2010 Ford Mini van is not circulating radiator fluid. We made sure fluid is full but the van heats up and only blows cold air. They radiator is not leaking so wondering if it is the thermostat or could it be that fluid in the radiator is frozen. It's 20 degrees here? Not sure of the quality of the antifreeze, just got the vehichle. I know it's a long shot and prolly more in is needed but thought, what the heck. any thoughts would be appreciated.