Heating Appliances: Construction, Working and Servicing
Heating Appliances: Construction, Working and Servicing
Heating Appliances: Construction, Working and Servicing tests the heating element, insulation, thermostat and safety cut-outs in irons, kettles, geysers, room heaters, ovens and induction cooktops, and the servicing checks.
Core rules and concepts
- Joule heating: heat produced is I²Rt. Elements are nichrome wire wound on a mica or ceramic former, insulated from the metal body but able to transfer heat.
- A bimetallic thermostat bends with temperature and opens or closes contacts to hold the set temperature. A thermal fuse is a one-shot safety device and must be replaced with the same temperature rating.
- A geyser has an element, thermostat, safety cut-out, inlet and outlet, and earthing; many storage types also have a sacrificial anode. An electric iron has a soleplate, element, thermostat and pilot lamp.
- Induction cooktops heat the pan by eddy currents; the vessel must be ferromagnetic. The glass top is not heated directly by the coil.
- Servicing: isolate, check element continuity and insulation resistance, test the thermostat, remove scale, check earthing, verify rated wattage and test for leakage.
Worked example
A 2 kW geyser takes 8.7 A at 230 V; its element should read about 26.5 Ω cold. A reading of infinity means an open element; a very low reading means a shorted turn or wrong element. If the element is good but water stays cold, test the thermostat and thermal cut-out for continuity. Reasoning: follow the path from supply through thermostat, cut-out and element before condemning the element.
Traps that cost marks
- Testing resistance on a live element.
- Fitting a higher-rated thermal fuse.
- Ignoring scale, which slows heating and overheats the element.
- Failing to refit the earth conductor.