EN ISO 21805:2023

SLOVENSKI STANDARD
01-junij-2023
Nadomešča:
SIST-TS CEN ISO/TS 21805:2019
Navodilo in priporočila za projektiranje, izbiro in vgradnjo prezračevalnih naprav
za zagotavljanje strukturne integritete namestitvenih prostorov, zaščitenih s
sistemi za gašenje s plinom (ISO 21805:2023)
Guidance and recommendations on design, selection and installation of vents to
safeguard the structural integrity of enclosures protected by gaseous fire-extinguishing
systems (ISO 21805:2023)
Anleitung für die Konstruktion, Auswahl und Installation von Entlüftungen zur
Gewährleistung der strukturellen Integrität von Gehäusen, die durch ortsfeste
Gaslöschanlagen geschützt sind (ISO 21805:2023)
Lignes directrices et recommandations relatives à la conception, à la sélection et à
l'installation d'évents pour préserver l'intégrité structurelle des enceintes protégées par
des systèmes d'extinction d'incendie à gaz (ISO 21805:2023)
Ta slovenski standard je istoveten z: EN ISO 21805:2023
ICS:
13.220.10 Gašenje požara Fire-fighting
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.

EN ISO 21805
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2023
EUROPÄISCHE NORM
ICS 13.220.10; 91.140.30 Supersedes CEN ISO/TS 21805:2019
English Version
Guidance and recommendations on design, selection and
installation of vents to safeguard the structural integrity of
enclosures protected by gaseous fire-extinguishing
systems (ISO 21805:2023)
Lignes directrices et recommandations relatives à la Anleitung für die Konstruktion, Auswahl und
conception, à la sélection et à l'installation d'évents Installation von Entlüftungen zur Gewährleistung der
pour préserver l'intégrité structurelle des enceintes strukturellen Integrität von Gehäusen, die durch
protégées par des systèmes d'extinction d'incendie à ortsfeste Gaslöschanlagen geschützt sind (ISO
gaz (ISO 21805:2023) 21805:2023)
This European Standard was approved by CEN on 7 August 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

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
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21805:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 21805:2023) has been prepared by Technical Committee ISO/TC 21
"Equipment for fire protection and fire fighting" in collaboration with Technical Committee CEN/TC
191 “Fixed firefighting systems” the secretariat of which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2023, and conflicting national standards
shall be withdrawn at the latest by September 2023.
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.
This document supersedes CEN ISO/TS 21805:2019.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: 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.
Endorsement notice
The text of ISO 21805:2023 has been approved by CEN as EN ISO 21805:2023 without any modification.

INTERNATIONAL ISO
STANDARD 21805
First edition
2023-02
Guidance and recommendations on
design, selection and installation
of vents to safeguard the structural
integrity of enclosures protected by
gaseous fire-extinguishing systems
Lignes directrices et recommandations relatives à la conception,
à la sélection et à l'installation d'évents pour préserver l'intégrité
structurelle des enceintes protégées par des systèmes d'extinction
d'incendie à gaz
Reference number
ISO 21805:2023(E)
ISO 21805:2023(E)
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 21805:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms.2
5 Use and limitations . 3
6 Safety . 4
6.1 Structural safety . 4
6.2 Personnel safety . 4
7 System design — Pressure-relief venting . 4
7.1 General . 4
7.2 Extinguishant characteristics . . 5
7.2.1 Positive and negative pressurization . 5
7.2.2 Pressure graphs . 5
7.3 Enclosure characteristics . 6
7.4 Pressure-relief vent paths . 6
7.5 Types of pressure-relief vents . 7
7.5.1 General . 7
7.5.2 Gravity vents . 7
7.5.3 Counterweighted flap vent . 7
7.5.4 Electrically-operated vents . 8
7.5.5 Pneumatically-operated vent . 8
7.5.6 Vent accessories . 8
7.6 Pressure-relief vent characteristics . 9
7.6.1 Vent efficiency. 9
7.6.2 Minimum opening pressure . 10
7.6.3 Minimum closing pressure . 10
7.6.4 Fire rating . 10
7.7 Vent location and mounting . 10
7.7.1 Vent location . 10
7.7.2 Vent mounting. 11
7.8 Pressure-relief vent area calculations .12
7.8.1 Use of agent-specific formulae .12
7.8.2 Vent area requirement (non-liquefiable gases) .13
7.8.3 Vent area requirement carbon dioxide . 16
7.8.4 Vent area requirements (liquefiable gases) . 16
7.8.5 Leakage . 22
7.9 Cascade venting calculations . 22
7.9.1 Example calculation 3: Cascade venting calculations for IG-541 (peak
discharge) . 23
7.9.2 Cascade vent arrangements . 24
7.9.3 Venting into adjacent enclosures . 25
8 System design — Post-discharge venting .27
9 Acceptance .27
10 Service and maintenance . .27
Annex A (informative) Development of agent-specific formulae for liquefiable gases.29
Annex B (informative) Method for development of agent-specific formulae for liquefiable
gases .34
iii
ISO 21805:2023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 21, Equipment for fire protection and
firefighting, Subcommittee SC 8, Gaseous media and firefighting systems using gas. in collaboration with
the European Committee for Standardization (CEN) Technical Committee CEN/TC 191, Fixed firefighting
systems, in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna
Agreement).
This first edition cancels and replaces the first edition (ISO/TS 21805:2018), which has been technically
revised.
The main changes are as follows:
— subclause 7.8.3 has been amended to cross-reference ISO 6183 for vent area calculations for CO ;
— Annex A has been added, providing guidance on how testing in order to derive the agent-specific
formulae;
— Annex B has been added, providing guidance on the procedure for developing coefficients for any
new agents in the ISO 14520 series.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
ISO 21805:2023(E)
Introduction
The guidance presented in this document is based on the results of a joint research programme
conducted in 2006 and 2007 by several fire protection system manufacturers and interested parties.
The programme of work consisted of several series of tests to evaluate the peak pressure response and
pressure-relief vent area effects for each agent addressed in this document. The key data used in the
development of this document were the values of peak enclosure pressure response (PMAX) at each
value of the volume-normalized pressure-relief vent area of the test enclosure, hereinafter referred to
as the “leakage-to-volume ratio” or LVR. Other test parameters (enclosure temperature, agent quantity,
discharge time and humidity) were held constant or varied in a specified manner. For each test series
employing a single agent, the several pairs of LVR and resultant PMAX values were graphically analysed,
and a best-fit correlation curve was determined.
The LVR vs. PMAX correlation curve for each agent or system forms the basis of the associated formulae
in cases where the discharge of the agent results in cooling the air temperature below its dew point.
Only halocarbon agents cause sufficient cooling to cause humidity-related effects on the peak enclosure
pressure. Thus, a correction for humidity effects is included in the formulae for estimating vent area
and maximum pressure on the discharge of the following agents:
— FK-5-1-12
— HFC-23
— HFC-125
— HFC-227ea
The humidity corrections used in this document are based on the results of tests conducted with HFC-
227ea at different conditions of humidity.
The resulting values for humidity correction will be assumed to be equally applicable to the agents FK-
5-1-12, HFC-125 and HFC-23 until further data or analyses indicate otherwise.
The correlations of LVR to maximum negative pressure and maximum positive pressure were based
on test work performed in a test chamber at a relative humidity (RH) of approximately 38 %. If the RH
in a protected enclosure differs from 38 % then a correction to the estimated maximum negative and
positive pressures can be required. See 7.8 and 7.9 for further information on the effect of humidity. The
temperature of the test enclosure was 21°C (nominal) for all tests that form the basis of the estimating
methods given in this document.
In conducting the research programme described above, a large number of different venting
arrangements were created in the test enclosure. The equivalent leakage area (ELA) for each test was
determined by a “door fan test” and data analysis. The average enclosure pressure in effect during
the many door fan tests varied from test to test. All values of ELA were normalized to an equivalent
enclosure differential pressure of 125 Pa. The resulting enclosure correlations of peak pressure vs. LVR,
and any resulting estimate of enclosure pressure-relief vent area, reflect a pressure-relief vent area
calculated at an effective enclosure pressure of 125 Pa for a vent with a discharge coefficient of 0,61.
The effectiveness of a gaseous total flooding firefighting system depends, in part, on retention of
the air-extinguishant mixture within the protected volume for a period of time. Retention of the
extinguishant-air mixture requires that gas exchange (“leakage”) between the enclosure and the
ambient environment be restricted. To limit the rate of gas exchange, the enclosure boundary should
have a high degree of integrity. To put it another way, the total of the areas of the various penetrations
in an enclosure’s bounding surfaces should be low, at least during the gas-retention period (hold time)
after the end of the extinguishant discharge.
The addition of a gaseous firefighting extinguishant to an enclosure having a limited pressure-relief
vent area will naturally result in a change of pressure therein. If the enclosure is sealed too tightly
during the extinguishant discharge, i.e. too little pressure-relief vent area, the pressure change could
exceed the structural strength of one or more of its bounding surfaces — windows, doors, walls,
v
ISO 21805:2023(E)
ceiling. Conversely, if the enclosure has too much pressure-relief vent area then gas exchange with the
ambient atmosphere will occur rapidly, leading to a short retention time of the extinguishant within the
protected volume.
Thus, the use of gaseous firefighting systems should address two performance considerations:
a) pressure management within the protected volume during the period of extinguishant discharge,
and;
b) retention of the extinguishant-air mixture within the enclosure for a specified period of time after
the completion of the discharge.
This document provides guidance for limiting pressure extremes in an enclosure during the discharge
of a clean agent fire extinguishing system. This document does not provide the information necessary to
determine all of the requirements related to the design, installation, service, maintenance, inspection,
test and/or requalification of fire suppression systems.
Some limitations and restrictions apply to the use of the formulae contained in this document. Please
refer to the text and notes that follow them.
The information in this document does not supersede the manufacturer’s guidance. The information
contained in this document is presented as being supplementary to the guidance provided by the
respective system manufacturers. Guidance from the system manufacturer should always be followed
and used for purposes of system design, installation, operation and maintenance.
It has been assumed in the preparation of this document that the execution of its provisions is entrusted
to people appropriately qualified and experienced in the specification, design, installation, testing,
approval, inspection, operation and maintenance of systems and equipment, for whose guidance it has
been prepared, and who can be expected to exercise a duty of care to avoid unnecessary release of
extinguishant.
vi
INTERNATIONAL STANDARD ISO 21805:2023(E)
Guidance and recommendations on design, selection and
installation of vents to safeguard the structural integrity
of enclosures protected by gaseous fire-extinguishing
systems
1 Scope
This document gives guidelines for fulfilling the requirements contained in ISO 6183:2022, 6.4.1 and
7.4.1 and ISO 14520-1:2023, 5.2.1 h) and 5.3 h), in respect to over- and under-pressurization venting and
post-discharge extract.
It considers the design, selection and installation of vents to safeguard the structural integrity of
enclosures protected by fixed gaseous extinguishing systems and the post-discharge venting provisions
where used.
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/
NOTE For the purposes of this document, the term “bar” signifies “gauge”, unless otherwise indicated.
Concentrations or quantities expressed in percentages (%) signify by volume unless otherwise indicated.
3.1
free pressure-relief vent area
sum of all free vent areas of the pressure-relief vents provided
Note 1 to entry: This is determined by the gross pressure-relief vent area multiplied by the vent efficiency.
3.2
gross pressure-relief vent area
total area of the pressure-relief vent
3.3
negative pressure
pressure in the protected room which is lower than the pressure immediately outside the enclosure
boundary
3.4
peak pressure
maximum pressure (positive and negative) generated within an enclosure caused by the discharge of
the gaseous agent
ISO 21805:2023(E)
3.5
positive pressure
pressure in the protected room which is higher than the pressure immediately outside the enclosure
boundary
3.6
enclosure strength
specified differential pressure limit for the protected enclosure
3.7
pressure-relief area
sum of the free pressure-relief vent area and the enclosure leakage area
3.8
pressure-relief vent
device that provides a flow path through an enclosure boundary to limit the pressure therein
3.9
authority
organization, office or individual responsible for approving equipment, installations or procedure
4 Symbols and abbreviated terms
A pressure-relief vent area (m )
2 2
A pressure-relief vent area to limit negative pressure to a specified P (cm or in )
N N
2 2
A pressure-relief vent area to limit positive pressure to a specified P (cm or in )
P P
A total pressure-relief vent area (m )
T
C agent design concentration (vol. %)
E positive pressure excursion
p,P
E negative pressure excursion
p,N
3 3
f flooding factor (m /m )
F
H relative humidity within the enclosure (%)
L enclosure positive pressure limit
e,p,P
L enclosure negative pressure limit
e,p,N
m minimum design quantity of agent (kg)
M molecular weight of the agent (kg/mol)
AGT
M molecular weight of air (0,029; kg/mol)
AIR
M is the mixture molecular weight of the agent (kg/mol)
H
P pressure (Pa or psf)
P maximum room strength (Pa)
max
P negative pressure (Pa or psf)
N
P positive pressure (Pa or psf)
P
ISO 21805:2023(E)
P and P represent either
N P
— design pressure limits for estimating A or A , or
N P
— estimates of maximum values of P or P for given values of A or A
N P N P
Q quantity of agent required at reference temperature of 20 °C (m )
R
R gas law constant, 8,314 (J/mol-K)
S specific volume of the agent at the design temperature (m /kg)
S specific volume of air (m /kg)
AIR
S specific volume of the agent at the reference temperature (m /kg)
R
t discharge time (s)
t gaseous firefighting system discharge time (s)
d
T temperature (K)
V volume of the protected space (m )
V specific volume of the agent at the design temperature (m /kg)
A
V specific volume of the homogenous agent-air mixture (m /kg), which is the inverse of the density
H
V specific vapour volume of extinguishant (m /kg)
V
w maximum mass flow rate of the agent
ρ agent-air mixture density
H
5 Use and limitations
This document is for the use by those competent in the design, installation, servicing and maintenance
of fixed gaseous firefighting systems. It also serves as guidance for those involved in the design,
construction and operation of buildings in which such systems are installed.
It does not replace the need for the person responsible for the design, construction and operation of the
building to fulfil their obligations in respect to providing adequate structural provisions.
Other trades and services are involved in the complete system and this document is limited to providing
the guidance outlined in the Scope.
After applying the enclosure peak pressure and pressure-relief vent area analysis of this document,
the user can potentially conclude that an enclosure can require additional pressure-relief vent areas to
avoid exceeding specified maximum pressure values upon discharge of a gaseous agent system. If that
is the case, it is recommended that the user advise the supplier of a supplemental venting device, which
can be specified and selected by use of this document.
The maximum pressure developed in an enclosure on the discharge of a clean agent fire extinguishing
system is affected by several characteristics of the system itself and the enclosure being protected. Of
particular importance are the thermodynamic properties of the agent and the discharge characteristics
of the hardware. Each of the following clauses contains correlation formulae that are specific to the
agent type and manufacturer’s hardware. The formulae can be used to make estimates of the following:
a) enclosure pressure-relief vent area, given a specified enclosure pressure limit;
ISO 21805:2023(E)
b) maximum positive or negative pressure developed in an enclosure given a stated or calculated
pressure-relief vent area.
NOTE The formulae in this document for halocarbon agents have a limited range of applicability based on
the parametric limitations of the data from which they were derived. Table 1 indicates the applicable limits of
design concentration, discharge time and enclosure pressure response for use in this document. The maximum
peak pressure estimates (both positive and negative) based on data obtained for each agent are given in Table 1.
CAUTION — It is physically possible to develop pressures greater than those covered by this
document during system discharges.
Table 1 — Summary of formulae application limits
Agent Minimum Maximum Minimum Maximum Maximum Maximum
agent agent discharge discharge over pres- under pres-
conc. conc. time time sure sure
vol. % vol. % Pa (pfs) Pa (pfs)
FK-5–1-12 4,2 6 6 10 239 (5) 1 197 (25)
HFC-23 18 30 6 10 1 437 (30) n/a
HFC-125 8 10,5 6 10 479 (10) 479 (10)
HFC-227ea 6,25 10,5 6 10 383 (8) 958 (20)
6 Safety
6.1 Structural safety
The provision of correctly designed and engineered pressure venting of enclosures protected by
gaseous fire-extinguishing systems is essential for preventing the possibility of failure of structural
integrity. This is essential for mitigating forces exerted by the changes in enclosure pressure when
gaseous fighting media are discharged into an enclosure.
6.2 Personnel safety
The operation of pressure-relief vents or extract systems requires the displacement of mixtures of air/
gaseous media from a protected enclosure to the atmosphere or another area not necessarily protected.
Safety issues can arise due to exposure to the extinguishants themselves or products of combustion
and/or extinguishant breakdown products. Also, any hazards arising from the operation of the over/
under pressurization vents themselves should be considered.
7 System design — Pressure-relief venting
7.1 General
The basic design principle is to limit the pressure excursions imposed on the structure of the protected
enclosure by the discharge of gaseous extinguishant to that within the limits the enclosure can
withstand.
A room integrity test can be used to determine the equivalent leakage area, or simply the "vent” area
that exists at the time of evaluation. The methods of this document can use the known or estimated
pressure-relief vent area to estimate the maximum pressure that will be developed on the discharge
of a clean agent system. If the estimated maximum pressure exceeds a specified design threshold, the
methods of this document may be used to estimate a pressure-relief vent area sufficient to limit the
development of pressure upon system discharge to an acceptable value.
ISO 21805:2023(E)
7.2 Extinguishant characteristics
7.2.1 Positive and negative pressurization
Consideration should be given to positive pressurization created by all extinguishants and additionally
to negative pressurization created by some extinguishants as shown in Table 2.
Table 2 — Pressure effects of gaseous extinguishant
Extinguishant name Positive pressure created Negative pressure created
FK-5–1-12 Yes Yes
HFC-125 Yes Yes
HFC-227ea Yes Yes
HFC-23 Yes No
IG 01 Yes No
IG 100 Yes No
IG 55 Yes No
IG 541 Yes No
a
CO Yes No
a
Negative pressure has been observed, with adverse effects. It can occur in certain cases where large quantities of CO
are released into a space having low leakage to ambient.
7.2.2 Pressure graphs
The graphs shown in Figure 1 illustrate the typical pressure excursions that would occur during
discharge within the protected area.
a) Inert gas
b) Inert gas (constant flow)
ISO 21805:2023(E)
c) Halocarbon gas
Key
X1 positive pressure
X2 negative pressure
Y time
Figure 1 — Typical pressure excursions
7.3 Enclosure characteristics
It is the client’s responsibility and not the responsibility of the fire protection system supplier to
determine room strength. The client should advise the allowable pressure differential the protected
enclosures can withstand without sustaining damage.
It is generally accepted that normal masonry construction can withstand 500 Pa, while lightweight
structures such as stud partitioning can withstand only 250 Pa. Both figures assume fixings at the
top and bottom. Certain structure types can have even lower limits, particularly suspended ceilings.
However, fire system engineers are not qualified to give guidance on room strengths, so it is up to the
client to provide this information. If the client does not make clear the allowable pressure the enclosure
will withstand, it is necessary to obtain their acceptance of the figures used.
Due to issues related to enclosures utilizing suspended ceilings, it is recommended that protection is
provided to volumes above and below the suspended ceiling where practical.
7.4 Pressure-relief vent paths
It is generally assumed that positive/negative pressure-relief vent paths will lead to/from the
atmosphere. Positive pressure-relief vent paths will assist in the safe transfer of the displaced air/
extinguishant volume to the atmosphere in the most efficient, uncomplicated manner as well as
ensuring air/extinguishant contaminated with fire by-products also finds a safe route to the outside air.
As positive pressure venting can involve the displacement of smoke, the possible effect on fire detection
systems along the vent path should be considered.
Under certain circumstances, it can be necessary to consider the use of adjacent spaces as the means
to dissipate the pressure condition, either directly as a function of the volume of that adjacent space or
where the adjacent space acts as a transit path to the atmosphere. Under the circumstances described
in the latter option, special venting considerations can be required to ensure the pressure condition is
not simply transferred to that adjacent space (see 7.9).
ISO 21805:2023(E)
7.5 Types of pressure-relief vents
7.5.1 General
There are various types of pressure-relief vents, which are normally closed to preserve the integrity of
the enclosure and which then open to relieve a pressure impulse and close again. These pressure-relief
vents can fall into several categories, which are described in the following subclauses.
7.5.2 Gravity vents
The blades for these vents are generally hinged on the top edge. They have no electric or pneumatic
actuation but rely totally on the enclosure pressure change to move the vent blades.
This type of vent can provide a free pressure-relief vent area significantly smaller than the gross
pressure-relief vent area. In addition, the vent design creates turbulent flow and therefore is likely to
create higher pressure loss for any given flow. This additional pressure loss should be factored into the
determination of the free pressure-relief vent area required.
Vents, if not fitted with an end stop, for example ‘cat flaps’, could relieve pressures in both directions.
However, these are not recommended unless they can avoid compromising the enclosure fire rating.
See Figure 2.
Figure 2 — Gravity vent
7.5.3 Counterweighted flap vent
This type of vent is configured with the hinge located just off from the centre of gravity so that when
positive pressure is exerted on the upstream side of the vent it allows the vent blades to pivot to their
fully open positions.
The vent can be designed such that there is a minimum operational release pressure, which will ensure
that nuisance movement is avoided.
Typically, these vents are more efficient (i.e. larger discharge coefficient, lower opening pressure, lower
intrinsic inertia) than gravity flap vents.
ISO 21805:2023(E)
7.5.4 Electrically-operated vents
This type of vent utilizes blade(s) operated by an electric motor.
This type of vent is reliant upon power at the time of the discharge. Therefore, if no other option is
available, there should be a protected power supply to the vent motor to ensure that failure of mains
does not leave the vent in the closed position.
This type of vent generally opens more slowly than other types of vent and correct operation can be
dependent on the sequence of activation and the time allowed for the vent to open fully.
See Figure 3.
Figure 3 — Electrically-operated vent
7.5.5 Pneumatically-operated vent
Pneumatically operated vents are actuated by pressure, normally that of gas flowing through the
pipework or by pilot containers or compressed air line.
7.5.6 Vent accessories
7.5.6.1 Security provisions
If the vent is located within an external wall at a low level it is feasible that the client will have some
concerns regarding forced entry. Therefore, it is likely that security bars can be fitted across the
aperture to retain the building security.
7.5.6.2 Insect screen
If there is concern that insects can potentially penetrate the building through the vent it can be
necessary to specify insect screens. However, these are made of fine mesh and can have a significant
impact on the free pressure-relief vent areas.
7.5.6.3 Weather louvres
When fitted on exposed, external faces of a building it is possible that rain can penetrate the opening
even with the vent in the closed position. In this case, a weather louvre can be fitted externally. However,
this can have a significant impact on the free pressure-relief vent areas.
See Figure 4.
ISO 21805:2023(E)
Figure 4 — Louvres
7.5.6.4 Decorative grilles
A decorative grille can be used to cover the inner face of the vent assembly. However, this can have a
significant impact on the free pressure-relief vent areas.
7.5.6.5 Limit switches
If electrically- or pneumatically-operated vents are inadvertently left in the open position they can
potentially become either a security risk or endanger the equipment within the space by the infiltration
of pollution from external sources. In this case, it can be desirable to fit limit switch(es) to monitor the
position of the vent and create a warning signal, either locally, or through the building management
system, or both.
7.6 Pressure-relief vent characteristics
7.6.1 Vent efficiency
Pressure-relief vents, of any type (see 7.5), control the flow of air by the movement of air control
elements (blades). The design of the blades and the extent to which they open at any given pressure
determines the free pressure-relief vent area of the vent at that pressure. For example, if a vent has a
nominal area of 1,0 m and an efficiency of 50 % at 100 Pa it will provide a free pressure-relief vent area
of 0,5 m at 100 Pa. The blades of the same vent can open further at higher pressures, perhaps having an
efficiency of 80 % at 250 Pa and thus provide a free pressure-relief vent area of 0,8 m at 250 Pa.
Examples of vent efficiencies for gravity and weighted vents are shown in Figure 5. It is therefore
recommended that free pressure-relief vent areas are specified at no less than 3 pressures, for example,
100 Pa, 250 Pa and 500 Pa.
ISO 21805:2023(E)
Vent efficiency will be reduced by the addition of other accessories in the vent path, e.g. weather louvres,
grilles, etc. Vent manufacturers should provide a safe assessment of the potential effect based on the
free pressure-relief vent area of the accessory proposed.
NOTE Vent efficiencies are provided by vent manufacturers.
Key
X efficiency (%)
Y pressure (Pa)
1 gravity
2 counter-weighted
Figure 5 — Efficiency of pressure-relief vents
7.6.2 Minimum opening pressure
The vent should be designed to have a minimum opening pressure to avoid nuisance opening. This
should be at least 50 Pa.
7.6.3 Minimum closing pressure
The vent should be designed to have a minimum closing pressure to ensure closure at the end of the
discharge. This should be at least 30 Pa.
7.6.4 Fire rating
Where vents are included in an enclosure, they should not reduce the fire rating of the structure and
should therefore be of equivalent fire rating.
7.7 Vent location and mounting
7.7.1 Vent location
The most favourable location for the vent is on an exterior wall of the building.
The vent should be located taking due account of the discharge nozzles and any objects both
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