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Q:

Heater only let's heat out when it wants to. It lets it blow heat for about 15 l on 2008 Ford F-350 Super Duty

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We drove fron San Angelo, TX NOV 26, to Fayetteville , AR for thanksgiving we just got into town and the temp drop out of the heated air coming out. It took about 20 min and I turn down the street my Aunt lives on and the heat came back on. It is still intermittent I need to grey it fixed before I go back to Texas on Sunday "
(1) Answer
I would start by replacing the t stat and see what happens If there is no change then check the blend door operation fords are famous for broken doors