ACCA Manual D is the recommended method to size your duct system. Before you can start to size your duct system, your home’s air conditioning equipment size must be known. This will require a load calculation (ACCA Manual J) to be performed on your home. Once you know the sizing requirements, you will know the air flow (CFM) that is required to make your system perform to the manufacturer’s specifications.
Fact is, most duct systems have been grossly undersized by most installers, leaving the system under performing and causing serious damage to the rest of your system. This includes premature rusting of the air handler, air handler motor failure, compressor failure and excessive power bills.
Air conditioning contractors have been using duct sizing calculators that are designed for sheet metal duct work.
Today’s fiberglass duct systems combined with flex duct are very restrictive in comparison to metal duct work, causing your fiberglass duct system to be undersized by approximately 12% and your flex duct system to be undersized by approximately 30%. Plus, in most cases, the flex duct system loss is much higher due to the fact of poor installation.