I have a Ford E-450 RV with a heater that does not seem to shut off. When the air control is set to "Vent" and the temperature contorl set to the coolest setting, the heat is pouring out of the vents.

Does anyone know if the heater lines are shut down via a valve if the controls are not in heat mode? Higher end vehicle like the Lincoln and Mercury's have such a valve.

Is there such things as plumbing diagrams for this vehicle?

 

 

This is exactly my problem in a 1999 E450. Did no one else ever have this problem? The AC seems to work okay, so don’t think it is the common defrost problem that receives all the replies.

 

The blend door actuator is broken. It directs air around or through the heater core. It can be tricky to replace. Look for a YouTube.

Diagram of the air flow is here:
https://www.ford-trucks.com/forums/1273073-easy-cheap-quick-fix-for-ford-defrost-vent-problems.html