89 f250 460. Brand new thermostat that is functioning properly, tested with an ir thermometer at the water neck. Flushed system with new coolant. The air from the defrost vents, or any others for that matter never get really warm, best is a luke warm. What should I be looking at to fix this.

 

What temp is thermostat? Check the 2 hoses, that go to heater core, are they hot, warm, luke warm, is radiator full, or do you have to keep filling ever so often?

 

Its a 195 t stat. Hoses are hot I would say, radiator stays full.

 

Clogged heater core is where to look next.

 

Damper Door

 

^^^^yup^^^^. common problem with these trucks.
coffee or soda cup on the dash and hit the brakes, the sticky liquid goes down the defrosted ducts and sits in the heater plenum. then dries sticking the blend doors where they are. if it was on ac when it dried, you will never get any heat.
remove the glove box and door, and feel along the top of the heater plenum while moving the heat/ac temp lever.
you will feel the cable trying to move the door. this will then allow you to dig in and hopefully free that door.

 

Thanks! I was just reading that the door might be a problem, now I know how to check it.