Hi there, I have a question about the sizing of return ducts and how it compares to the inlet port size on HVAC equipment. I am also curious about options for adding additional return capacity using my existing duct configuration.
I have a 3.5-ton unit with a return inlet that measures 12" x 19" (228 sq in). For this tonnage, most rules of thumb would suggest a ~20" round flex return line based on cfm requirements, which equates to about 315 sq in. Since the equipment has a smaller inlet area than the duct, what sort of issues could occur? Would the air velocity at the HVAC unit simply be greater than in the trunk line, assuming that the blower can maintain its rated airflow based on the static pressure in the return line? I presume this isn't a big deal, as this series of HVAC equipment has the same return inlet size all the way up to 5 tons.
My duct system has a 16" main return trunk, which necks down to 14" and then splits into two 12" ducts and two 12" x 12" filter registers. There are no other return paths in the house. The system is noticeably choked down and the grilles make a lot of noise. I am trying to determine if additional return branches can be added to the main 16" trunk, or if I need to budget for a larger trunk that goes all the way back to the equipment. How might I determine this?
First-time poster. Thank you for the help. :)
I have a 3.5-ton unit with a return inlet that measures 12" x 19" (228 sq in). For this tonnage, most rules of thumb would suggest a ~20" round flex return line based on cfm requirements, which equates to about 315 sq in. Since the equipment has a smaller inlet area than the duct, what sort of issues could occur? Would the air velocity at the HVAC unit simply be greater than in the trunk line, assuming that the blower can maintain its rated airflow based on the static pressure in the return line? I presume this isn't a big deal, as this series of HVAC equipment has the same return inlet size all the way up to 5 tons.
My duct system has a 16" main return trunk, which necks down to 14" and then splits into two 12" ducts and two 12" x 12" filter registers. There are no other return paths in the house. The system is noticeably choked down and the grilles make a lot of noise. I am trying to determine if additional return branches can be added to the main 16" trunk, or if I need to budget for a larger trunk that goes all the way back to the equipment. How might I determine this?
First-time poster. Thank you for the help. :)
source https://hvac-talk.com/vbb/showthread.php?2224142-Return-duct-sizing-trunk-line-vs-equipment-inlet-cfm-velocity&goto=newpost
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