Sounds like a bad thermostat but if all your coolant is going into your overflow bottle could also be a bad radiator cap. Both of which are inexpensive to replace. The thermostat is just a bit messy to change out (drain all coolant), remove the thermostat replace with new one.
But if your car is overheating with the heat on in your car? Is the radiator fan coming on? How old is your radiator? Is it filthy? How old is your coolant?
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