Repair-It

For heat, the 9k can do 6-12.5k depending on fan speed, outside air temp using 5F (as the low end and realistically I tend not to use them when much colder than upper 20s) and 68F as the inside temp. The 18k can do 6-22k depending on fan speed and same other conditions.

If I use 27F which really is the coldest I would ever try to heat the house with these things, and substituting the 5th gen specs for my existing 4th gen just for estimation purposes, I am looking at:
18k: max 18k on turbo fan (which I never use) or 13k on high fan
9k: max 9.3k on turbo fan or 8.3k on high fan

I will have two 18s and 3 9s when all said and done. About 64k BTU with everything screaming, which I would never done unless my furnace broke. On high fan the max would be 51k BTU. My furnace should put out around 97.5k BTU with 80% efficiency and another 10% loss through ducts (no idea on accuracy). The furnace would certainly cycle at 27F so perhaps on turbo I could just about keep the house at 68F? Not sure about on high fan though. If 51k was enough that would imply a roughly 52% duty cycle for my furnace at 27F outside. Maybe? I wish I had that data.

Ordered 27k with 18+9 as the 10% discount was about to expire. This will be the most costly, hardest to install, ugliest aesthetically (outside view) and adds arguably the least benefit of all the zones. Almost didn't pull the trigger last night. But I just want to get to a full install, not "3 tons and 3 zones plus a window A/C and sometimes the downstairs gets a little warm."