Combustion performance testing of flexible elastomeric insulated air ducts verifies that rubber-plastic foam duct systems meet the fire safety classifications required by GB 8624, EN 13501-1, and ASTM E84 frameworks before installation in occupied buildings. This technical article explains specimen preparation, test items, standard test methods, and acceptance criteria used by accredited third-party laboratories to evaluate ignitability, flame spread, heat release, smoke production, flaming droplets, and smoke toxicity.
Technical Overview
Flexible elastomeric insulated air ducts are pre-insulated duct products built from a closed-cell rubber-plastic (NBR/PVC) foam insulation layer, a helical steel-wire or polymer frame, and internal and external jackets such as aluminum foil, polyester film, or glass fabric. They combine air conveyance and thermal insulation in a single assembly and are widely used in commercial HVAC, residential ventilation, and clean-environment engineering. Because these ducts pass through ceilings, wall cavities, and service shafts within occupied spaces, their combustion behavior is a primary fire-safety concern, and documented classification is required by fire codes prior to installation.
Combustion performance testing evaluates ignitability, flame propagation, heat release, smoke production, flaming droplets and particles, and smoke toxicity, and assigns a classification under the relevant regulatory framework: GB 8624-2012 in China, EN 13501-1 in Europe, and ASTM E84 or UL 723 in North America. The Chinese product standard GB/T 17794 for flexible cellular rubber-plastic thermal insulation products links directly to GB 8624 classes, while UL 181 governs factory-made flexible air ducts in the North American market. All tests described below are routinely carried out by accredited third-party laboratories.
Testing Samples
Foam Insulation Material Specimens
Specimens are cut directly from the elastomeric foam insulation layer of the duct wall. Testing the base foam in isolation establishes the intrinsic fire behavior of the polymer compound, including its limiting oxygen index and its response to small-flame ignition.
Composite Duct Wall Specimens
Because the finished duct is a layered composite, most classification tests are performed on specimens that retain the end-use configuration: foam core with aluminum foil, film, or fabric facings bonded in their production orientation. Facings can significantly alter flame spread and dripping behavior, so results for the bare foam alone are not accepted as substitutes.
Finished Duct Assemblies
For product-level conformity such as UL 181 listing, complete duct sections including seams, closure systems, and suspension provisions are subjected to the full suite of duct tests in addition to surface burning characterizations.
Sample Conditioning and Dimensions
Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 48 hours, or as specified by the individual test standard. Typical specimen dimensions are as follows:
- Single burning item test (GB/T 20284 / EN 13823): long wing 500 mm × 1500 mm and short wing 1000 mm × 1500 mm, at production thickness.
- Small-flame ignitability test (GB/T 8626 / ISO 11925-2): 250 mm × 90 mm strips, tested at product thickness.
- Limiting oxygen index (GB/T 2406.2 / ISO 4589-2): 150 mm × 10 mm specimens, product thickness up to 10.5 mm.
- Smoke density test (GB/T 8627): sets of 25.4 mm × 25.4 mm specimens at product thickness.
- Steiner tunnel test (ASTM E84 / UL 723): 7.62 m × 0.51 m panels representing the maximum thickness and density offered.
Testing Items
A complete combustion performance assessment for flexible elastomeric insulated ducts typically comprises the following items, selected according to the target classification scheme:
- Ignitability under small-flame impingement, including flaming droplets and particles.
- Lateral flame spread over the exposed surface.
- Fire growth rate index (FIGRA) and total heat release within 600 seconds (THR600s).
- Smoke growth rate index (SMOGRA) and total smoke production within 600 seconds (TSP600s).
- Smoke density rating (SDR) determined by the chamber method.
- Smoke production toxicity grading (AQ1, AQ2, AQ3) for the decomposition products.
- Limiting oxygen index (LOI) of the foam compound.
- Flame spread index (FSI) and smoke developed index (SDI) for North American acceptance.
For GB 8624-2012 classification, the resulting class designation carries additional suffixes for smoke production (s1, s2, s3), flaming droplets (d0, d1, d2), and smoke toxicity (t0, t1, t2), producing a full designation such as B1 (B-s1, d0, t1).
Testing Methods
Small-Flame Ignitability Test (GB/T 8626 / ISO 11925-2)
A vertical specimen is exposed to a small premixed burner flame applied to its lower edge or surface for 15 seconds (Class E verification) or 30 seconds (Class B, C, D verification). The specimen passes if the flame tip does not reach the 150 mm gauge mark within 20 seconds and no flaming droplets ignite the filter paper below, which supports d0 or d1 grading.
Single Burning Item Test (GB/T 20284 / EN 13823)
The SBI test exposes a corner assembly of two wings to a 30 kW propane burner for approximately 20 minutes. Oxygen-consumption calorimetry yields FIGRA and THR600s, while a photometric system yields SMOGRA and TSP600s. For European and Chinese classifications, Class B requires FIGRA0.2MJ ≤ 120 W/s and THR600s ≤ 7.5 MJ with flame spread not reaching the specimen edge; Class C allows FIGRA0.2MJ ≤ 250 W/s and THR600s ≤ 15 MJ. Smoke grades require SMOGRA ≤ 30 m²/s² and TSP600s ≤ 50 m² for s1, or SMOGRA ≤ 180 m²/s² and TSP600s ≤ 200 m² for s2.
Smoke Density Test (GB/T 8627)
Specimens are burned inside an enclosed chamber under radiant heating with pilot ignition, and the attenuation of a light beam is converted into a smoke density rating on a 0 to 100 scale. Flexible foam insulation products intended for B1 use are commonly required to achieve an SDR no greater than 75.
Smoke Toxicity Test (GB/T 20285)
Specimens are thermally decomposed under controlled conditions in a tube furnace, and the generated smoke is evaluated for acute inhalation toxicity. Materials are graded AQ1, AQ2, or AQ3, which correspond to the t0, t1, and t2 toxicity suffixes used in GB 8624-2012 classifications.
Limiting Oxygen Index (GB/T 2406.2 / ISO 4589-2)
A vertical specimen burns downward in a flowing oxygen-nitrogen mixture, and the minimum oxygen concentration that sustains flaming combustion for three minutes or over 50 mm of specimen length is recorded. Product standard GB/T 17794 typically requires a limiting oxygen index of at least 32% for B1-class foam and at least 26% for B2-class foam.
Steiner Tunnel Test (ASTM E84 / UL 723)
A 7.62 m specimen is exposed to a controlled gas flame in a horizontal tunnel furnace. The flame spread index is benchmarked against red oak flooring (index 100), and the smoke developed index is measured photometrically. Interior finish classes are defined as Class A (FSI 0 to 25), Class B (26 to 75), and Class C (76 to 200), with smoke developed index generally limited to 450. UL 181 Class 1 flexible air ducts must additionally demonstrate an FSI not exceeding 25 and an SDI not exceeding 50 at the maximum representative thickness.
Cone Calorimeter Testing (ISO 5660-1)
A 100 mm × 100 mm specimen is irradiated at 25 to 50 kW/m² in an open calorimeter, providing peak heat release rate, total heat release, and smoke production rate data. Although not a classification test, cone calorimeter results are valuable for formulation development and for project specifications that impose heat release limits.
Applications
Verified combustion performance enables flexible elastomeric insulated ducts to be specified with confidence across demanding building services, including:
- Supply, return, and exhaust air connections between air handling units, VAV terminals, and diffusers in offices, hotels, and shopping centers.
- Hospital and cleanroom ventilation where low smoke production supports tenability during fire events.
- Data center and telecommunication facility cooling, where low flame spread and low smoke acidity protect sensitive equipment.
- Residential central air conditioning and fresh-air systems, particularly in retrofit installations with limited plenum space.
- Industrial plants and marine HVAC installations subject to additional authority requirements.
Design compliance is demonstrated against GB 50016 and GB 50736 in China, EN 13501 based codes in Europe, and model building codes referencing ASTM E84 and UL 181 in North America.
FAQ
What fire class is mandatory for flexible elastomeric insulated ducts in China?
For most comfort HVAC applications, the duct and its insulation must achieve at least B1 under GB 8624-2012, corresponding to European-equivalent Class B or C performance. Systems with stricter roles, such as smoke exhaust ductwork, require Class A non-combustible materials, which elastomeric foam cannot achieve, so flexible foam ducts are restricted to supply, return, and general exhaust systems where B1 or B2 grades are accepted by the design code.
What is the difference between B1 and B2 grades?
B1 corresponds to Class B or C in GB 8624-2012, requiring FIGRA0.2MJ no greater than 250 W/s, controlled total heat release, and no flame spread to the specimen edge in the SBI test, together with a limiting oxygen index of about 32% or higher. B2 corresponds to Class D or E, verified mainly by the small-flame test with an oxygen index of about 26%. B1 products additionally demonstrate restrained smoke production and are the default choice for public buildings.
Which tests are required for export to the European market?
Classification under EN 13501-1 requires EN ISO 11925-2 ignitability and EN 13823 SBI results, yielding a designation such as B-s1, d0 or C-s2, d1. Flame-retardant elastomeric ducts commonly target Class B or C, and buyers increasingly request the s1 smoke grade for occupied spaces.
Do the foil or film facings need separate testing?
No, facings are tested as an integral part of the composite specimen in their production orientation. Because jackets can either suppress or accelerate flame spread and dripping, any change to the facing, adhesive, or foam formulation invalidates the previous classification and requires retesting.
How many samples are needed and how long does testing take?
A full GB 8624-2012 B1 assessment typically requires specimens for the small-flame test, the SBI assembly, smoke density, smoke toxicity, and oxygen index, generally totaling several square meters of product at production thickness. Including conditioning, testing, and reporting, accredited third-party laboratories usually complete the program within two to four weeks.
Is the classification valid for all thicknesses of the product family?
No. Combustion classifications are thickness-dependent and are only valid for the configuration tested. Thicker sections generally produce higher heat release, so manufacturers must either test the maximum thickness offered or hold separate reports for each thickness range, as required by GB 8624-2012, EN 13501-1, and the extended application rules of each scheme.