Turned on the heat on the first chilly morning and it's barely warm. I'm figuring it's either a clog or airlocked. If I flush it, what's better, water or air? The only thing that concerns me about using air is that I might damage the heater core and from what I can tell, it seems that the core can't be replaced without pulling the dash. Regardless of what I use, should I flush in the opposite direction of flow?