A heater (10) for heating an aerosol-forming substrate. The heater (10) comprises a heating element configured to heat an aerosol-forming substrate, the heating element comprising at least one PTC thermistor (24, 25, 260, 261, 262, 263, 264, 265, 27). The resistance of the at least one PTC thermistor (24, 25, 260, 261, 262, 263, 264, 265, 27) increases when the temperature of the at least one PTC thermistor (24, 25, 260, 261, 262, 263, 264, 265, 27) increases within a stable temperature range. The lower end of the stable temperature range is a reference temperature (CT) at which the resistance of the at least one PTC thermistor (24, 25, 260, 261, 262, 263, 264, 265, 27) is twice the minimum resistance value of the at least one PTC thermistor (24, 25, 260, 261, 262, 263, 264, 265, 27).