They should not be located in the same area as the furnace, nor Duct Work Supplies are located on outside walls. When Cooling: You must have 400 CFM of air flow per Ton of A/C (1 Ton equals 12,000 BTU) Approximately 1 CFM of air is required to heat or cool 1 to 1.25 sq.According to major grille manufacturers, returns. 2: Sizing return grilles is easy just use the 1 sq.ft. Installed ducts rarely perform like they do on paper they must be adjusted individually after installation, using proper testing and balancing methods and No.if the return duct is over 30 feet long and has a few elbows in it.

4 square feet of return duct will result in a velocity of 500 feet per minute. Residential gas furnaces are available in a number of sizes and are measured in British thermal units, or BTUs, per hour. Due to the undersizing of the return and the available sta- tic pressure of the blower, the blower will try to pull air from the attic space if leaks are present in the duct system. Assume that the return is improperly sized and can handle only 1,000 cfm. For example, con- sider a 3-ton system that requires 450 cfm per ton for cooling (1,350 cfm).

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Right-size the HVAC system if a 3-ton unit is enough to satisfy the cooling load, do not install a Historically, duct systems have been sized to fit into the home at the expense of proper airflow.

Return ducts should not be sized according to Table 4-10 or Table 4-11 purely as a way to avoid the 1.

Click here to read the blog post about duct sizing.

Tech Chart: Duct Size Chart Wednesday, July 1, 2015.

Ducted fan design volume 1 pdf