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Heating, Air Conditioning, Fridge, HVAC/Ovaled Metal Duct - air flow problem


I recently had a new 3 Ton down flow  System installed; Lennox G61MP-36B-071, Lennox XC14-036-230-1, Lennox CR33-30/36B-F coils. The metal plenum on top of the furnace measures 16 3/4" W x 25" deep x 13 1/2 " high. It has an 18" metal return air duct that enters on top of it for the return air entrance. There is also a new 18" return air flex duct in the attic that I know has problems.   My question : the installer squeezed the sides of the 18" metal duct where it enters the top of the plenum - so it now forms a dovetailed oval that is 15 3/4" wide x approximately 19 1/2" deep ( the depth from front to back was difficult to measure, but this is close) will this cause an air flow problem?

There is a big difference between changing the shape, and changing the opening size.  The area of the measured space you gave is bigger then that of the round duct.  In some cases, there is just on other way to install ducts without changing the shape which does not actually change the size of the opening.  You should be ok, as long as it was not pinched too much.


Heating, Air Conditioning, Fridge, HVAC

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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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