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.

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Originally Posted by dadsvz

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.

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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.