I need a little expert advice with an AC problem I'm having. My house is a 1995 build and was not originally configured for AC. It has a 135k 5 ton Carrier furnace, an adequate number of registers (every window), two large floor returns on the main floor and one small high wall return upstairs. The high wall return is reduced to about 2" x 5" as it passes the staircase in my wall cavity. The house is 1770 sqft above grade and 1050 of that is on the main floor. I had my ducts and returns professionally cleaned in the fall.
Just last summer when I moved in, I installed a Rheem 2.5 ton R410A unit and A coil. The house takes forever to cool down if it cools down at all. An example would be today:
-I installed a brand new filter before conducting this study.
-I live in Denver and it was a 90F day today with lots of sun.
-By 4pm my house was 80F inside.
-I turned on the AC and measured my Delta T at the furnace. My return was 65F and the output was 42F giving a 23 degree yield. Not bad I thought.
-At 545pm (After 1hr 45m) the temperature at the main floor thermostat was 79.5F. The return still measured 65F, however the output had surprisingly dropped to 40F.
-Both times I measured, my mainfloor registers showed ~45F while my upstairs registers showed ~50F.
I have trouble understanding why my house isn't cooling down when its apparent that the AC unit itself is performing great and actually improving as it runs. One observation I've made (and I do not own an airflow meter) is that the upstairs vents have very poor supply pressure in comparison to the downstairs vents. All vents in the house are in-floor. This still doesn't explain to me how the main floor temperature would remain so high.
My initial thought was that the massive floor returns were simply recirculating the same cold air over and over again, but that was dis-proven by my return measurements being 65F both times. My original plan was to pull more high return air from the second floor.
Before I go running new ductwork and tearing my house apart, I thought I'd see if anyone has any input to offer. Thanks in advance!