Failure Modes of Domestic Electric Heating Devices

Balancing Thermal Comfort with Device Reliability

By Pier-Olivier Morin, P.Eng.

In cold-climate countries like ours, residential heating is a significant concern. Designers have long focused on addressing challenges related to the performance and safety of heating devices, as well as ensuring occupant comfort.

From the second half of the 20^th century onward, various types of electric heating devices became widely used in Canada. Generally, these devices are safe, but several types of failures can, unfortunately, lead to fires.

This article examines the most common failure modes identified by forensic experts tasked with determining the causes of fires involving such devices.

Three Modes of Heat Transfer

There are three modes of heat transfer: conduction, convection, and radiation (Figure 1).

Figure 1 – Modes of heat transfer

Conduction is the transfer of heat through solid materials. For example, a pot on a functioning stove element will transfer heat to its metal handle by conduction.

Convection transfers heat by the movement of fluids. This is the most common heat transfer method in domestic heating devices. An electric baseboard heater, for instance, heats the surrounding air, causing it to rise as it becomes less dense. Cooler air moves in to take its place, creating a continuous air circulation pattern that warms the room.

Radiation transfers heat by electromagnetic waves. The sun is the best example: it emits heat as electromagnetic waves that travel through the vacuum of space and the Earth’s atmosphere. Heat is felt when these waves strike an object and warm it.

Failure of Heating Elements

All convection-based heating devices contain at least one heating element, which can be categorized as either sheathed or unsheathed.

Sheathed elements typically consist of a resistive wire encased in a metal sheath, often covered with fins to increase the surface area in contact with the air and improve thermal efficiency. The space between the resistive wire and the sheath is usually filled with magnesium oxide powder, which is a good conductor of heat but also provides electrical insulation. If the electrical insulation between the wire and the sheath fails, a current can flow, creating an arc that produces molten metal particles with enough heat to ignite a fire (Figure 2).

Figure 2 – Failure of a sheathed element

Unsheathed elements, as the name suggests, leave the resistive wire exposed to air. Failures involving electrical arcing can occur if one or more resistive wires touch each other or other grounded metal parts of the device (Figure 3). The heat generated by the arc is sufficient to cause a fire.

Figure 3 – Fused resistive wires

Poor Electrical Connections

The components of an electric heating system, including heaters and thermostats, are particularly susceptible to failure due to poor electrical connections because of the high currents they carry. Thermostats and heaters are typically connected to the home’s electrical wiring using wire connectors (commonly known as Marrette). The torque applied to these connectors must be sufficient to ensure good contact between the conductors. If not, the resistance at the connection point can increase, generating enough heat to ignite a fire (Figure 4). This phenomenon is known as resistive contact.

Figure 4 – Resistive contact testing with a wire connector

Obstruction

Electric heating devices are equipped with safety mechanisms to prevent overheating in the event of an obstruction. In an electric baseboard heater, for example, this safety feature is a capillary tube located inside the device near the heating element (Figure 5). If an object obstructs the heater and causes the capillary to overheat, the gas inside the capillary expands and triggers the diaphragm, cutting off the power and stopping the heater.

However, this thermal protection mechanism has a limited lifespan. If the obstruction persists over an extended period, the safety mechanism may fail. Without a functioning thermal protection, the heater may overheat and ignite any nearby objects or materials.

Figure 5 – Capillary tube and diaphragm in an electric baseboard heater

Electrically Heated Floors

Electrically heated floors rely on the principle of conduction to heat the floor covering, which transfers the heat to the ambient air primarily by radiation. These systems typically consist of a heating cable or film installed beneath the floor covering and include a thermostat to regulate the room temperature. In addition to the temperature sensor built into the thermostat, another  temperature sensor is typically installed beneath the floor covering near the cables. The thermostat regulates the room air temperature and the floor sensor provides the high limit thermal protection.

However, if a large object covers a section of the floor for an extended period of time, heat transfer may be restricted. If this obstruction is not near the sensor, it may not detect the overheating. As a result, the floor covering and the obstructing object may be damaged by heat, creating a fire risk (Figure 6).

Figure 6 – Thermal damage to floor covering due to obstruction

Overloads

Compatibility of heating system components is crucial to ensure their proper operation and occupant safety. The voltage (volts), current (amperes), and maximum power (watts) ratings for each component must be strictly adhered to. These specifications can usually be found on the device’s nameplate, installation manual or technical data sheet.

In the example shown in Figure 7, an electronic thermostat rated for 3 000 watts was controlling two baseboard heaters with a combined power of 4 500 watts. This overload caused the thermostat to fail, resulting in a fire.

Figure 6 – Thermostat damaged by power overload

For any questions following a fire caused by the failure of a domestic electrical heating device, do not hesitate to consult the electrical and fire experts at CEP Forensic.