I have a 2006 F150 and my wife says when outside temps are low, teens to 20 deg. there's no heat, then when it warms up 30+ there is heat again. I haven't had a chance to dig into yet so I'm looking for some ideas before I do. My coolant level is good and tested to -20 below or better.

 

If the weather stays cold you might have to do the old semi trick. Manually limit some of the air flow to the front of the radiator. Don't totally fasten it to the front of the radiator, just block some of the airflow. (It takes quite awhile for mine to heat up too) You can always take it out. Really cold winters (wife complained about the lack of heat) I used a piece of cardboard in front of the radiator on her car. But it fit in without being fastened in, and that made it so easy to appease her. Just make sure everything else is ok first, like the thermostat.
If you truly want an answer to this question, start a new thread. Your issue is really off the op's. you will get more of a response starting your own.

 

I have this same problem. Is there any pics or video of what needs to be replaced or where exactly it is.
Thanks