A vacuum leak in the HVAC system will change where the air flow is directed. Under low vacuum situations to the blend doors, i.e. going up hill, and acceleration, the air flow will default to defrost.
If you are experiencing low or no heat only at idle, I would first make sure the cooling system reservoir is at the "FULL HOT" mark on the tank. If a vehicle looses heat at idle, it is an indication that the heater core is starving for coolant. When a cooling system is low on coolant, the heater core is the first thing to loose coolant. This is inherent with the design to, number one: let you know there is a problem, and number two: keep coolant in the engine block to prolong overheating damage. The reason this happens is because the heater core is the highest point in the cooling system, therefore, care must be taken while re-filling the cooling system to ensure all air is purged out. Once the radiator cap is installed it is nearly impossible for this to happen. Have a gallon of the proper coolant handy and remove the radiator cap. Run the engine at approximately 1500 RPM until the engine reaches operating temperature, (you will notice coolant moving in the radiator, and probably bubbles too). Continue at 1500 RPM while making sure the radiator is full at all times. Brief and quick increases in RPM will help to dislodge any air pockets trapped in the system. Once you're satisfied all air is purged, make sure the radiator is full and reinstall the cap. NOW you can let the engine idle. Fill the reservoir to "FULL HOT" and take her for a test drive. If this doesn’t cure the problem, there is a possibility that the heater core is restricted, in which case reverse flushing it should help. Diagnosis for the core is done while at operating temperature and 1500 RPM. Both inlet and outlet hoses at the heater core should be very close to the same temperature. They will be very warm, but not so warm that you can’t touch them. (You shouldn’t notice a temperature difference touching them with your hand.) Good luck and I’ll watch for replies/questions.