The ductwork is the most misunderstood (and often dismissed) part of a HVAC system and yet it often is the primary reason why homes are uncomfortable. In the summer, air velocity is needed to mix the air and remove the heat. the lack of air velocity at the registers create stale air and it is the primary cause for hot spots and cold spots in a home.
Ductwork must always be sized based on the room heating and cooling load; to remove BTUs require CFM not FT²; a 200 FT² room full of windows facing south may need 600 CFM while a 400 FT² room with no windows may only need 150 CFM.
Ductwork should NEVER ever be sized based on a room FT²
Do not spend money on a high efficient HVAC unit if you cannot afford a proper duct job; the equipment will never achieve its rated efficiency.
Sizing the ducts without a load calculation is a sure path to bad air flow leading to an uncomfortable house and high energy bills.
There are also unforeseen ductwork cost associated with oversized HVAC system. Larger HVAC units produce more CFM requiring larger ducts (both on the supply side and on the return side).
Never replace an existing A/C unit for a new one larger without resizing the ductwork and always make sure that there is enough returns. After all a HVAC is a closed loop system, the blower can only put out what it takes in. Not enough return air affect the efficiency of the system as well as the comfort of the house.
Do it right the first time, ductwork is the most difficult and the most expensive part of the HVAC system to replace (when it can be replaced).
To see the relationship between load calculation and duct size calculation click on the picture below.