Thermal comfort evaluation of storage electric heaters is essential for verifying that off-peak heat-release appliances deliver stable, uniform, and healthy warmth in accordance with ISO 7730 comfort criteria. This article explains the evaluation framework, sample categories, key testing items, laboratory methods, and typical applications based on ISO, IEC, EN, and GB standards. Engineers, product developers, and procurement specialists can use this guide to understand how accredited third-party laboratories assess PMV and PPD indices, radiant temperature asymmetry, and 24-hour thermal stability in thermal storage heating appliances.
Technical Overview
Storage electric heaters, also referred to as thermal storage heaters or heat-storage radiators, convert electrical energy into heat during off-peak charging periods and store it in high-density ceramic, magnetite, or phase-change material (PCM) cores. The stored heat is then released gradually over many hours, either through natural convection across insulated core channels or through fan-assisted discharge. Because the heat output varies continuously throughout a charge-release cycle, the perceived comfort depends not only on steady-state air temperature but also on dynamic factors such as radiant asymmetry from hot front surfaces, vertical temperature gradients, air movement, and the rate of indoor temperature drift.
Thermal comfort evaluation quantifies these effects using the internationally recognized PMV (Predicted Mean Vote) and PPD (Predicted Percentage Dissatisfied) indices defined in ISO 7730. The PMV index predicts the mean thermal sensation of a large group of occupants on a seven-point scale from -3 (cold) to +3 (hot), while the PPD index predicts the percentage of occupants likely to be dissatisfied. ISO 7730 establishes three comfort categories: Category I requires a PMV between -0.2 and +0.2 with a PPD below 6 percent, Category II requires a PMV between -0.5 and +0.5 with a PPD below 10 percent, and Category III requires a PMV between -0.7 and +0.7 with a PPD below 15 percent.
In addition to whole-body comfort, the evaluation addresses four local discomfort mechanisms that are particularly relevant to storage heaters:
- Draught rate (DR) caused by buoyancy-driven or fan-driven air movement near the occupant, most critical at ankle level.
- Vertical air temperature difference between head and ankle heights, which increases when warm air stratifies above a slowly discharging heater.
- Warm floor sensation and floor surface temperature, relevant for heaters mounted close to the floor.
- Radiant temperature asymmetry produced by the hot front panel or grille of the heater relative to cooler room surfaces.
The performance side of the evaluation draws on IEC 60531, which specifies methods for measuring the performance of household electrical thermal-storage room heaters, including charging characteristics, heat output, and discharge behavior. Safety-related surface temperature limits are governed by IEC 60335-2-61 and the general requirements of IEC 60335-1, while Chinese national frameworks apply GB 4706.1 for general safety and GB/T 18048 and GB/T 18049, which correspond to ISO 7726 and ISO 7730 respectively.
Testing Samples
Thermal comfort evaluation is applicable to a wide range of storage heater constructions. Samples are typically submitted with rated voltage, rated input power, charge duration, and manufacturer-defined discharge settings. Representative sample categories include the following:
Core-Based Natural Convection Storage Heaters
These are the classic designs in which a ceramic or magnetite brick core is charged overnight and releases heat passively through top and bottom air channels. Comfort evaluation focuses on the slow temperature drift, stratification, and the elevated surface temperature of the front panel during peak discharge.
Fan-Assisted Storage Heaters
These units incorporate an insulated core with an electrically controlled fan and damper that discharge stored heat on demand. Because the fan can generate local air velocity above comfort thresholds, draught rate and vertical temperature distribution testing at occupant positions is essential.
PCM Storage Heaters
Phase-change material units store latent heat at relatively low and stable temperatures, offering a flatter release curve. Testing emphasizes temperature stability across the phase-change plateau and the reduction of radiant asymmetry compared with brick-core units.
Combined Storage and Direct-Acting Heaters
Hybrid appliances integrate a storage core with an additional direct-acting convective element for peak-demand top-up. Comfort evaluation covers both operating modes, including the transition between stored-heat discharge and direct heating, where overshoot and control accuracy are common concerns.
Smart and Adaptive Storage Heaters
Models equipped with weather-compensated charging controllers, occupancy sensors, or app-based scheduling are evaluated for control accuracy, predicted-charge algorithm behavior, and their ability to maintain the target operative temperature band across a simulated occupancy day.
Before testing, all samples are preconditioned at rated charge for at least one complete charge-discharge cycle to stabilize core behavior, and instrumentation ports are fitted without altering the airflow geometry of the enclosure.
Testing Items
The evaluation program combines global comfort indices, local discomfort parameters, and appliance-specific thermal performance measurements. Typical testing items include:
- PMV (Predicted Mean Vote) index calculated at seated and standing occupant positions.
- PPD (Predicted Percentage Dissatisfied) index derived from PMV.
- Operative temperature, computed as the weighted combination of air temperature and mean radiant temperature.
- Air temperature distribution across the occupied zone at 0.1 m, 0.6 m, 1.1 m, and 1.7 m heights.
- Vertical air temperature difference between ankle (0.1 m) and head (1.1 m seated or 1.7 m standing) levels.
- Mean radiant temperature measured with black-globe sensors at occupant positions.
- Radiant temperature asymmetry in the vertical and horizontal planes using plane radiant temperature sensors.
- Air velocity and turbulence intensity at ankle and neck levels for draught rate determination.
- Relative humidity within the occupied zone, with a typical comfort expectation of 30 to 70 percent.
- Surface temperature mapping of accessible heater panels, grilles, and control covers.
- Twenty-four-hour heat release curve and cumulative delivered heat, per IEC 60531.
- Charging energy, charge acceptance, and storage efficiency over a complete cycle.
- Operative temperature stability, overshoot after charge completion, and drift rate per hour.
- Thermostat and charge-controller accuracy, including setpoint deviation and timing drift.
- Acoustic noise contribution from fan-assisted discharge, where applicable.
Testing Methods
Test Chamber and Environmental Conditions
Testing is performed in a calibrated climatic chamber or instrumented test room conforming to the environmental requirements of IEC 60531 and the measurement framework of ISO 7730. The chamber is stabilized at a reference condition, typically 20 °C dry-bulb with controlled relative humidity, and the heater is operated through complete charge-discharge cycles. The analysis assumes a standard occupant scenario, for example a sedentary metabolic rate of 1.2 met and winter clothing insulation of 1.0 clo, in accordance with ISO 8996 and ISO 9920.
Instrumentation and Measurement Accuracy
All sensors conform to ISO 7726 for accuracy and response characteristics: shielded air temperature sensors with radiation error below 0.2 K, black-globe thermometers for mean radiant temperature, omnidirectional anemometers capable of resolving velocities down to 0.05 m/s for draught analysis, plane radiometers for radiant asymmetry, and capacitive humidity sensors. Data acquisition logs all channels continuously at intervals of one minute or finer throughout the multi-day test sequence.
Measurement Point Layout
Sensor arrays are positioned at defined occupant locations around the heater, including a primary measurement point at typical seating distance and secondary points at 0.5 m and 1.0 m lateral offsets. Vertical masts capture the temperature profile from ankle to head height, and floor surface thermocouples record under-heater and adjacent floor temperatures. Plane radiant sensors are oriented toward and away from the heater front to quantify asymmetry.
Charge-Discharge Cycle Protocol
The sample is charged at rated input for the manufacturer-specified charge period and then allowed to discharge under the defined release condition. The full 24-hour cycle is repeated at least three times to confirm reproducibility. Comfort parameters are recorded continuously, and hourly averages are used to plot PMV, PPD, operative temperature, and draught rate as functions of time since charge completion, revealing whether comfort remains within the selected ISO 7730 category throughout the release window.
Computation of Comfort Indices
PMV and PPD are computed from the six standard input parameters: air temperature, mean radiant temperature, air velocity, humidity, clothing insulation, and metabolic rate. Local discomfort indices are evaluated separately: draught rate from air temperature, velocity, and turbulence intensity; vertical temperature difference from the mast data; and radiant asymmetry from the plane radiant temperature measurements. Temperature drift rates are checked against the ISO 7730 guidance that steady-state criteria remain applicable for changes below approximately 2 K per hour.
Supplementary Methods
Where deeper insight is required, testing may be supplemented by a heated thermal manikin to quantify segmental heat losses and equivalent temperatures, and by subjective assessment panels conducted in accordance with ISO 10551 using standardized thermal sensation, comfort, and preference scales. Field verification in instrumented demonstration rooms is also available to bridge laboratory reference conditions and real occupied environments.
Applications
Thermal comfort evaluation of storage electric heaters serves multiple stakeholders across the product lifecycle:
- Product development and optimization, guiding core sizing, insulation design, damper control, and fan curve selection to flatten discharge temperature profiles.
- Performance benchmarking between storage heater models and against direct-acting or hydronic alternatives.
- Market access support, providing the performance data required under IEC 60531 and EN 60531 frameworks and related regional energy-efficiency and labeling programs.
- Building energy simulation, supplying validated 24-hour heat release curves as input for dynamic whole-building energy models and load calculations.
- Utility demand-side management, qualifying appliances for time-of-use and off-peak heating programs by demonstrating comfortable heat delivery across the discharge window.
- Procurement and tender documentation, giving housing authorities, developers, and facility managers objective comfort criteria for acceptance.
- Academic and industrial research on PCM storage media, adaptive charging algorithms, and human-centered heating control strategies.
FAQ
What acceptance criteria are applied in the evaluation?
The most common acceptance target is ISO 7730 Category II, which requires a PPD below 10 percent together with local limits such as a draught rate below 10 percent, a vertical air temperature difference below 3 K between head and ankle, floor temperatures between 19 °C and 29 °C, and radiant asymmetry below the standard limits for warm vertical surfaces. Higher-grade projects may specify Category I, while Category III is used where moderate deviations are acceptable.
How does comfort evaluation of storage heaters differ from direct-acting heaters?
Direct-acting heaters can often be assessed at steady state, whereas storage heaters must be evaluated dynamically across complete charge-discharge cycles because their heat output continuously declines. The evaluation therefore extends over multiple 24-hour periods and gives additional weight to temperature drift, release-curve shape, and the high radiant asymmetry associated with hot core-adjacent surfaces.
Which standards govern the assessment?
Comfort indices and local criteria are defined in ISO 7730, measurement instrumentation in ISO 7726, subjective methods in ISO 10551, metabolic rate and clothing values in ISO 8996 and ISO 9920, appliance performance in IEC 60531 and EN 60531, and safety-related surface temperature limits in IEC 60335-1 and IEC 60335-2-61. In China, GB/T 18048 and GB/T 18049 mirror the ISO comfort standards, and GB 4706.1 covers general appliance safety.
How long does a complete evaluation take?
A standard program including chamber stabilization, sample preconditioning, and three reproducible 24-hour charge-discharge cycles typically requires five to seven working days. Programs adding thermal manikin measurement or subjective assessment panels require additional scheduling time.
Is subjective testing with human participants required?
No. The analytical PMV/PPD method defined in ISO 7730 is fully accepted for compliance and development purposes. Subjective assessment per ISO 10551 is an optional supplement used when manufacturers want to validate comfort perception for specific user groups or innovative control strategies.
Can laboratory results be extrapolated to real installations?
Laboratory results characterize the appliance under controlled reference conditions and provide comparable, reproducible data. For project-specific conclusions, the heat release curves and comfort metrics can be transferred to dynamic building simulation, and in-situ verification in instrumented rooms can confirm field performance under actual envelope, ventilation, and occupancy conditions.