First of all, many thanks to those who responded to a question I had several weeks ago. I followed your advice and got an energy audit done.
Here's my current situation: I have a 3300 sq. ft. (with the basement), open floorplan home. The main floor and basement is heated and cooled by a Goodman gas furnace and a/c. The upstairs (fully exposed to the main floor) is heated and cooled by a Trane heat pump, 17 SEER. (I'm sorry that I don't know the sizes of these units.) The heat pump was just repaired (bad capacitor).
My concern: No matter how mild the heat is outside, the upstairs runs constantly.
On Thursday, with a local high temp (I live in Maryland) of only 87, my upstairs unit could not reach the set point of 78 degrees in the later afternoon after being set to 83 when we're at work. When I got home, I watched the thermostat read 81 degrees for 2.5 hours with the unit constantly running.
On Friday, with a local high temp of only 81, I experimented with a set point of 74 degrees (after an "away" set point of 79 degrees). When I got home in the mid afternoon, I observed my unit run for 3 straight hours, but the thermostat temperature dropped down from only 79 to 77.
On Saturday, with a local high of only 83 degrees, I set the thermostat to a steady 79 degrees all day. Even though the upstairs was actually kept at 79 all day, in the afternoon the upstairs unit was running almost constantly, with an approximate run time of 30 minutes followed by only a 5 minute off period before turning back on again.
I'm not educated in these things, but common sense seems to tell me that these numbers might be appropriate for a 100 degree day, but not temperatures in the low 80s. The a/c company told me that both units are working great and that they are fully charged. They checked the ductwork and found no leaks. The energy audit revealed that my house is "tight" (and if I did any more tightening, I'd have to install a system to bring in more outside air to avoid the dangers of stale air or carbon monoxide). My insulation is adequate, my house having been built in 2000, but like most owners, I could always add more insulation. There are "typical" problems of some air leakage round the baseboads, outlets, etc.
I have another company coming out Tuesday to look at the upstairs unit. What sort of things should I have them check? If we assume that the unit is working correctly and that my insulation is adequate, is there something else that could explain my unit working and working and working to get to or keep a set point not much cooler than the outside? Thanks so much!