Average heat release rate determination is a fundamental fire performance test that quantifies the thermal energy emitted by a material per unit time under controlled combustion conditions, providing critical data for fire hazard assessment and material classification. This technical article explains the oxygen consumption principle, relevant international standards such as ISO 5660-1 and ASTM E1354, sample requirements, test procedures, and typical applications across building materials, polymers, and transportation industries.
Technical Overview
Heat release rate (HRR) is widely regarded as the single most important variable in fire hazard analysis, as it directly governs fire growth, temperature rise, smoke production, and the availability of time for occupant escape. The average heat release rate (also called mean heat release rate, or MHRR) represents the thermal energy released per unit time by a burning specimen, averaged over a defined time interval or over the entire test duration. It is expressed in kilowatts (kW) or normalized by exposed surface area in kilowatts per square meter (kW/m²).
Unlike the instantaneous or peak heat release rate, which captures transient flashover-type behavior, the average heat release rate reflects the sustained contribution of a material to fire development. Regulatory frameworks and classification systems frequently reference averaged values computed over specific windows, such as the first 180 seconds or the first 300 seconds of a test, because these intervals correlate strongly with the early stages of fire growth when evacuation is most critical.
Modern determination of heat release rate is based on the oxygen consumption principle. Extensive research by Huggett established that for most organic materials, approximately 13.1 MJ of heat is released per kilogram of oxygen consumed, regardless of the fuel chemistry. By measuring oxygen, carbon dioxide, and carbon monoxide concentrations in the combustion exhaust stream, together with the exhaust flow rate, a cone calorimeter or room calorimeter can compute heat release rate in real time with an accuracy typically within ±10%.
Testing Samples
Building Materials and Construction Products
Rigid boards, sandwich panels, gypsum products, wall and ceiling linings, flooring, facade claddings, and thermal insulation materials are commonly evaluated. These products are often tested as part of reaction-to-fire classification schemes required by building codes.
Polymeric Materials and Plastics
Thermoplastics, thermosets, engineering resins, flame-retardant compounds, and polymer nanocomposites are characterized to compare formulations and quantify the effectiveness of additive or reactive flame retardants.
Textiles, Upholstery, and Furnishings
Fabrics, curtains, mattress facings, seat cushions, and foam padding can be tested individually or in representative layered composites to assess the fire behavior of finished furniture assemblies.
Cables and Wire Insulation
Sheathing compounds, insulation materials, and complete cable cross-sections are evaluated to support fire safety assessments for power, control, and data cables installed in bundles in buildings, tunnels, and industrial plants.
Transportation Interior Materials
Materials used in rail vehicle, marine, and aviation interiors are tested to demonstrate compliance with sector-specific flammability requirements, often under radiant heat fluxes representative of post-crash fire scenarios.
Testing Items
A comprehensive heat release rate test campaign typically reports the following measurands:
- Average heat release rate over defined intervals (for example, 60 s, 180 s, and 300 s after ignition) and over the full test duration, in kW/m².
- Peak heat release rate (pkHRR), identifying the maximum instantaneous energy output.
- Total heat release (THR) or total heat evolved, in MJ/m², representing the cumulative energy released.
- Effective heat of combustion, in MJ/kg, derived from heat release and specimen mass loss.
- Time to sustained ignition under the applied irradiance level.
- Mass loss rate, in g/s, determined by continuous load-cell weighing.
- Smoke production rate and total smoke release, measured optically in the exhaust duct.
- Residual mass fraction and combustion residue characteristics after test completion.
Testing Methods
Cone Calorimeter Method (ISO 5660-1, ASTM E1354, GB/T 16172)
The cone calorimeter is the most widely used apparatus for average heat release rate determination. A square specimen of 100 mm × 100 mm, with thickness up to 50 mm, is mounted horizontally on a weighing device beneath a conical heater. The heater imposes a uniform irradiance of 25, 35, 50, or 75 kW/m² onto the specimen surface. An electric spark igniter initiates piloted ignition, and all combustion gases are drawn into an exhaust duct where oxygen, carbon dioxide, carbon monoxide concentrations, smoke opacity, and duct flow are continuously recorded. Heat release rate is computed every second from oxygen depletion using the oxygen consumption equations. Tests typically run until the specimen is fully consumed or all flaming has ceased for a defined period. Average values are then calculated over the specified integration windows.
Single Burning Item Test (EN 13823)
For construction products in the European classification system, the SBI test exposes two vertically mounted wing panels to a triangular propane burner. Heat release rate is measured by oxygen consumption from the exhaust hood, and averaged heat release data feed directly into the FIGRA and THR parameters used for Euroclass classification.
Room Corner Test (ISO 9705)
This full-scale reference method evaluates wall and ceiling linings in a standard room with a door opening, using a gas burner ignition source. Total heat release rate of the room fire is recorded, and average values over 60-second intervals are used for classification and modeling validation.
OSU Rate of Heat Release Apparatus (FAA 14 CFR Part 25)
Aircraft cabin interior materials are tested in the OSU calorimeter, in which a 150 mm × 150 mm specimen is exposed to radiant heat and the heat release rate is averaged over the first two minutes and over the first five minutes of exposure, with regulatory peak limits.
Quality Assurance of Results
Accredited third-party laboratories verify measurement traceability through gas analyzer calibration with certified gases, methane check burns to validate heat release measurement within tolerance, load-cell calibration for mass loss, and regular verification of heater flux uniformity using a water-cooled heat flux gauge. Specimens are conditioned to constant mass at 23 °C and 50% relative humidity prior to testing, and replicate tests are performed to address material variability.
Applications
Average heat release rate data support a broad range of engineering and regulatory purposes:
- Reaction-to-fire classification of construction products under building regulations in Europe, North America, China, and other jurisdictions.
- Comparative screening of flame-retardant formulations during research and product development.
- Input data for fire modeling and performance-based fire safety design, including zone models and computational fluid dynamics simulations.
- Compliance demonstration for transportation sectors, including railway rolling stock, ships, and aircraft interiors.
- Fire hazard assessment of consumer products such as furniture, mattresses, and electrical appliances.
- Insurance risk evaluation and forensic reconstruction of fire incidents.
- Verification of conformity for quality control in manufacturing of fire-rated materials.
FAQ
What is the difference between average and peak heat release rate?
Peak heat release rate is the maximum instantaneous value recorded during the test and indicates the worst-case thermal pulse, while average heat release rate is the integrated mean over a defined period, reflecting the sustained energy contribution. Classification criteria and fire models rely on both parameters, but averaged values generally correlate better with overall fire growth behavior.
Which irradiance level should be selected for testing?
The choice depends on the intended fire scenario. An irradiance of 35 kW/m² is the most common default in ISO 5660-1 and approximates the heat flux experienced by a surface in a developing room fire. Higher levels, such as 50 or 75 kW/m², are used to simulate severe post-flashover exposure, while 25 kW/m² may represent milder early-stage conditions.
How many specimens are required for a reliable result?
Most standards recommend testing a minimum of three replicate specimens. For heterogeneous or anisotropic materials, additional replicates and specimen orientations may be necessary to characterize variability adequately.
How is average heat release rate calculated?
The instantaneous heat release rate curve, recorded at one-second resolution, is integrated over the specified time interval and divided by the interval duration. When normalized by the initially exposed surface area, the result is reported in kW/m².
Can results from different test methods be compared directly?
Not directly. Cone calorimeter, SBI, and room corner tests differ in scale, geometry, ignition source, and ventilation, so average heat release values must only be compared within the same test configuration and standard. Correlations between small-scale and large-scale results should always be supported by scenario-specific validation.
Why is the oxygen consumption method preferred over direct calorimetry?
The oxygen consumption technique is essentially complete in capturing the combustion energy, requires no water cooling jacket, remains accurate across widely varying fuels, and permits compact apparatus design. Its universality, based on the near-constant 13.1 MJ/kg oxygen heat of combustion, has made it the foundation of virtually all modern heat release rate standards.
