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Heater/Defrost in van doesn't blow hot air when parked and idling. What's wrong?

2001 Chevy Venture. Defrost only blows hot air when van is moving. The faster the better. What is wrong with the heater?

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With this information I can think of two possible reasons (assuming the thermostat and coolant flow control valve to heater are working properly).
If your blower fan is not working and the system is set for fresh air intake instead of recirculate, then air will only flow out of the system while the vehicle is moving. Under this scenario if the system is set to recirculate then air will never flow out of the vents.
If your blower fan is working but part of the coolant system feeding the heater is partially obstructed, then not enough hot coolant is flowing to the heater at engine idle. When the engine is at higher RPM the coolant pump is flowing more coolant through the system and adequate hot coolant is flowing through the partially obstructed tubing to heat the air flowing from the vents. Using recirculate setting will cause the air to be hotter.

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