Heat release determination is a key performance test for thermal storage electric heaters, quantifying how much of the thermal energy stored during the charging period is actually delivered to the room during discharge. This article explains the test principle, applicable standards such as IEC 60335-2-30 and IEC 60675, sample requirements, measurement procedures, and calculation methods commonly used by accredited third-party laboratories and manufacturers.
Technical Overview
A thermal storage electric heater is a room heater that converts electrical energy into heat during a programmed charging period, stores that heat in a high-density core material such as magnesite or ceramic bricks, and releases it gradually to the room over an extended discharge period. This operating principle allows the heater to take advantage of off-peak electricity tariffs while providing continuous space heating throughout the day.
Heat release determination is the quantitative measurement of the rate and total amount of thermal energy discharged from the appliance under controlled laboratory conditions. The test evaluates whether the heater delivers the heat output declared by the manufacturer, whether the release profile is uniform over the discharge cycle, and whether excessive energy is lost to the surroundings or retained unproductively in the core. The results support safety assessment, energy efficiency classification, energy labeling declarations, and design optimization.
The safety framework for these appliances is defined by IEC 60335-1 and IEC 60335-2-30, together with their national adoptions such as GB 4706.1 and GB 4706.23. Performance measurement guidance for electric room heaters, including storage types, is provided by IEC 60675 and its equivalent national standards. Testing is typically performed by accredited third-party laboratories in climate-controlled calorimetric facilities.
Testing Samples
Sample Types and Categories
Heat release determination is applicable to a wide range of thermal storage heating products submitted as complete, production-representative units. Typical sample categories include:
- Static thermal storage heaters with manual charge control.
- Automatic storage heaters with thermostatic or ambient-temperature-compensated charge regulators.
- Combined storage and direct-acting heaters with an auxiliary convector section.
- Fan-assisted storage heaters with controlled discharge via an integral fan.
- Storage radiator cores and modular storage heating units for fixed installation.
Sample Preparation and Conditioning
Samples must be complete with all charge controllers, thermostats, dampers, fans, and insulation as supplied for normal use. Before testing, the appliance is stabilized at ambient temperature, typically 20 °C ± 2 °C, for a period sufficient to ensure the storage core is fully cooled and thermally uniform. Rated voltage and rated power input are confirmed by measurement, and the charge control is set according to the manufacturer's instructions. Any transport damage, missing accessories, or unauthorized modifications disqualify the sample, as they would invalidate the energy balance of the test.
Testing Items
The heat release determination test program covers the following measurement items:
- Charging energy input: total electrical energy consumed during the full charge period at rated voltage.
- Instantaneous heat release rate: thermal power delivered to the surrounding environment as a function of time during discharge.
- Total heat release: cumulative thermal energy discharged over the complete measurement cycle, typically 24 hours.
- Discharge duration and release profile: uniformity of heat output across the cycle, including peak and residual output.
- Stored residual energy: heat remaining in the core at the end of the discharge period.
- Casing and structural losses: energy dissipated through insulated enclosure surfaces.
- Charge control performance: correlation between charge regulator setting and stored energy.
- Surface temperatures during charge and discharge, referenced to the safety requirements of IEC 60335-2-30.
Testing Methods
Calorimetric Chamber Method
The reference method uses a thermally insulated, climate-controlled test chamber or calibrated calorimetric enclosure in which the heater is placed after charging. Ambient air temperature, wall temperatures, and air movement are maintained within strict tolerances. The heat released by the appliance is determined either calorimetrically, by measuring the mass flow rate and temperature rise of air drawn through the enclosure, or by an energy balance on the chamber derived from calibrated heat-loss coefficients.
Charging Procedure
The sample is connected to a calibrated power analyzer and charged at rated voltage for the charge period specified by the manufacturer, which simulates a typical off-peak supply window. The electrical energy input is continuously recorded. For automatically controlled heaters, the test is repeated at several charge control settings to characterize the relationship between control position and stored energy.
Discharge and Heat Release Measurement
Immediately after charging, the heater is transferred to the test chamber or the chamber measurement phase is initiated. Temperature sensors positioned at the air inlet and outlet, together with airflow measurement or the calibrated chamber coefficient, allow the instantaneous heat release rate to be computed at regular intervals throughout the discharge period, normally a full 24-hour cycle. Damper positions, fan operation, and thermostat actions follow the manufacturer's defined use conditions.
Data Processing and Calculations
The instantaneous heat release rate at each interval is calculated as the product of air mass flow, specific heat capacity of air, and the inlet-outlet temperature difference. Integrating this rate over the discharge period yields the total heat release. The result is cross-checked against the electrical charge input using an energy balance, in which the input energy equals the sum of released heat, residual core energy, and enclosure losses. Deviations beyond the acceptance limits require investigation of instrumentation or chamber calibration.
Applications
Heat release determination data serve multiple purposes across the product lifecycle:
- Compliance demonstration for safety and performance conformity assessment under IEC 60335-2-30, GB 4706.23, and IEC 60675 based procedures.
- Verification of manufacturer-declared heat output values for energy efficiency labeling and product datasheets.
- Qualification of appliances for off-peak tariff programs and demand-side management schemes.
- Research and development benchmarking of core materials, insulation systems, and charge control strategies.
- Factory quality control and batch consistency verification for serial production.
- Technical documentation supporting tenders, procurement, and building heating load calculations.
FAQ
What standards apply to heat release determination of thermal storage electric heaters?
Safety requirements are specified in IEC 60335-1 and IEC 60335-2-30, adopted nationally as GB 4706.1 and GB 4706.23. Performance measurement methods for electric room heaters, including storage types, follow IEC 60675 and equivalent national standards. Accredited third-party laboratories apply these frameworks in combination with calibrated calorimetric facilities.
How long does a complete test take?
A single charge-discharge cycle normally covers 24 hours, and the standard test program usually requires at least two or three repeat cycles plus preconditioning. Including instrument setup, calibration checks, and repeated charge control settings for automatic models, the full test typically takes one to two weeks.
What ambient conditions must be maintained during the test?
The test environment is held at a reference ambient temperature of approximately 20 °C with tightly controlled tolerances on temperature stability, air velocity, and surrounding surface temperatures, because heat release results are highly sensitive to ambient conditions and chamber heat-loss characteristics.
How many samples are required?
One production-representative unit is generally sufficient for performance determination, though additional units may be requested when safety tests that are destructive or temperature-intensive are performed on the same project, since such tests can alter the thermal characteristics of the storage core.
Why is heat release determination important for energy efficiency assessment?
The test quantifies how effectively stored electrical energy is converted into useful room heating. A low ratio of released heat to charge input indicates excessive losses or poor charge control, which directly affects running costs, energy label accuracy, and the economic viability of off-peak storage heating installations.
