No defrost while idling to warm up car in winter

We have a 2006 Aveo hatchback with 4 doors. For the most part it's been a great little car that we bought for my daughter. But I have one issue I can not fix in regards to the heater / defrost function in the winter and I'm stumped.

During the winter she starts her car and leaves it in idle to warm it up before driving it, the defrost fan runs fine, but almost no heat comes out of the defrost vents, it feels like lightly cooled air, and the windows will still be iced up. As soon as you rev up the car then air in the defrost heats up and the windows will then defrost within 5-10 minutes, unfortunately about the time she pulls up to the house. I have let the car warm up for 15-45 minutes and the air never gets hot enough to defrost the windows until you step on the gas pedal and get the rpm's over 1500 - 2000 rpm. I've replaced the thermostat and upgraded the thermostat housing, all that is working correctly. Fan belt is tight so I'm assuming no water pump issues unless it's slowly failing. Are there any vacuum tubes that could be losing vacuum and causing the hot air not to switch on?

any help is appreciated. Thanks

nine times out of ten this is a water circulation issue...air in the coolant system, rad cap defective, low coolant ect....increased engine rpm creates pressure in the cooling system and circulation begins...can be too low an idle speed as well

The question is....is there hot air coming out of the floor vents and it's just the defrost vent that blows cold ? Or are all vents blowing the same temp ? And they all get hotter with increased engine speed.....

Does the coolant gauge show the engine is actually up to temperature?

Sounds like low coolant levels to me, but I would imagine you have already checked that with the thermostat replacement.

I agree with doax. Could it also be a heater core issue.

Correct me if im wrong aren't there two kinds of thermostats that he could run it sounds like its opening to early and not heating up properly. You live in Alaska do you use a block heater at all?