THere is no air blowing when the heat is on. The furnace does come on but no air blows. If I turn the heat off and manually turn the fan on the air blows. If I leave the fan switch in the on position and turn the heat on the air stops.

THere is no air blowing when the heat is on. The furnance does come on but no air blows. If I turn the heat off and manually turn the fan on the air blows. If I leave the fan switch in the on position and turn the heat on the air stops.
You didn't give me an age, manufacturer, efficiency, to go on. But it sounds like the heat fan speed relay in the circuit board has gone bad. You will need a new PC board.