Appliances Renovation

Digital Reliable Hobs and ovens

These are among the most frequently used items in the home. Generally, they are very reliable, but when a problem does occur, it is usually at the most inconvenient time. Over the years, ovens and hobs have become increasingly complex with the addition of digital timers, touch controls and other refinements.

It is essential to remember that these appliances are connected directly to the mains supply and, as with any repair, cleaning or service, must be completely isolated. In this instance, the main double-pole isolation switch for the appliance must be switched off and also the fuse for the cooker/hob circuit removed from the main distribution board. If an MCB is fitted in place of a fuse, ensure it is in the off position. Do not proceed with any repair or service if both these requirements are not met and also make sure you advise others who may use the switch of your intentions.

Hob elements

The hob works by means of heating elements (usually four). These can be of various patterns and may be either double or single operation. The fixing plate on the element gives details of wattage, and a simple inspection of the element will determine size and style — double or single and number of turns or rings the element has.

Continuity testing is simple. If a replacement is required, it is essential that all the relevant criteria of the old element be met by the new one that is, wattage, size, number of turns, etc. Some elements are sold as separate items, while others come complete with their own dished recess. The latter are normally the type of element that can be tilted or swivelled for cleaning purposes.

As with all heaters, it is most important that the push- on or screw-on connections to the element are fitted securely and all cables and covers are sound. The cable used within the appliance is heat resistant and shrouded for protection. Make sure all cable runs are routed correctly and not trapped or allowed to foul sharp or hot surfaces. All metal parts must be part of the earth path and the appliance must be earthed in accordance with the manufacturer’s instructions.

When a replacement element is required, it may be possible to fit an economizer element. These are available in most of the popular sizes and are completely interchangeable. The fitting of such elements in place of the standard type can result in a saving of up to 10 per cent in consumption.

Temperature control

The more popular hobs use spiral radiant elements in either double or single form. Temperature control is by means of an energy regulator. Manufacturers may use different wattages, sizes and styles (double or single) of element, so it is essential that only the correct replacement is obtained and fitted. They are widely available, but do make sure you obtain one of the highest quality.

Make a note of all wiring positions and routes before removal. Some replacement controls are sold with sticky labels for the purpose of marking the wiring. This is especially helpful with older appliances where the new unit may have a different wiring configuration as a result of improvements in switch design and operation.

The function of the regulator is simply to turn the supply voltage to the element on and off periodically. The temperature variation is, therefore, in direct proportion to the rate of the on/off sequence, which is controlled by means of a bi-metal strip. Within the unit is a large bi-metal plate with a small heating element secured to it. On early regulators a small wound element was used and open-circuit faults were common. The new control, shown here, uses a thick-film resistor to heat the bi-metal plate and prevents problems associated with the earlier type. The control includes a switching action at the ‘off’ position. When turned on, the live contact is allowed to rest on a cam linked to the shaft of the unit. The cam varies the distance required for the bi-metal to move before operation of this switch takes place, that is, the more deflection required, the longer power is supplied to the element. When the bi-metal plate has moved enough to cause the switch to go open circuit, thus removing power from the element, the same switching action removes the supply to the small element/resistor, allowing the bi-metal plate to cool and reset the switch. This process repeats for as long as the switch is in the ‘on’ position. The varying lift of the cam is proportional to the visual heat setting indication on the hob switch control panel: low, medium and high.

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