The object of the present invention is a thermostat (1) having calibration locking means that may be unlocked at user discretion, in order to increase the performance of the water heater whereon said thermostat (1) is installed. Said locking means consist of joining elements (7.41) fracturable as a result of the application of a rotational force on the adjustment knob (7, 7.1) by which the operating temperatures of such thermostat (1) are calibrated. Said adjustment knob (7, 7.1) is also provided with visual and/or sound identification means (7.10, 7.40) in order to facilitate the user in the task of recovering the calibration point T.box initially preset, even after the fracture of said joining elements (7.41).
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