If you flip your windows AC unit around, then you get the exact amount of heat energy removed by the AC in the form of hot air going into your room. This is cal

If you flip your windows AC unit around, then you get the exact amount of heat energy removed by the AC in the form of hot air going into your room. This is called a heat exchanger. It was just cheaper to use gas furnace heater when most home was built and it is still true in many places.