I think i have a problem with air flow. the return opening is 12" by 23" and the furnace is rated at 1600 cfm. The AC cycles on and off every 15 minutes respectfully. The thermostat is set at 79 but if i set it lower it takes longer to cool the house (1700 sq ft). I have a york h1zd model condenser that is 13 seer. it is a 410a system with a suction pressure of 127 and a head pressure of 245. I think i have adequate ducts coming out of the supply. i have four 6" ducts, one 10" and one 14" trunk coming out of the supply plenum. i have a feeling that the unit is starving for air. comments? suggestions?
more information --
The furnace and coil sit on a platform in the attic. the unit lays on it's side and an opening the size of 13" x 23" is cut out on side that lays on the platform. a 20" x 30" return plenum attaches to the bottom of the platform and a 20" x 30' return grill attaches to the bottom of that plenum. i think i need to add a 10" flex duct from the plenum to the unit since i can't cut the opening any larger.