Why does my 98 Lincoln navigator blow out cold air instead of hot air?

My 2002 Lincoln LS did the same thing, and began to overheat very quickly just after starting.

I had thermostat and water pump replaced, just in case.

But, when the car was cool, I simply pressed the hoses leading to the engine pumping coolant through them, which for some reason, opened the thermostat, and allowed coolant to flow as expected.

The hot air began flowing again through the vents.