Return Duct Sizing

Under sized return air ducting is one of the most prevalent system installation problems. The root cause of many other system problems can be traced to airflow deficiencies caused by undersized return duct. (IE: incorrect system pressures, insufficient cooling and/or heating, coil frost up, poor super heat, possible refrigerant slug back on the compressor, and noise in the air handler or duct work).

We would recommend the return duct be a minimum of 30% larger than the correctly sized supply duct.

Example:

• 20" x 8" supply duct = 160 sq inches supply
• 160 sq inches x 1.30 = 208 sq inches return
• 208 sq inches return = 26" x 8" return duct size

This is an excellent rule of thumb and can be useful when trying to diagnose problems with an existing installation.