Ford F150 Heater Only Blows Cold Air Already Checked

My 1997 ford f150 heater will not blow hot air. i have replace the switch that controls the hot and cold air. i do not know what else to do. i do not heat.I have a problem with my heater blowing cold air. the heater core is hot and hot air blows out directly at the heater core, but all the vents are blowing cold air..05 6.0 heater blowing cold air 6.0l power stroke diesel '04 eb explorer 4x4 4.6 ltx m/s2 (trans replaced at 53k miles) '05 eb excursion 2wd 6.0 ltx m/s2, diesel. Truck (f-150)ford heater blows cold air instead of hot air?. The a/c works fine but the heater blows cold air. i live in houston so i only found out when this winter though it did work last year. i suspect it is the flapper. My a/c is blowing cold but only thru the defrost vents. first it was blowing normally from the dash but during acceleration or under load, the a/c would randomly blow. Ford f150 air intake systems breathe bursts of power into your truck's engine. gusts of clean cool air are just what the doctor ordered, as far as you and your engine. Question - 2009 f150 heater blows pass side only - 3f. find the answer to this and other ford questions on justanswer..Question type: maintenance & repair when i turn the heat on in my car it only blows cold air. the ac works just fine and the fan does blow at full power but no.