HVAC unit has a single blower fan, an evaporator downstream of the blower and a heater downstream of the evaporator, wherein each zone outlet includes a temperature mixing door for controlling portions of hot and cold air and an output valve for controlling a zonal output flow rate. A method is devised to control the discharge of temperature-conditioned air from a plurality of zone outlets of an automotive HVAC system via such an HVAC unit by the steps of reading an operator-requested zonal discharge blower level for each of the zone outlets; converting each zonal discharge blower level request to a zonal flowrate request; calculating a total requested output flowrate as a summation of all zonal flowrate requests; and adjusting a blower voltage to a minimum voltage required for generating the total requested output flowrate.