Surface Temperature Rise Testing of Thermal Storage Electric Heaters

Surface temperature rise testing is a critical safety evaluation for thermal storage electric heaters, verifying that accessible surfaces, handles, and air outlets do not exceed the temperature limits specified in IEC 60335-1, IEC 60335-2-61, and the corresponding GB 4706 standards. By measuring temperature rise across complete charge-storage-discharge operating cycles, manufacturers can demonstrate protection against burn injuries, ignition of adjacent materials, and premature insulation aging. This article explains the applicable standards, sample requirements, test items, test methods, and frequently asked questions related to surface temperature rise testing of thermal storage heaters.

Technical Overview

A thermal storage electric heater is an appliance that uses electricity during a charging phase to heat a high-density storage core, typically constructed from magnesite or ceramic bricks, and then releases the stored heat gradually into the room over many hours, often taking advantage of off-peak electricity tariffs. Because the appliance operates unattended for long periods, remains hot well after the charging cycle ends, and is installed in occupied living spaces, the temperatures of its accessible surfaces are a primary safety concern.

Surface temperature rise testing quantifies how much the temperature of each accessible part exceeds the ambient temperature when the heater is operated under the most unfavorable normal operating conditions. The test is a core element of the heating test of IEC 60335-1, supplemented by IEC 60335-2-61, and provides the technical basis for evaluating:

• Burn risk to users, particularly children, elderly people, and persons with reduced thermal sensitivity
• Risk of igniting adjacent combustible materials such as flooring, curtains, and wooden wall linings
• Long-term aging of electrical insulation, supply cords, and non-metallic components
• The correct functioning of charge limiters, thermostats, and other thermal controls

The principal standards governing this evaluation include:

• IEC 60335-1, Household and similar electrical appliances – Safety – Part 1: General requirements
• IEC 60335-2-61, Particular requirements for thermal-storage room heaters
• EN 60335-1 and EN 60335-2-61, the European adoptions of the IEC standards used for CE marking under the Low Voltage Directive
• GB 4706.1 and GB 4706.23, the Chinese national standards covering general safety of household electrical appliances and particular requirements for room heaters

Testing Samples

Complete Appliances

Representative production units submitted for type testing, including free-standing floor storage heaters, wall-mounted storage heaters, high-heat-retention models with enhanced insulation, and combination units that integrate a storage core with a direct-acting convector section. Samples must be complete with all fittings, feet or wall brackets, charge controls, and installation accessories, because the mounting configuration strongly influences surface temperatures.

Assemblies and Components

Where design verification is required before a complete unit exists, key assemblies such as storage cores, heating elements, charge regulators, damper mechanisms, insulation packages, air outlet grilles, and control knobs can be evaluated individually or in thermal mock-ups, with the results used to refine the enclosure and insulation design.

Material Specimens

Specimens of non-metallic materials, including enclosure plastics, knob and handle materials, terminal block insulators, and cable sheaths, may be tested to confirm their suitability at the operating temperatures recorded during the surface temperature rise test, for example through ball-pressure and glow-wire verification at the temperatures measured in service.

Testing Items

Typical measurement points and evaluation items include:

• Temperature rise of accessible enclosure surfaces, including top panels, side panels, front covers, and base areas near the floor
• Temperature rise of handles, knobs, and other parts gripped in normal use, distinguishing parts held continuously from parts touched only briefly
• Temperature of air outlet grilles and the discharged air stream, including the effect of automatic or user-adjustable damper settings
• Temperatures of the storage core, element terminations, and internal wiring, including connections to thermal controls
• Temperature rise of the supply cord, the cord anchorage region, and any accessible switchgear
• Heat transferred to mounting surfaces, adjacent walls, and the floor in a standardized test corner
• Surface temperatures under abnormal operating conditions, such as charge regulator failure, blocked air outlets, or a stalled fan in fan-assisted models
• Recording of the temperature values used as the basis for subsequent material verification tests

Testing Methods

Instrumentation and Test Setup

Fine-gauge thermocouples, typically Type K or Type J with wires of small diameter to minimize heat conduction error, are bonded to the selected measurement points using thermally conductive adhesive or high-temperature cement. The heater is installed exactly as described in the installation instructions, for example at the specified minimum clearances, in a test corner constructed from plywood finished with matte black paint, with thermocouples embedded in the walls and floor to capture heat transferred to adjacent building surfaces. The ambient temperature is maintained within the range specified by the standard, generally 20 °C plus or minus 5 °C, and is measured at defined positions away from the direct radiation and convection plumes of the appliance.

Test Procedure

The heater is supplied at rated voltage, or at the most unfavorable voltage within the tolerance band allowed by the standard, and operated through complete operational cycles:

• Charging phase: the appliance is charged until the charge limiter or other thermal control terminates or reduces the charge, representing a fully charged core
• Retention phase: temperatures are monitored as stored heat migrates through the insulation and enclosure
• Discharge phase: heat release is recorded, with dampers or output controls set to the most unfavorable positions, until temperatures fall or a further charge cycle begins, as required by the particular standard

Temperatures are recorded continuously, and the temperature rise of each point is calculated as the measured temperature minus the ambient temperature at the moment of measurement. The maximum temperature rise at each point is then compared with the applicable limit.

Evaluation Criteria

Limits are taken from the general requirements of IEC 60335-1 as modified or supplemented by IEC 60335-2-61. Representative limits include 35 K for bare metal parts gripped continuously in normal use, 45 K for porcelain or glass, and 60 K for plastic, with higher values permitted for parts touched only briefly. The particular standard additionally constrains the temperatures of grilles, air outlets, and enclosure surfaces in a manner that accounts for the sustained heat output characteristic of storage heaters, and the temperatures of the test corner walls and floor must remain within specified limits to demonstrate that adjacent building materials cannot be ignited or degraded. Internal wiring, insulation systems, and electronic components are assessed against their own material classes using the measured values.

Supplementary Abnormal Testing

Because a storage heater contains a large quantity of stored thermal energy, abnormal operation tests are especially significant. Conditions such as a failed charge regulator, obstructed air outlets, a jammed damper, or a stalled fan in fan-assisted designs are simulated, and surface temperatures are monitored to confirm that safety margins remain acceptable even under single-fault conditions.

Applications

Surface temperature rise testing of thermal storage heaters supports a wide range of engineering and compliance activities:

• Product certification and market access, including CE marking in Europe and CCC certification for appliances placed on the Chinese market
• Design validation during product development, such as optimizing insulation thickness, reflective radiation shields, internal air gaps, and enclosure geometry
• Quality control in serial production and verification during periodic factory audits
• Technical documentation for utility off-peak heating programs, public procurement, and housing authority tenders
• Failure analysis and incident investigation following complaints of burns, discoloration, or scorching near installed heaters

Accredited third-party laboratories perform these tests under ISO/IEC 17025 management systems, providing manufacturers with internationally recognized test reports for global market access.

FAQ

Why are limits expressed as temperature rise rather than absolute temperature?

The standards define limits as temperature rise above ambient so that results remain comparable regardless of the laboratory climate. The burn hazard experienced by a user depends on the actual surface temperature, and expressing limits as a rise over a controlled ambient ensures that this hazard is evaluated consistently at any test site.

Which standards apply to thermal storage electric heaters?

The general safety requirements are given in IEC 60335-1, adopted in Europe as EN 60335-1 and in China as GB 4706.1, while IEC 60335-2-61 and the corresponding room heater standard GB 4706.23 provide the particular requirements, including test corner configuration, cycle definitions, and any modified surface temperature limits specific to storage heaters.

How long does the test take?

Because the appliance must be driven through full charge, retention, and discharge cycles, testing typically spans one to several days for each configuration. Additional time is required for abnormal operation tests and for material verification tests that use the temperatures recorded during the heating test.

Can testing be performed on prototypes?

Yes. Thermal measurements on prototypes and thermal mock-ups are widely used during development to refine insulation and enclosure design. However, only tests on representative production samples performed by accredited third-party laboratories are accepted for certification purposes.

What happens if a surface exceeds the limit?

Exceeding a limit constitutes non-compliance. Corrective actions typically include increasing insulation thickness, adding radiation shields, enlarging internal air gaps, relocating controls, or modifying outlet grille geometry, after which the test is repeated to demonstrate conformity.