effectiveness.
• Primary ventilation requirements - the requirement that sets the target IAQ level or amount of air replacement.
• Descriptive requirement - a requirement stated as a technical solution, such as a minimum opening area.

Who uses SIST-TS CEN/TS 18351:2027?

SIST-TS CEN/TS 18351:2027 is used by ventilation engineers, building services designers, architects, and project teams working on non-residential buildings. It is also relevant to quality managers, commissioning teams, and building operators who need to check that a natural or hybrid system is designed, installed, and maintained in line with the intended IAQ strategy.

It is particularly useful for offices, educational buildings, industrial buildings, retail buildings, leisure buildings, and healthcare buildings, because the definition of non-residential covers those groups. It also supports users who need to define controls, sensors, and maintenance provisions for automatically controlled vents and BMS-linked systems.

What changed in SIST-TS CEN/TS 18351:2027 from the previous edition?

SIST-TS CEN/TS 18351:2027 is identical to CEN/TS 18351:2026.

Which standards are used with SIST-TS CEN/TS 18351:2027?

SIST-TS CEN/TS 18351:2027 points to related standards and publications that supply input data, calculation methods, and supporting guidance.

• EN 16798-1 - provides possible levels of IAQ requirements used as design input.
• EN 16798-3 - complements the document for non-residential mechanical ventilation and room-conditioning design.
• EN 16798-7 - provides methods for calculating air flow rates in buildings; Annex A uses its graphic method.
• CEN/TS 18335 - covers ventilative cooling for acceptable thermal comfort.
• FprEN 15665 - is the residential counterpart for ventilation design.
• EN 12599 - supports commissioning and handover procedures.
• EN 16211 - supports on-site measurement of air flow rates.
• EN 16798-17 - gives inspection guidance for ventilation and air conditioning systems.
• EN 16798-5-1 and EN 16798-5-2 - are referenced for control factors in related mechanical systems.
• EN ISO 7730 and EN ISO 52000-1 - provide related energy and thermal environment framework references.
• EN IEC 60335-2-103 - is relevant to the safety of window and door drives used in controlled vents.
• ISO 16813 - is cited in the project-stage guidance in Annex C.

What does the SIST-TS CEN/TS 18351:2027 document contain?

SIST-TS CEN/TS 18351:2027 contains a full design framework rather than a single calculation method. It includes definitions, abbreviations, design principles, staged design procedures, and examples for project use.

The informative annexes add practical material for design and implementation. Annex A shows how to use a graphic approach for opening areas, Annex B gives example detailed requirements for components and controls, and Annex F gives control factors. Annex G defines the occupied zone, Annex H explains ventilation effectiveness and air flow patterns, and Annex I classifies hybrid ventilation systems.

It also includes examples for the parts of a project that surround design. Annex C outlines project stages, Annex D covers handover and maintenance, and Annex E gives a commissioning checklist for natural and hybrid ventilation systems. For a design team, those annexes help connect the calculation stage with the real tasks of testing, operation, and upkeep.

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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