18 inch flex duct 4ton system - 1440 CFM

kbr0125April 27, 2013


Currently getting my 4ton XL20I/XC80 Trane system installed, they replaced the old return that was a 16" that was bent and squished. so they did some framing and opened it up to fit a 18 inch r-6 flex return line, it is about 10-12 feet in length connected to a 20x30x1 return filter/grate. is this large enough to support the 4ton system with out causing damages? they did a CFM test with a cool hand held fan and it was reading 1440 CFM, they are coming back to try to straighten it up to get more CFM out of it but will it decrease the efficiency or hurt the system if it is getting 1440 CFM and not the 1600 every one recommends?

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The 18 inch flexible duct is large enough. The problem is the filter area is too small and you are using a one inch filter. You need a minimum area of 25 x 30. If possible you should retrofit the opening to accept a 4 inch filter.

The down side of not increasing the air flow is extra wear on the blower motor and a noisy return.

    Bookmark   April 27, 2013 at 11:05AM
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The flex needs to be pulled tight and run as straight as possible to really be effective. It would have been best to run round hard duct for as many feet as possible, and then used flex. 20" flex would have been better, and as said, the filter grille area is too small. I would add a media filter at the unit instead and eliminate restrictive 1" filters at the grille(s). Can they add another return? That is likely your best bet.

    Bookmark   April 27, 2013 at 11:42AM
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Manual D calls for a Max of 700 ft/min on a flex duct RA trunk or branch, your moving 815 at 1440 cfm. If your in a high humidity area the 360 cfm per ton is fine, if your in a dry area like I am in Arizona not so good. My concern would be what static ( total external static) the system is running at to give you your 1440 cfm, if it is high you should resolve what ever is causing it.

If your system is using a VS motor, and it should be with the 20i the life and the efficiency of the motor will be reduced with high statics. And yes you do have an undersized RAFG, as Ryan has said if at all possible add a second RA.

    Bookmark   April 28, 2013 at 5:01PM
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