I bough this 2012 2500 6.7 early in the yr, everything worked fine, now after the summer of using my ac, i got to use my heater and i can switch from vent, floor, defrost, and have great flow but doesnt NOT blow hot. Anyone have tips or pointers

 

2 options:
this can be a temperature blend door problem...ie the hot air is there but not directed thru the hvac box.
possible faults...
-bad door actuator
-broken blend door
-stuck blend door

or a coolant problem...ie lack of heat exchange from the coolant to the heater core.
the heat exchange can be from a number of reasons
-stuck open thermostat not letting the coolant get hot
-blocked heater core.
-low coolant level