When I start my 97 F150 w/4.6l, the heat is just warm until the thermostat opens - then air turns cooler. If I turn the temp down it will come out cold. The controls seem to work correctly.

 

sounds like a thermostat issue. I'd try replacing that first.

What is your engine temperature doing?

 

I have replaced the thermostat. Which was stupid of me. If the thermostat is open - it just takes longer to get heat but when the engine does finally get to temp I should have heat.

 

Not necessarily. If your thermo is stuck open your heat won’t be as hot as it should be. The car wont hit full operating temp. Where is the temp gauge reading?

Did you feel both the top and bottom radiator hose to see if the thermo is opening and working? Are you sure you put the correct temperature thermostat in the car?