Sara Childs set the thermostat in her Forest Hill sunroom to 75. "After a while, I was too warm, so I checked the thermostat." It was set on "heat" so, she selected "cool," and soon it was comfortable. But she was puzzled: "Shouldn't 75 degrees be 75 degrees, hot or cold?" It is. But October sunshine warmed the room to more than 75. Your heater can only heat, so it shut off. By selecting "cool," you switched on the AC to cool the room back to 75.