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In order to understand the foundational principles of duct sizing, a thorough study of ACCA Manual D is a great place to start. This is an industry standard for duct design. In it, you'll learn the principles behind accounting for indoor coil and air filter pressure drops and how to select the best duct fittings.
When it comes to sizing ducts, however, it's evident that very few contractors have the free time to perform the needed steps or to reverse engineer the existing installation. Experience tells us that field installation conditions often don't follow the perfection of a modeled design - this creates varying results and makes the design inaccurate.
To help you overcome this challenge, I would like to share three proven steps that many successful contractors use each day to resize existing duct systems.
EQUIPMENT SELECTION
The first step begins at the air-handling equipment. If you have a system equipped with a weak blower, restrictive coil, or undersized air filter, scratch trying to resize the ducts. These issues must be corrected first. Even a perfect duct system can't overcome these obstacles.
The air-handling equipment you use should ideally have a maximum-rated total external static pressure (TESP) of .70 inches or greater. This is enough fan capacity to move the required airflow in most residential applications, but it's good to note that a variable-speed blower is typically needed.
Many residential fans are only rated to operate at a maximumrated TESP of .50 inches. This blower won't be able to move the required fan airflow when static pressure exceeds the .50 inches rating. To verify this, you'll need to look at nameplate data and the fan performance tables for the equipment.
Once you...