Just got a new 3 ton Lennox XP14 to replace an older 3 ton Lennox. To get the SEER and EER up it was matched with a 3.5 ton, CBX27UH-042 air handler. I have two returns with a damper just after the main return in an attempt to draw more air from a to hot/cold Great rm. This was not a problem before with a 3 ton AH and I was able to even out the temps with the damper. Speed on the new AH has been double checked and is correct. Only other change is I went from a 20x20x1 mid grade filter, at the AH, to a 20x25x4 Honneywell MERV 11, at the AH.
With the damper wide open I have no noise problem but have a 4-5 deg temp difference from one end of the house to the other. Tech ran system with no filter and door on and still had same noise. Tried again with door off and noise went away. He is suggesting I change the 12 inch flex on a 14x14 grill to a 14 inch flex on a larger grill. Will this be enough to work? I'm wondering if I have enough area anyway. Grill area of main return is 23x13 though the duct board that leads from it is 12x20 then makes a 2' run to a 24x26x16 " box that the 12 inch flex also leads to. Flex is maybe 25' long and sits on top of attic insulation the whole way and makes a lazy S turn to the 14x14 grill. If 14 inch flex is enough what size grill should I get 14x?
House is 1800 sq. ft. and the Great rm has a 16' cathedral ceiling and two sky lights that face WNW.