I have a 97' built 3 story (all 3 above ground), 2100sqft townhouse with a single AC system. The top level of the house has cathedral ceilings (no attic). The air ducts and all but one return are all flex pipe. The unit inside (gas furnace) is a York Diamond 90 (P3URD12N07501A), the unit outside has a model number of H1RC030S06A.
The main problem that we're experiencing is that the unit is not able to cool off or even maintain temperature when it gets over 90 degrees (Maryland). In the summer I have the air distribution (adjustments at the ducts that sit on top of the unit, that split the flow between levels) set in such way that the first two levels are 20% open, while top level is 100% open. When temperatures are around 80-85 the AC unit is able to maintain 78 degrees, turning on and off as necessary. When the temperature is 95-100 (we had a week of 100 degree days), the AC unit will turn on in the morning and not turn off until 3-4AM at night. During the day it easily gets to 80 on the main level, and 84 on the top level.
If my temperature reading are correct (on a 95 degree day), the air coming out of the vents is anywhere from 68 degrees on the registers that are the closest to the main duct, to 72-73 in the top level room furthest from the main duct. Aside from temperature I would say the air flow is marginal at best, but I really don't know how to quantify this.
My understanding is that I have a 2.5 ton unit outside, as far as inside I have no idea, I am assuming it should match up as it's the original that was put in by the builder. In comparison my neighbor (townhouse next to mine) has a 3.5 ton unit, their townhouse is 500sqft larger. It seems to me that my unit is undersized based on what I've been reading online. Assuming 700sqft per ton I it should be at least a 3. Considering that it's a very tall townhouse (40ft to the roof, above ground) and cathedral ceilings (so much more heat is absorbed on the top level).
As far as the temperature readings I provided earlier, they seem to be 12-10 degrees below room temperature, which as far as I understand should be more in the 15 to 20 below range. I also don't know if there is simply enough air flow from the blower. I am assuming a larger unit also means a larger blower? The bottom line is I need to remedy the situation, and looking for some seasoned opinions from those in the know.
Edit: I would like to also note that the system has been serviced, the coil cleaned, the condenser cleaned and topped off with freon, all within the last year.