heater blowing cool air while idling

Are you sure all the air is out of the system? With the engine cold, remove the radiator cap, is the radiator full to the top? If not top off the coolant in the radiator and the expansion tank the rest will take care of itself. You could also have a partially plugged heater core. I hope this helps

If what Mike says doesn’t solve issue, replace thermostat and refill (air out of system). And pressure test system. I also want to say, “when idling” is this when it is first cold in the morning? If so give it time to warm up while driving. Lastly if all this doesn’t help, look at your coolant temperature sensor for a fault.