Heater blowing half cold half hot in my lexus hiace 2009 model

Is it that only half the vents are blowing cold and the others are blowing hot? Or is it that half the time you are driving they are blowing cold and the other half of the time they are blowing hot? If half of the vents are blowing cold, you may have a blend door issue, if its going from cold to hot and vise versa there may be an issue with the HVAC components (compressor&clutch, high pressure switch, ex)