Second question: The house is generally quite humid. Since the tech told me everything checks out regarding both units' operation, could the humidity simply be a result of this being an older house? I can think of several contributing factors:
-the windows are original (wood)
-some of the exterior doors don't seal very well
-the blown insulation in the attic is covering up the soffet vents (I need to rake the stuff back to uncover them)
-I live in a humid climate (northern Virginia)
The house I came from is in the same general area but was a recent construction. I never measured the indoor humidity there but it was comfortably dry. In the house I'm in now it's upwards of 70-75% according to the humidistat on the little dehumidifier I bought from the hardware store.
Are there any simple steps I can take to improving my system's dehumidification, or do I just need to deal with it until I can afford to replace windows and doors? Should I consider retrofitting dehumidifiers to the air handlers themselves (big expense)?
Hopefully, the little dehumidifier's humidistat is not accurate; 70 to 75 is a very high humidity level.
The A/C systems appear to be oversized as runtimes are too short.
In a humid climate it needs long runtime cycles.
Get a room t-stat with a temp differential setting.
[PDF] E G O User's Manual
The Robertshaw 9400 is a single-stage thermostat designed to control ... Adjustable temperature differential: .5°F to 3.0°F (0.5°C to 1.5°C) .... The display will return to the current room temperature after a few seconds. Item 9555 .... Indoor fan connection. Yes. Energizes with Y & W terminals or with FAN ... non-programmable...
Just an example; there may be better differential room t-stats...
Slowing the blowers down to 350-cfm per ton of cooling should help some.
Also, may be too high an air infiltration rate; try to getting it down to 0.375-ACH - air changes per hour.