Do you mean that at idle, it gives no air or no hot air.? If it is no air , then it would likely be a fan problem in the heater system. If it is no hot air, then it is most likely a vaccuum problem
Check the antifreeze level.
Is the blower motor working ??
maybe your thermostat is stuck in the closed position or your heater is stuck on the on position
Low on coolant. Top it up.
