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Old 11-30-2013, 10:56 PM   #1
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Cold Air Return Sizing


Hello, I recently tore out most of my older, rusty and bad ductwork in my home. My home is approximately 120 years old and has a 3ton, 100k btu, 1200cfm, 92% efficient gas furnace. I did NOT do an in depth j then d calculations for the home. I just did a rough 1-1.25 cfm per sq. ft to determine room needs for supply ducts. From online data tables for the round rigid duct I will be using my supply will be totaling about a 1000 cfm or more in high end. The original return boot and stack was a 8x20 duct, so instead of panning of floor joists that were hard to get at and clean I ran more 8x 20 rect. duct to floor grills. I put two centrally located grills in the home near the open stairwell. One is 8x30 and the other 12x24. Also a smaller 4x14 in the bottom stair rise. My question is: Is the 8x20 return duct enough? From what I found the 8x20 should move about 900 cfm. Should I add another return line to one of the return grills and hook it up on the other side of the furnace with another boot and filter? I will be using high quality filters also.
Kirk

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Old 12-01-2013, 07:09 AM   #2
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Cold Air Return Sizing


You need enough supply and return to move at least 1200 CFM, or the furnace will lock on on its high limit switch. So your 8X20 is too small.

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Old 12-01-2013, 08:26 AM   #3
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Cold Air Return Sizing


Thank you; with out scrapping the 8x20 pipe I have, can I just add another return boot and filter to the other side of furnace? I heard they do that for larger furnaces who need more air.
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Old 12-01-2013, 10:00 AM   #4
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Cold Air Return Sizing


Yes, you can bring another return into the other side of the furnace.
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