English version machine-translated from the Vietnamese original — under review.
Introduction to ISO 21925 and EN 1366
In the field of fire safety, ISO 21925 and EN 1366 are both test standards for assessing the fire resistance of fire dampers in ventilation systems. However, they are issued by different organizations: ISO 21925 is an international standard issued by the International Organization for Standardization (ISO), while EN 1366 is a series of European standards issued by the European Committee for Standardization (CEN). Both standards aim to test the ability to prevent fire from spreading through duct openings in the event of a fire, but their scope and application may differ. To understand the relationship between them, it is necessary to consider the technical scope of each standard, the compliance requirements under legal regulations, and whether they can be considered equivalent or interchangeable. This article focuses on comparing ISO 21925 with EN 1366, particularly in terms of compliance and the documents assessing the equivalence of the two standards, in order to determine whether ISO 21925 can replace, or be considered equivalent to, EN 1366 under current legal regulations or testing requirements.
Overview of ISO 21925 and EN 1366
ISO 21925 (Fire resistance tests – Fire dampers for air distribution systems) is currently published in several parts, notably:
- ISO 21925-1:2018 – Specifies the fire resistance test method for mechanical fire dampers (mechanical dampers). This standard consolidates the requirements of the former ISO 10294 series into a single document and aims at harmonization with regional/national standards for fire damper testing
- ISO 21925-2:2021 – Test method for intumescent fire dampers (intumescent dampers). This is a passive type of fire damper with no moving mechanism, which uses intumescent material that expands when exposed to heat to seal the duct when a fire occurs. ISO 21925-2 describes how to test and assess the fire and smoke resistance of this type of fire damper, similarly to ISO 21925-1 but for non-mechanical fire dampers.
. The content covers installing a fire damper specimen in a fire-resisting wall or floor and testing it in a furnace to assess the integrity and insulation criteria over a specified fire duration, similar to the objective of EN 1366-2. ISO 21925-1 applies to mechanical fire dampers (those with blades and a closing mechanism) and does not apply to dampers used only for smoke control (smoke control dampers)
EN 1366 is the series of European standards on fire resistance testing of building service installations (Fire resistance tests for service installations). The series consists of several parts, each covering a different type of element. Among them:
- EN 1366-2: Fire dampers – Part 2 of EN 1366 specifies the fire resistance test method for mechanical fire dampers mounted in fire-separating walls or floors. The test assesses the ability of the fire damper to prevent fire, smoke and hot gases from spreading from one fire compartment to another through the ductwork system
- EN 1366-12: Non-mechanical fire barrier for ventilation ductwork – Part 12 of EN 1366 specifies tests for non-mechanical fire dampers in ventilation ductwork (including intumescent fire dampers or fire-stopping solutions with no moving parts). EN 1366-12 is used to test devices that EN 1366-2 does not cover. In fact, EN 1366-2 refers to EN 1366-12 for non-mechanical fire dampers
. EN 1366-2 requires the fire damper specimen to be installed in a standard fire-resisting partition/wall and tested in a furnace in accordance with EN 1363-1. This standard does not apply to fire dampers installed in suspended ceilings and, in particular, does not apply to non-mechanical fire dampers (non-mechanical dampers)
, meaning that to assess an intumescent damper, Part 12 must be used rather than Part 2.
Thus, the technical scope of ISO 21925-1 is similar to that of EN 1366-2 (both cover mechanical fire dampers), while ISO 21925-2 corresponds to EN 1366-12 (covering non-mechanical types). Both sets of standards aim to assess how long a fire damper can withstand fire (through the criteria E (Integrity), I (Insulation), and possibly S (Smoke leakage) where applicable). The introduction of ISO 21925 reflects ISO's effort to harmonize testing requirements internationally. According to the introduction of ISO 21925-1: *"ISO 21925-1 is expected to contribute to promoting the harmonization of requirements in the regional and national standards relating to the testing of fire dampers according to this standard"*
. In other words, ISO wishes to create a common standard that regions (such as Europe) can recognize or reference, reducing duplication of standards.
Compliance Requirements and Legal Regulations
Although the technical objectives are similar, which standard must be complied with depends on the legal regulations of each market. In particular, in Europe and the UK, the regulations on construction products make compliance with European standards mandatory for the purposes of certification and CE marking. Specifically:
- The EU Construction Products Regulation (CPR) requires that fire dampers be tested and certified to the corresponding EN standards before being placed on the European market. In the UK (and similarly in the EU), from 1/7/2013, all fire dampers installed in buildings must carry the CE mark, complying with the requirements set out in EN 15650:2010
- ISO 21925 in the legal context: At present, ISO 21925 is not on the EU list of harmonized standards for fire dampers; therefore ISO 21925 cannot be used in place of EN 1366-2 when CE certification is needed or legal requirements in Europe must be met. There is no European regulation that allows "submitting ISO test results" in place of EN test results in this field. For example, a UK guidance document on fire safety in hospitals (HTM 05/02) also requires fire dampers to be tested to BS EN 1366-2 and classified to BS EN 13501-3 when installed for escape routes or fire risk areas
- Other regions of the world: Outside Europe, acceptance of ISO or EN standards depends on each country's regulations. Some countries may choose to adopt ISO 21925 as a national standard (for example as BS ISO 21925, DIN ISO 21925,...). However, even where it is adopted nationally, to meet export requirements or projects specified to European standards, ISO test results will most likely still have to be retested to EN. To date, there has been no widespread indication that the authorities of any country have fully replaced EN 1366-2 with ISO 21925-1 in their regulations. International test laboratories today are often accredited to test to both systems of standards; for example, a European laboratory (FIRES, Slovakia) lists both EN 1366-2:2015 and ISO 21925-1:2018 in its list of accredited test methods, but the choice of which standard to test to depends on the requirements of the client or the target market. In short, ISO 21925 is not automatically accepted in legal contexts that require EN 1366 unless specific regulations allow it.
. EN 15650 is the product standard specifying the basic requirements for fire dampers, and this standard makes mandatory reference to EN 1366-2 for furnace testing as well as to EN 13501-3 for classification of fire performance
. In other words, to obtain CE Marking, a manufacturer must have fire damper test results to EN 1366-2 (for mechanical types) and a classification to EN 13501-3, and then follow EN 15650 for certification. A document from the manufacturer BSB (UK) also emphasizes: *"For a fire damper to comply with the CPR and be CE marked, the product must comply with EN 15650, which includes testing to EN 1366-2, classification to EN 13501-3 and extended application to EN 15882-2"*
. This shows that EN 1366-2 is the mandatory standard within the European legal framework for testing mechanical fire dampers. Similarly, EN 1366-10 is required for smoke control dampers, and EN 1366-12 for non-mechanical dampers, where relevant.
. These examples confirm that within the EU/UK, EN 1366-2 is the legal benchmark, while ISO 21925-1 serves only as a technical reference where needed and is not used as evidence of compliance.
Comparison and Assessment of Equivalence between ISO 21925 and EN 1366
In terms of technical content, ISO 21925-1:2018 and EN 1366-2:2015 have similar purposes and methods – both test mechanical fire dampers by installing them in a fire-resisting construction and heating one side to assess their resistance to fire/smoke. ISO 21925-1 was published later and therefore integrates many aspects that previously could only be covered by EN 1366-2 together with related standards. For example, ISO 21925-1 has an Annex C dedicated to testing the thermal release mechanism of the fire damper
. This is a test of the reliability and operation of the thermal fuse or heat sensor used to close the fire damper when the temperature rises. In the European system of standards, testing of the thermal release mechanism is not directly part of EN 1366-2 but is addressed through the requirements of the product standard EN 15650 and other electromechanical standards (for example, durability testing of the thermal release mechanism may refer to separate standards such as the former ISO 10294-4, or to environmental standards such as EN 60068 for salt corrosion
). The fact that ISO 21925-1 integrates this test shows that the ISO standard is more detailed in some respects, but in terms of the main result (the fire resistance period achieved against the E, I, S criteria) there is no major difference in substance compared with EN 1366-2.
Similarly, ISO 21925-2:2021 on intumescent dampers has an objective close to that of the European EN 1366-12:2014+A1:2020. Each standard sets out its own test configurations and assessment criteria for non-mechanical dampers. Since EN 1366-2 does not apply to non-mechanical dampers, technically, to fully replace the EN 1366 parts relating to fire dampers, ISO 21925-1 and ISO 21925-2 must be used together (corresponding to EN 1366-2 and EN 1366-12). If a project requires compliance with "all fire damper testing requirements to European standards", then ISO 21925-1 alone will not be sufficient for special cases such as intumescent dampers – this is a point to note when comparing the scope of the two sets of standards.
Although the purpose and test methods are similar, there is currently no official document declaring ISO 21925-1 to be "fully equivalent" to EN 1366-2. Standardization bodies such as CEN and ISO have also not issued any joint statement on the cross-acceptance of test results between these two standards. Normally, when an ISO standard is adopted in Europe, it is published as an EN ISO (an international standard adopted as a European standard). In this case, however, ISO 21925-1 has not been adopted as a corresponding EN ISO, and CEN continues to maintain its own EN 1366-2. This means that, procedurally, the two documents exist in parallel and have not been merged. Even in EN 15882-2:2022 (the standard on extended application of test results for fire dampers), although the drafting committee mentions ISO 21925-1 in the bibliography
, EN 15882-2 itself still mainly serves results obtained to EN 1366-2.
Regarding equivalence in actual testing, technical experts consider that test results to ISO 21925-1 and EN 1366-2 are essentially equivalent in terms of performance assessment, since both are based on the same test principles (the standard ISO 834/EN 1363 time–temperature curve, the same E, I,… criteria). However, detailed differences in the test procedure (for example: the number of specimens to be tested, the installation configuration, or other secondary requirements) may mean that the results are not directly interchangeable. For instance, if EN 1366-2 requires testing at two different sizes, large and small, and in both horizontal and vertical orientations, whereas ISO 21925-1 may combine the tests in one specimen or differ slightly in its conditions, the authorities may not accept ISO results being used to derive an EN classification. Therefore, in practice, if a project or regulation requires "to EN 1366-2", an EN 1366-2 test is mandatory, unless the competent authority agrees in advance that an ISO 21925-1 certificate is accepted as a substitute.
Official Documents and Research on Equivalence
To date, official documents from standards bodies or regulators directly comparing ISO 21925 with EN 1366 are very limited. There is no publication from ISO or CEN affirming that the two standards are "fully equivalent". However, some guidance documents and technical publications do mention both:
- ISO (TC 92/SC 2), in the introduction to ISO 21925-1, alluded to the objective of harmonization with regional standards, indicating that the long-term direction is for regions such as Europe to consider aligning their standards with ISO
- Industry guidance documents: For example, an article by HVC (an air outlet manufacturer in the UK) lists ISO 21925-1 (Annex C) as a relevant document for testing the thermal release mechanism of fire dampers
- Academic research: Some studies may compare the performance of equipment under different standards (for example, comparing the US UL 555 with EN 1366-2 and ISO 21925). However, these studies usually focus on technical aspects (differences in test results) rather than on legal documents regarding equivalence. No scientific publication in specialist journals has yet been found that directly concludes that ISO 21925 can replace EN 1366 in terms of compliance.
. Nevertheless, this is still only an expectation, not yet actual practice.
, alongside the principal EN standards. This shows that the engineering community perceives ISO 21925 as a supplementary technical document, rather than as a replacement for the position of EN 1366-2 within the CE system. Similarly, international seminars (for example those of the FCIA – the US firestop association) list both ISO 21925 and EN 1366-2 as common test standards for fire dampers, but do not declare them interchangeable; they only provide multi-standard reference information
From the above sources, a point of consensus can be seen: ISO 21925 and EN 1366 have the same testing purpose, but their legal roles and recognition differ. ISO 21925 is international in nature and can be used where there is no regulatory obligation to use EN. Meanwhile, EN 1366 is the "statutory standard" in Europe for fire damper testing. In the absence of documents or agreements on mutual recognition, ISO 21925 cannot automatically be considered equivalent to EN 1366 in terms of compliance.
Conclusion
ISO 21925 and EN 1366 have similar technical content for fire damper testing, but ISO 21925 cannot yet be considered a direct replacement for, or fully equivalent to, EN 1366 in the context of current legal regulations. As evidence, in Europe, EN 1366-2 (together with EN 15650 and EN 13501-3) is required by law for the certification and design of fire dampers
, whereas ISO 21925-1, although of reference value, is not accepted for CE marking. Similarly, EN 1366-12 is used for intumescent dampers, while ISO 21925-2 has not yet been incorporated into European regulations. There is currently no official statement from the standards committees allowing test results to be interchanged between these two sets of standards. Therefore, ISO 21925 cannot fully replace EN 1366 when legal or tender requirements calling for EN standards must be met, unless the competent authority specifically allows it after assessing the case.
In the long term, however, ISO's publication of 21925 with the objective of harmonization may pave the way for cooperation between ISO and CEN. If CEN assesses that ISO 21925 fully meets its requirements, an EN ISO 21925 version may appear in the future, merging the two systems of standards. Only then will ISO 21925 truly be considered legally equivalent to EN 1366. At present, organizations and companies need to comply strictly with the standard requirements specified by law or by the project owner. If the requirement is EN 1366, testing should be carried out to EN 1366; and if a national standard allows ISO 21925, it must be ensured that this has the approval of the competent authorities. In summary, ISO 21925 cannot yet universally replace EN 1366, even though technically the two standards are very close to each other. Using ISO 21925 in a context that requires EN should be carefully considered and formally approved to avoid the risk of non-compliance with regulations.
References: The information in this article is compiled from the standard ISO 21925-1:2018
, descriptions of the standards EN 1366-2 and EN 1366-12
, guidance on CE marking requirements for fire dampers in Europe
, as well as HVAC industry technical documents relating to fire damper testing
. These sources give a comprehensive picture of the relationship and differences between ISO 21925 and EN 1366 in practice. Standards organizations and regulators are still monitoring the situation and may adjust the standards in the future to achieve better alignment between the systems.
Alex Dao
Research & Development Department - Star Asia Company
