I live in a 2 story house, 3550 sq ft, that has 2 units. One for each floor. Unit for downstairs is reaching the end of it's life cycle and probably needs replacing. Trying to figure out what size do I need. The current one is 5 ton and the AC guy says I can probably get away with only a 4 ton.

The old unit is the original that was installed when the house was built in 1999. Downstairs area is about 2100 sq ft and has vaulted ceilings in a lot of it. House holds temps pretty well, but the windows are all old, too.

Not sure if it helps, but they installed a 4 ton on my new single story house which is approximately 3000 sqft. I haven't been through a Texas summer in this house yet, so I really am not sure if it is oversized or not, but I am concerned with how air tight and well insulated the house is, and if that will cause short cycling.

Right now with it being 75 outside, I'm seeing it cycle on and off like this to maintain 71 inside: 14 min on, 7 min off, 14 min on, 7 min off, etc. Not sure if this is normal given the weather. Seems like that is about three on cycles per hour.

https://www.indeed.com/career-advice/career-development/manual-j-calculation

In an older existing house, there's about as many assumptions you need to make to run a manual J as there are to say it's between 4 and 5 tons. If you have a broad side of the house/and or lots of windows facing west with no shade and you want to maintain 65 degrees on a 105F day while not also running you upstairs unit, then do the 5 ton. If neither of those apply, then 4 ton will most likely keep you at 78F on a 105F day (also depends on how open your upstairs is to air falling down to first floor and how much you're cranking that baby up there.

According to the j calculations my downstairs is sized for about 3.5 tons. The AC guy said that the 5 ton will cool down fast, but not run long enough to remove the appropriate moisture from the air. This apparently creates inefficiency and affects "comfort."

I try to maintain around 75 in the upstairs and 74 downstairs. The worst spot in the house is the upstairs living area, which isn't that big, but gets a lot of sun in the afternoons.

Do a two-stage 4 or 5 and you will be good either way.

I wanted to ask a related question as I'm not sure if my furnace is oversized. How long should I expect my gas furnace to run? I've been noticing with it being 45 F outside, and I set my thermostat to 70 F, the furnace will run about 2 min to heat up, then blow hot air for 5 min before it shuts off. Then it may take 20 min before it runs again since my new house has pretty good insulation and is sealed up well. Is this short cycling? What sort of run times do you all get?

Almost all furnaces are oversized in the south. It doesn't matter. It's not like AC where dehumidification is important.

I would need to do some monitoring on mine to compare, but one suggestion that I have, since I see you're concerned about short cycling on both cooling and heating, is to increase the deadband. If the deadband is 2 degrees, at a 73 setpoint, the AC will kick on when the temp hits 75, and run until it gets down to 73. Your thermostat may use a different word than deadband.

I'll look into that. Does the deadband affect the lower temp too? If I set it to 2 degrees for 72 F, does the A/C cool down to 70?

On the flip side, I think for heating you'd want just the lower deadband active so it turns on at 70 F if I had it set to 72 F, but I'd probably want it shutting off at 72 F rather than 74 F.

I'll echo having someone do a manual j. You're conditioning cubic feet, not square footage and that's only one factor out of many.