I have a 4 ton unit on a simple 3 BR 1 bath one story house.
On the plenum I have a 12" metal duct supplying the living room, 10" metal supplying dining room and kitchen (2 vents off the same line), 10" metal ducts (2 vents off the same line) supplying 2 bedrooms, 9" flex duct for the 3rd bedroom, and 7" flex duct for the bathroom.
When the heat is on, the air coming out of the 12" metal in the living room and the 10" in the dining room and kitchen is extremely warm >90 deg.
However, the air out of the 10" metal for the 2 bedrooms and the 2 flex ducts is cold <65 deg.
All of the heating elements are good.
Connections at the plenum are snug (even had the plenum rebuilt). No discernible difference in air pressure from the vents in any room.
I've had several A/C repair specialists out and everyone seems stumped.
It's difficult for me to get my head around b/c all of the air entering the plenum is roughly equally warm having passed over the heating coils right?
There's no leak in the ducts and even if there was it'd be quite a coincidence since the vents blowing cold air are on 3 separate lines (7" flex, 9" flex, 10" metal).
The house is basically rectangular and the plenum sits in the middle of the attic. The hot air vents are north, the 2 flex ducts (cold air) are east and west of the plenum and the 10" metal cold air blowing duct runs south off the plenum.