sounds like there is no hot water from the engine going into the heater core.....
maybe the heater tap is old and jamming up? that would be a good place to start. get some one to operate the climate control from hot to cold and watch the heater tap to see if it switches. if it doesnt, then there is no water going into the core and no hot air!
or there is no vacumn going to the heater tap? that would cause the tap not to switch.