I wish to update my 37 year old HVAC system in my house and am finding that it is not a simple venture. The problem is that the system (all original) is a commercial Trane Climate Changer air handler with 6 zones and air flow up to 7,500 cfm. Heat source is a boiler (300,000 btu input) and the motor for the air handler runs at 1,200 rpm during heating and 1,800 rpm during cooling. The boiler is oversized because it also supplies another Trane air handler (Torrivent) for a 1,000 sq. ft. solarium. Cooling is from two 5 ton Trane air conditioning units but the second is never called on. The system is switched to either heat or cool at one time. Air flow is the same in each zone when heating is called for or not and when cooling is called for or not. The house is ~3,500 sq. ft plus ~1,200 sq. ft. lower level which we use often (rec. room) for a total of 4,700 sq. ft. (not including the solarium). We are located in North Central US and have large overhangs with no direct sun inside (except the solarium).
The basic problem is that I have read that ducts can be too small and also too large. If we convert to residential unit(s) we won't have the airflow that the ducts and vents were designed for. I asked for 3 bids to start and they ranged from 1 unit to 3 units including zoning. I am confused at this point and am unable to proceed. I realize that this is not an easy question but what do we have to consider when updating our system especially in regard to the duct/register size?