2000 F350 heater not working when in over drive

Coolant may be low or might be air in the cooling system. Was the air burped out after refilling? Was the heater set on hot when burping the system?

Not sure how to burp system. How do I do that?

With the radiator cap off, let the vehicle run so the thermostat opens. It may push fluid out the radiator but that is ok, that would be the air working its-way out. Have extra coolant to be able to pore back in once the level drops in the radiator. Do not rev up the engine or you will get a shower with the cap off. Have the heater temp set on high. Let the thermostat cycle a few times you can tell by watch the level in the radiator and by feeling the top radiator hose. It gets hot when the thermostat opens. Having the front end of the vehicle elevated helps.

Thanks - we'll give that a try and see if it solves the problem

Sounds good.