Welcome! Here are the website rules, as well as some tips for using this forum.
If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Hello, thanks for providing such a forum. Wish I'd found it sooner. :~)
Can anyone clear up what is the ballpark minimum return size required for a 1400CFM air handler, assuming adequate supply side, sealed ducts, etc.?
Specifically, at issue is a 2/3 reduction in return duct capacity after a new heat pump/air handler installation. The installed unit is a Trane XR15 3.5-ton heat pump (4TWR5042G1000A) and XR single-speed ECM air handler (GAM5B0C42M31SB). The air handler is mounted in a garage HVAC closet of our 40-year-old, 1.5 story, partially underground, 2400 sqft home (the home being partially below grade was the rationale for undersizing by 0.5-ton). Ducts were sealed 10 years ago.
The house was built with 24x24 and 12x12 returns which meet at the base of the air handler, for a total of 720 sq.in. Trane specs the AH at ~1400 CFM, which requires 700 sq.in. by the "2CFM per sq.in" rule-of-thumb for AH sizing. Therefore duct size should have been adequate.
But due to the greater height of the new air handler, the installer downsized the returns into a 9" high x 24" wide box under the air handler. That's 216 sq.in., or 432CFM per the 2x rule. The HVAC company owner says this is more than enough by a significant margin.
To confuse me further, Trane's recommends a 20x22 filter for this AH model, which is only about 430 sq.in. internally, or 860CFM.
So the rule-of-thumb's 700 sq.in, the contractor's 216 sq.in., or Trane's own 430 sq.in. . . . what is adequate for 1400CFM? ?