we have a 06 F-740 in work with a cat C-7 engine.
the past week or so it has started a no heat situation. this thing would make enough heat after 5 minutes running to de ice the windshields in winter.
now all of a sudden, it takes at least 45 minutes of driving to get anything besides cold air out of the heater.
once you get heat, it will stay hot and cook you out of the cab, but until to comes on you are cold.

i looked all over, and there are no water shutoffs i can see.

the heater controls work, and as you flip the vent control you can hear the blend doors open/close.

any ideas on what to look for?

 

Check the coolant level. When to low, it wont circulate through the heater.

 

first thing i did.