why does the heater only blow cold air,motor is working and switch is on hot, on 1994 Buick LeSabre

when the heater controll is switched on heat, what opens the water flow to the water heater, is it manual or electrnonic

Asked by for the 1994 Buick LeSabre
Sounds like the blend door is not opening to the heater core... Either broken blend door or blend door actuator. Need to find out which. Is the engine temp. getting up to regular operating temperatures?