The heat will not get hot in the front or the back. on 1998 Ford Expedition

I have not been able to get the heat in my truck to get hot. The temperature control knob on the dash board lets me know when the engine is hot enough to turn the heat on, and I hear it blow when I turn it on. I had the thermostat changed, the heater coil blew out in case it was plugged. I also had the temperature control knob changed, the clutch fan changed and still no heat. Please help.

Asked by for the 1998 Ford Expedition
There is a blend door in the heater box that adjusts the amount of heated/cooling air that goes through the heater. If the blend door is stuck in one position then it will not change with the temp control is moved. Check to see if the cables or motors are moving at the heater box when the temp control is moved. Even it it is, there is no guarantee that the door inside the heater is moving. The other thing to check is to see if the vehicle is equipped with a hot water valve. If so, make sure it's opening and allowing hot coolant through. A broken or stuck closed hot water valve will not allow hot coolant to circulate through the heater core and it will blow cold.