The combistat is a temperature gauge and a high limit cut off switch in one. This gauge also controls the heat and the heater on this machine. To engage the heat, you must turn the knob in the middle of the gauge to a higher temperature than the black indicator line on the gauge. When the red line is at or below the black line the heater will not come on. When the red line is above the black line the heater will engage. You can control the maximum temperature by setting the red line to the max desired temperature and then heater will cut-off when the temperature is achieved.