How old is the car, did this happen suddenly or gradually, or have you only just got the car ?
Has any work been done on cooling system ? Such as coolant change because an airlock in heater pipes or matrix can prevent proper flow and low output. Is the cabin filter clear ?
I know that some posters have had problems with the motors / solenoids and linkages that move the flaps for temperature and which vents the air blows out of, but they are buried inside the dashboard so not easy to check.
Does the car have climate control is there a temperature display on the dash.
There is a temperature sensor behind a little are of perforated dashboard below and to the left of steering wheel, this is to match cabin temperature to what you select and may be FUBAR.