Why does the AC light automatically turn on when the heater is on?

[StSienna]

It is normal for the AC light to turn on automatically when the heater is activated. This is a common setting in many vehicles, where the compressor starts running as soon as the blower is turned on. If you want to use the heater but the AC light is on, you can press the AC button to turn off the compressor. The compressor does not need to run when using the heater, and its operation will increase fuel consumption. The principles of cooling and heating in a car's air conditioning system are straightforward. When heating, the high-temperature coolant from the engine flows through the heater core, and the air blown by the blower also passes through the heater core, allowing the air conditioning vents to blow warm air. When cooling, the engine drives the compressor to operate. The compressor continuously compresses the refrigerant and delivers it to the evaporator. Inside the evaporator, the refrigerant expands and absorbs heat, cooling the evaporator. The cooled evaporator then cools the air blown by the blower, allowing the air conditioning vents to blow cold air.