The thermostat that controls the surface temperature is designed to allow the surface to get up to 170-180°F. The heated shelf is designed to have enough reserve power to overcome reasonable variations in temperature and air movement. It is not uncommon for these appliances to be exposed to cool ambient temperatures or excessive air movement, both of which may cause the surface temperatures of the heated shelf to be lower. This is because the surrounding environment pulls heat away from the heated shelf faster than the element can compensate.
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