Engine overheated now heater blowing cold

When I talked about the parts replaced I said the thermostat. But the engine temp hand still falls to the left. I bought a new thermostat for it about a week or 2 after it overheated. But why is the heater blowing cold as you drive down the road and burn you up sitting still or idling the engine temp goes up slowly almost half way up where has been since the day I bought it.