A duct type air conditioning system with a variable capacity blower in which the maximum blower capacity is established at initialization of the system at the optimum blower capacity. The optimum capacity is established by varying the capacity of the blower and measuring the air flow volume and air flow noise. The optimum capacity is inputed into the control system through a central thermostat which has a liquid crystal display associated therewith. The system installer interfaces with the control system by a dialog which occurs through the liquid crystal display. The optimum capacity of the blower is stored in a memory device, and the control system variably controls the capacity of the blower so as not to exceed the optimum capacity.