Bimetallic strips are two pieces of different metals stuck or riveted together.

Upon heating the bimetallic strip starts bending towards a direction that depends on the coefficient of thermal expansion of individual strips

Gaggia bimetallic thermostats fail due to metal fatigue and heat. As the metal in the tstat heats and bends it’s being made stiffer and more brittle, and as it’s heated the metal is weakened.


This is referred to as metal fatigue. Over time the opening and closing points are reduced, resulting in lower operating temperatures.

 

Thermostats of this type seldom fail outright, but when they do it’s invariably in the closed position which effectively leaves the heaters running uncontrolled.

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