They are also referred to as control knobs and temperature knobs, they control either the temperature or are used to switch the Ideal boiler on or off and can be found on the front of the boiler. They normally have a shaft that protrudes from the knob directly through the front of the boiler onto the printed circuit board. The shaft when turned, turns a potentiometer that's located on the printed circuit board and if the knob is turned too hard the shaft is designed to break hence protecting the Ideal printed circuit board from damage and an expensive repairs.
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Ideal 111904 Mexico Control Knob

Part: 111904

Price: £11.47
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Ideal 013241 Concord Super Control Knob

Part: 013241

Price: £14.92
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Ideal 172491 Icos Isar Knob

Part: 172491

Price: £5.35
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Ideal 170859 Concord Control Knob

Part: 170859

Price: £14.86
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Ideal 173562 Evo Knob

Part: 173562

Price: £6.22
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Ideal 177384 Independent Knob

Part: 177384

Price: £7.41
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Ideal 172554 Mini Control Knob

Part: 172554

Price: £3.96
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Ideal 171929 Classic Knob

Part: 171929

Price: £8.21
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Ideal 177387 I Mini Logic Knob

Part: 177387

Price: £8.09
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Ideal 172442 Boxer Control Knob

Part: 172442

Price: £11.35