I do not think there is a set rule since every home has different heat/cool loads. I’ll probably come down to what’s is an acceptable temperature and comfort for the home and family. This is what I’ve learned from my homes.
I’m last home in Lake Worth was two floors and very open. The house was built with two 2ton units The second floor could never keep up until I replaced it with a 2.5 ton. Some HVAC experts don’t agree with me that the second floor needs to be the larger unit. Most go by floor space that might be ok for snow country but not for the gulf coast states. The heat gained by the first floor does rise, therefore the second floor unit is handling both floors for cooling. My home had the master bedroom on the first floor.
For 26 years owning the home, I found that the thermostat had be set two-three degrees higher (depending on the season) for the second floor than the first. If the both were set to the same temperature, the second floor would run all day and only cycle at nighttime. It also prevented the first floor from running much and cause the master bedroom to be warm since the AC didn’t run much. With both set to the same temperature, the second floor unit would cool the first floor by the cold air dropping and it usually satisfied the first floor thermostat but the second floor remained hot.
The temp difference did make for a more comfortable home. During the heat of summer, my second floor unit ran most of the day even with the 2.5 ton and both floors were fairly comfortable with the first floor unit cycling properly. It made for a comfortable humidity level. It wasn’t a very well insulated home and I had a lot of glass.
Now with WiFi and programable thermostats you can set it up for time of day for when the rooms are occupied. Check out the Honeywell T9 thermostat. You can add remote sensor, give them a priority level on the program and these sensor have motion detection so temps will adjust when the space is occupied.
My current home is also two floors but one 3.5 ton on the second floor with the thermostat on the first. It works pretty good as long as I keep the fan mode in “circulation” mode. A minimum of 35 minutes an hour the fan will run. It pulls excess heat from the second floor and circulates the air and mixes with the cooler first floor air making it not perfect temperatures between the floors but acceptable at least during the day light hours. Night time the temps are fairly even as long as the fan is running.
