There was a pro on this site a little while ago that said his company insulates ductwork in attics with spray foam. You may want to look into that in conjunction with an 80% furnace. You are likely losing 30% efficiency on your system due to the ductwork being in the attic, so use the price difference between the 80% & the 90% and get your increased efficiency from air sealing & insulating that duct system. Sounds good in theory at least.
An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............
If you can dress so a temperature swing of a few degrees within the comfort zone is okay, I'd install a room thermostat with a temp swing setting and then go with a single stage furnace.
Here is what I come up with on sizing:
1) Observation of current furnace:
At 32°F outside (per HVAC-Calc the low temp for our area is 10°F) and inside set to 69°F, it runs for 5-6 minutes, waits 13-15 minutes, then does it over again.
You can control the off time & on time to a more efficient cycling ratio in both the heating & cooling modes.
Of course, that is what I call short cycling; runtimes are too short with too many cycles per hour; it wastes energy reheating the plenum & duct system & cycles off about the time it is reaching its steady-state efficient heat delivery condition.
At 32°F outside & Inside set to 69°F, it runs for 5-6 minutes, waits 13-15 minutes, then does it over again.
I have mine set to run about one cycle per hour at night with around 14 to 18-miniutes of runtime each cycle. In the day-time the off times may be 2 to 4-hours & when the sun is shining it may not cycle until evening.
At 32-F outdoors or somewhat lower, with the TH Set at 67-F it drops about 3-F before it cycles on. It appears to be very efficient at those settings; using that or a similar room t-stat setting I'm perfectly comfortable, I don't know why I'd ever need a 2-stage furnace.
Ended up going with a 80% 75k VS 2-stage. To vindicate the HVAC-Calc and other data below, the house is cozier than ever, and even at 17°F outside I have only seen it running on low heat. Thanks for the help back then.
Infiltration set to 'average': 25.5k cool and 44.2k heat
Infiltration set to 'excellent': 23.1k cool and 34.6k heat
(house is standard vinyl-sided split-foyer built in 1992 with a vapor barrier, but a few leaky spots.)
Systems are less efficient and will fail sooner when running on low stage constantly