Originally Posted by AviatorTroy
There is fluid circulating through your heater core all the time, it just blows hot air out when the door opens inside the duct.
unless there's a valve on the heater water line... then it only blows hot air when the core is hot. No hot water flowing, no air gets hot as it goes through the core.
You might be right about the GM system of making hot air, but such is not the case on my old Benz or any of my previous toyotas.