I just purchased a 2005 F-350 6.0 Diesel and when the truck idels there is no heat but when I drive it the heat comes on right away. At work when this happend to our 7.3 it had a bad head gasket.

 

The head gasket blew today! The heater will not put out any heat. Then I noticed the fan was running under the hood but the temp gauger was OK. All of a sudden when I turned a bell rang and I looked it read WATER IN FUEL then it read CHECK YOUR GAGES the water was in the red so I stopped and let it cool off. You could see the steam blowing out of the head. With the cap off the radiator restore it stopped blowing out.
I called the FORD towing 1-800 number what a joke! This is a waste of time. They told me there is no towing on Sundays so leave it unlocked and put the key under the seat. OK $50,000.00 truck unlocked????? Good advice! I had my garage get it and took it to my house.I hope my Dealer is better than Ford Towing.

 

That sucks. I (fortunately) ain't a diesel guy, but I hope your dealer gets you squared away ASAP.